locking: WW mutex cleanup
Make the WW mutex code more readable by adding comments, splitting up functions and pointing out that we're actually using the Wait-Die algorithm. Cc: Ingo Molnar <mingo@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Gustavo Padovan <gustavo@padovan.org> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Sean Paul <seanpaul@chromium.org> Cc: David Airlie <airlied@linux.ie> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Kate Stewart <kstewart@linuxfoundation.org> Cc: Philippe Ombredanne <pombredanne@nexb.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: linux-doc@vger.kernel.org Cc: linux-media@vger.kernel.org Cc: linaro-mm-sig@lists.linaro.org Co-authored-by: Thomas Hellstrom <thellstrom@vmware.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> Acked-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Ziljstra
Thomas Hellstrom
Родитель
eab9766931
Коммит
55f036ca7e
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@ -32,10 +32,10 @@ the oldest task) wins, and the one with the higher reservation id (i.e. the
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younger task) unlocks all of the buffers that it has already locked, and then
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tries again.
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In the RDBMS literature this deadlock handling approach is called wait/wound:
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In the RDBMS literature this deadlock handling approach is called wait/die:
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The older tasks waits until it can acquire the contended lock. The younger tasks
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needs to back off and drop all the locks it is currently holding, i.e. the
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younger task is wounded.
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younger task dies.
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Concepts
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--------
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@ -56,9 +56,9 @@ Furthermore there are three different class of w/w lock acquire functions:
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* Normal lock acquisition with a context, using ww_mutex_lock.
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* Slowpath lock acquisition on the contending lock, used by the wounded task
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after having dropped all already acquired locks. These functions have the
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_slow postfix.
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* Slowpath lock acquisition on the contending lock, used by the task that just
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killed its transaction after having dropped all already acquired locks.
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These functions have the _slow postfix.
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From a simple semantics point-of-view the _slow functions are not strictly
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required, since simply calling the normal ww_mutex_lock functions on the
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@ -220,7 +220,7 @@ mutexes are a natural fit for such a case for two reasons:
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Note that this approach differs in two important ways from the above methods:
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- Since the list of objects is dynamically constructed (and might very well be
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different when retrying due to hitting the -EDEADLK wound condition) there's
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different when retrying due to hitting the -EDEADLK die condition) there's
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no need to keep any object on a persistent list when it's not locked. We can
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therefore move the list_head into the object itself.
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- On the other hand the dynamic object list construction also means that the -EALREADY return
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@ -6,7 +6,7 @@
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*
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* Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
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*
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* Wound/wait implementation:
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* Wait/Die implementation:
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* Copyright (C) 2013 Canonical Ltd.
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*
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* This file contains the main data structure and API definitions.
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@ -28,9 +28,9 @@ struct ww_class {
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struct ww_acquire_ctx {
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struct task_struct *task;
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unsigned long stamp;
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unsigned acquired;
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unsigned int acquired;
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#ifdef CONFIG_DEBUG_MUTEXES
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unsigned done_acquire;
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unsigned int done_acquire;
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struct ww_class *ww_class;
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struct ww_mutex *contending_lock;
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#endif
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@ -38,8 +38,8 @@ struct ww_acquire_ctx {
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struct lockdep_map dep_map;
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#endif
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#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
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unsigned deadlock_inject_interval;
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unsigned deadlock_inject_countdown;
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unsigned int deadlock_inject_interval;
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unsigned int deadlock_inject_countdown;
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#endif
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};
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@ -102,7 +102,7 @@ static inline void ww_mutex_init(struct ww_mutex *lock,
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*
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* Context-based w/w mutex acquiring can be done in any order whatsoever within
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* a given lock class. Deadlocks will be detected and handled with the
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* wait/wound logic.
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* wait/die logic.
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*
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* Mixing of context-based w/w mutex acquiring and single w/w mutex locking can
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* result in undetected deadlocks and is so forbidden. Mixing different contexts
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@ -195,13 +195,13 @@ static inline void ww_acquire_fini(struct ww_acquire_ctx *ctx)
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* Lock the w/w mutex exclusively for this task.
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*
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* Deadlocks within a given w/w class of locks are detected and handled with the
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* wait/wound algorithm. If the lock isn't immediately avaiable this function
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* wait/die algorithm. If the lock isn't immediately available this function
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* will either sleep until it is (wait case). Or it selects the current context
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* for backing off by returning -EDEADLK (wound case). Trying to acquire the
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* for backing off by returning -EDEADLK (die case). Trying to acquire the
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* same lock with the same context twice is also detected and signalled by
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* returning -EALREADY. Returns 0 if the mutex was successfully acquired.
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*
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* In the wound case the caller must release all currently held w/w mutexes for
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* In the die case the caller must release all currently held w/w mutexes for
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* the given context and then wait for this contending lock to be available by
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* calling ww_mutex_lock_slow. Alternatively callers can opt to not acquire this
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* lock and proceed with trying to acquire further w/w mutexes (e.g. when
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@ -226,14 +226,14 @@ extern int /* __must_check */ ww_mutex_lock(struct ww_mutex *lock, struct ww_acq
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* Lock the w/w mutex exclusively for this task.
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*
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* Deadlocks within a given w/w class of locks are detected and handled with the
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* wait/wound algorithm. If the lock isn't immediately avaiable this function
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* wait/die algorithm. If the lock isn't immediately available this function
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* will either sleep until it is (wait case). Or it selects the current context
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* for backing off by returning -EDEADLK (wound case). Trying to acquire the
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* for backing off by returning -EDEADLK (die case). Trying to acquire the
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* same lock with the same context twice is also detected and signalled by
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* returning -EALREADY. Returns 0 if the mutex was successfully acquired. If a
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* signal arrives while waiting for the lock then this function returns -EINTR.
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*
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* In the wound case the caller must release all currently held w/w mutexes for
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* In the die case the caller must release all currently held w/w mutexes for
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* the given context and then wait for this contending lock to be available by
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* calling ww_mutex_lock_slow_interruptible. Alternatively callers can opt to
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* not acquire this lock and proceed with trying to acquire further w/w mutexes
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@ -256,7 +256,7 @@ extern int __must_check ww_mutex_lock_interruptible(struct ww_mutex *lock,
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* @lock: the mutex to be acquired
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* @ctx: w/w acquire context
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*
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* Acquires a w/w mutex with the given context after a wound case. This function
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* Acquires a w/w mutex with the given context after a die case. This function
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* will sleep until the lock becomes available.
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*
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* The caller must have released all w/w mutexes already acquired with the
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@ -290,7 +290,7 @@ ww_mutex_lock_slow(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
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* @lock: the mutex to be acquired
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* @ctx: w/w acquire context
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*
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* Acquires a w/w mutex with the given context after a wound case. This function
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* Acquires a w/w mutex with the given context after a die case. This function
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* will sleep until the lock becomes available and returns 0 when the lock has
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* been acquired. If a signal arrives while waiting for the lock then this
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* function returns -EINTR.
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@ -244,6 +244,17 @@ void __sched mutex_lock(struct mutex *lock)
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EXPORT_SYMBOL(mutex_lock);
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#endif
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/*
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* Wait-Die:
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* The newer transactions are killed when:
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* It (the new transaction) makes a request for a lock being held
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* by an older transaction.
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*/
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/*
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* Associate the ww_mutex @ww with the context @ww_ctx under which we acquired
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* it.
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*/
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static __always_inline void
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ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
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{
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@ -282,26 +293,53 @@ ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
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DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
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#endif
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ww_ctx->acquired++;
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}
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static inline bool __sched
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__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
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{
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return a->stamp - b->stamp <= LONG_MAX &&
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(a->stamp != b->stamp || a > b);
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ww->ctx = ww_ctx;
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}
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/*
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* Wake up any waiters that may have to back off when the lock is held by the
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* given context.
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* Determine if context @a is 'after' context @b. IOW, @a is a younger
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* transaction than @b and depending on algorithm either needs to wait for
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* @b or die.
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*/
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static inline bool __sched
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__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
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{
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return (signed long)(a->stamp - b->stamp) > 0;
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}
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/*
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* Wait-Die; wake a younger waiter context (when locks held) such that it can
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* die.
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*
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* Due to the invariants on the wait list, this can only affect the first
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* waiter with a context.
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* Among waiters with context, only the first one can have other locks acquired
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* already (ctx->acquired > 0), because __ww_mutex_add_waiter() and
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* __ww_mutex_check_kill() wake any but the earliest context.
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*/
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static bool __sched
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__ww_mutex_die(struct mutex *lock, struct mutex_waiter *waiter,
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struct ww_acquire_ctx *ww_ctx)
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{
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if (waiter->ww_ctx->acquired > 0 &&
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__ww_ctx_stamp_after(waiter->ww_ctx, ww_ctx)) {
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debug_mutex_wake_waiter(lock, waiter);
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wake_up_process(waiter->task);
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}
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return true;
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}
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/*
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* We just acquired @lock under @ww_ctx, if there are later contexts waiting
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* behind us on the wait-list, check if they need to die.
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*
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* See __ww_mutex_add_waiter() for the list-order construction; basically the
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* list is ordered by stamp, smallest (oldest) first.
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*
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* The current task must not be on the wait list.
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*/
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static void __sched
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__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
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__ww_mutex_check_waiters(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
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{
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struct mutex_waiter *cur;
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@ -311,30 +349,23 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
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if (!cur->ww_ctx)
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continue;
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if (cur->ww_ctx->acquired > 0 &&
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__ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
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debug_mutex_wake_waiter(lock, cur);
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wake_up_process(cur->task);
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}
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if (__ww_mutex_die(lock, cur, ww_ctx))
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break;
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}
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}
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/*
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* After acquiring lock with fastpath or when we lost out in contested
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* slowpath, set ctx and wake up any waiters so they can recheck.
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* After acquiring lock with fastpath, where we do not hold wait_lock, set ctx
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* and wake up any waiters so they can recheck.
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*/
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static __always_inline void
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ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
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{
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ww_mutex_lock_acquired(lock, ctx);
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lock->ctx = ctx;
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/*
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* The lock->ctx update should be visible on all cores before
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* the atomic read is done, otherwise contended waiters might be
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* the WAITERS check is done, otherwise contended waiters might be
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* missed. The contended waiters will either see ww_ctx == NULL
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* and keep spinning, or it will acquire wait_lock, add itself
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* to waiter list and sleep.
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@ -348,29 +379,14 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
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return;
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/*
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* Uh oh, we raced in fastpath, wake up everyone in this case,
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* so they can see the new lock->ctx.
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* Uh oh, we raced in fastpath, check if any of the waiters need to
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* die.
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*/
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spin_lock(&lock->base.wait_lock);
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__ww_mutex_wakeup_for_backoff(&lock->base, ctx);
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__ww_mutex_check_waiters(&lock->base, ctx);
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spin_unlock(&lock->base.wait_lock);
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}
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/*
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* After acquiring lock in the slowpath set ctx.
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*
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* Unlike for the fast path, the caller ensures that waiters are woken up where
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* necessary.
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*
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* Callers must hold the mutex wait_lock.
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*/
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static __always_inline void
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ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
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{
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ww_mutex_lock_acquired(lock, ctx);
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lock->ctx = ctx;
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}
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#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
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static inline
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@ -646,37 +662,73 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock)
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}
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EXPORT_SYMBOL(ww_mutex_unlock);
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static __always_inline int __sched
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__ww_mutex_kill(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
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{
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if (ww_ctx->acquired > 0) {
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#ifdef CONFIG_DEBUG_MUTEXES
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struct ww_mutex *ww;
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ww = container_of(lock, struct ww_mutex, base);
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DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
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ww_ctx->contending_lock = ww;
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#endif
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return -EDEADLK;
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}
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return 0;
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}
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/*
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* Check whether we need to kill the transaction for the current lock acquire.
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*
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* Wait-Die: If we're trying to acquire a lock already held by an older
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* context, kill ourselves.
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*
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* Since __ww_mutex_add_waiter() orders the wait-list on stamp, we only have to
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* look at waiters before us in the wait-list.
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*/
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static inline int __sched
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__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
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__ww_mutex_check_kill(struct mutex *lock, struct mutex_waiter *waiter,
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struct ww_acquire_ctx *ctx)
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{
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struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
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struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
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struct mutex_waiter *cur;
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if (ctx->acquired == 0)
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return 0;
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if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
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goto deadlock;
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return __ww_mutex_kill(lock, ctx);
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/*
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* If there is a waiter in front of us that has a context, then its
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* stamp is earlier than ours and we must back off.
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* stamp is earlier than ours and we must kill ourself.
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*/
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cur = waiter;
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list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
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if (cur->ww_ctx)
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goto deadlock;
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if (!cur->ww_ctx)
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continue;
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return __ww_mutex_kill(lock, ctx);
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}
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return 0;
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deadlock:
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#ifdef CONFIG_DEBUG_MUTEXES
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DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
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ctx->contending_lock = ww;
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#endif
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return -EDEADLK;
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}
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/*
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* Add @waiter to the wait-list, keep the wait-list ordered by stamp, smallest
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* first. Such that older contexts are preferred to acquire the lock over
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* younger contexts.
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*
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* Waiters without context are interspersed in FIFO order.
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*
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* Furthermore, for Wait-Die kill ourself immediately when possible (there are
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* older contexts already waiting) to avoid unnecessary waiting.
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*/
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static inline int __sched
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__ww_mutex_add_waiter(struct mutex_waiter *waiter,
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struct mutex *lock,
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@ -693,7 +745,7 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
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/*
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* Add the waiter before the first waiter with a higher stamp.
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* Waiters without a context are skipped to avoid starving
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* them.
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* them. Wait-Die waiters may die here.
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*/
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pos = &lock->wait_list;
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list_for_each_entry_reverse(cur, &lock->wait_list, list) {
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@ -701,34 +753,27 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
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continue;
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if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
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/* Back off immediately if necessary. */
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if (ww_ctx->acquired > 0) {
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#ifdef CONFIG_DEBUG_MUTEXES
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struct ww_mutex *ww;
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/*
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* Wait-Die: if we find an older context waiting, there
|
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* is no point in queueing behind it, as we'd have to
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* die the moment it would acquire the lock.
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*/
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int ret = __ww_mutex_kill(lock, ww_ctx);
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ww = container_of(lock, struct ww_mutex, base);
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DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
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ww_ctx->contending_lock = ww;
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#endif
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return -EDEADLK;
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}
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if (ret)
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return ret;
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break;
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}
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pos = &cur->list;
|
||||
|
||||
/*
|
||||
* Wake up the waiter so that it gets a chance to back
|
||||
* off.
|
||||
*/
|
||||
if (cur->ww_ctx->acquired > 0) {
|
||||
debug_mutex_wake_waiter(lock, cur);
|
||||
wake_up_process(cur->task);
|
||||
}
|
||||
/* Wait-Die: ensure younger waiters die. */
|
||||
__ww_mutex_die(lock, cur, ww_ctx);
|
||||
}
|
||||
|
||||
list_add_tail(&waiter->list, pos);
|
||||
|
||||
return 0;
|
||||
}
|
||||
|
||||
|
|
@ -772,7 +817,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
|
|||
*/
|
||||
if (__mutex_trylock(lock)) {
|
||||
if (use_ww_ctx && ww_ctx)
|
||||
__ww_mutex_wakeup_for_backoff(lock, ww_ctx);
|
||||
__ww_mutex_check_waiters(lock, ww_ctx);
|
||||
|
||||
goto skip_wait;
|
||||
}
|
||||
|
|
@ -790,10 +835,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
|
|||
waiter.ww_ctx = MUTEX_POISON_WW_CTX;
|
||||
#endif
|
||||
} else {
|
||||
/* Add in stamp order, waking up waiters that must back off. */
|
||||
/*
|
||||
* Add in stamp order, waking up waiters that must kill
|
||||
* themselves.
|
||||
*/
|
||||
ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
|
||||
if (ret)
|
||||
goto err_early_backoff;
|
||||
goto err_early_kill;
|
||||
|
||||
waiter.ww_ctx = ww_ctx;
|
||||
}
|
||||
|
|
@ -815,7 +863,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
|
|||
goto acquired;
|
||||
|
||||
/*
|
||||
* Check for signals and wound conditions while holding
|
||||
* Check for signals and kill conditions while holding
|
||||
* wait_lock. This ensures the lock cancellation is ordered
|
||||
* against mutex_unlock() and wake-ups do not go missing.
|
||||
*/
|
||||
|
|
@ -824,8 +872,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
|
|||
goto err;
|
||||
}
|
||||
|
||||
if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
|
||||
ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
|
||||
if (use_ww_ctx && ww_ctx) {
|
||||
ret = __ww_mutex_check_kill(lock, &waiter, ww_ctx);
|
||||
if (ret)
|
||||
goto err;
|
||||
}
|
||||
|
|
@ -870,7 +918,7 @@ skip_wait:
|
|||
lock_acquired(&lock->dep_map, ip);
|
||||
|
||||
if (use_ww_ctx && ww_ctx)
|
||||
ww_mutex_set_context_slowpath(ww, ww_ctx);
|
||||
ww_mutex_lock_acquired(ww, ww_ctx);
|
||||
|
||||
spin_unlock(&lock->wait_lock);
|
||||
preempt_enable();
|
||||
|
|
@ -879,7 +927,7 @@ skip_wait:
|
|||
err:
|
||||
__set_current_state(TASK_RUNNING);
|
||||
mutex_remove_waiter(lock, &waiter, current);
|
||||
err_early_backoff:
|
||||
err_early_kill:
|
||||
spin_unlock(&lock->wait_lock);
|
||||
debug_mutex_free_waiter(&waiter);
|
||||
mutex_release(&lock->dep_map, 1, ip);
|
||||
|
|
|
|||
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