74 строки
3.2 KiB
Plaintext
74 строки
3.2 KiB
Plaintext
RT-mutex subsystem with PI support
|
|
----------------------------------
|
|
|
|
RT-mutexes with priority inheritance are used to support PI-futexes,
|
|
which enable pthread_mutex_t priority inheritance attributes
|
|
(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details
|
|
about PI-futexes.]
|
|
|
|
This technology was developed in the -rt tree and streamlined for
|
|
pthread_mutex support.
|
|
|
|
Basic principles:
|
|
-----------------
|
|
|
|
RT-mutexes extend the semantics of simple mutexes by the priority
|
|
inheritance protocol.
|
|
|
|
A low priority owner of a rt-mutex inherits the priority of a higher
|
|
priority waiter until the rt-mutex is released. If the temporarily
|
|
boosted owner blocks on a rt-mutex itself it propagates the priority
|
|
boosting to the owner of the other rt_mutex it gets blocked on. The
|
|
priority boosting is immediately removed once the rt_mutex has been
|
|
unlocked.
|
|
|
|
This approach allows us to shorten the block of high-prio tasks on
|
|
mutexes which protect shared resources. Priority inheritance is not a
|
|
magic bullet for poorly designed applications, but it allows
|
|
well-designed applications to use userspace locks in critical parts of
|
|
an high priority thread, without losing determinism.
|
|
|
|
The enqueueing of the waiters into the rtmutex waiter tree is done in
|
|
priority order. For same priorities FIFO order is chosen. For each
|
|
rtmutex, only the top priority waiter is enqueued into the owner's
|
|
priority waiters tree. This tree too queues in priority order. Whenever
|
|
the top priority waiter of a task changes (for example it timed out or
|
|
got a signal), the priority of the owner task is readjusted. The
|
|
priority enqueueing is handled by "pi_waiters".
|
|
|
|
RT-mutexes are optimized for fastpath operations and have no internal
|
|
locking overhead when locking an uncontended mutex or unlocking a mutex
|
|
without waiters. The optimized fastpath operations require cmpxchg
|
|
support. [If that is not available then the rt-mutex internal spinlock
|
|
is used]
|
|
|
|
The state of the rt-mutex is tracked via the owner field of the rt-mutex
|
|
structure:
|
|
|
|
lock->owner holds the task_struct pointer of the owner. Bit 0 is used to
|
|
keep track of the "lock has waiters" state.
|
|
|
|
owner bit0
|
|
NULL 0 lock is free (fast acquire possible)
|
|
NULL 1 lock is free and has waiters and the top waiter
|
|
is going to take the lock*
|
|
taskpointer 0 lock is held (fast release possible)
|
|
taskpointer 1 lock is held and has waiters**
|
|
|
|
The fast atomic compare exchange based acquire and release is only
|
|
possible when bit 0 of lock->owner is 0.
|
|
|
|
(*) It also can be a transitional state when grabbing the lock
|
|
with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
|
|
we need to set the bit0 before looking at the lock, and the owner may be
|
|
NULL in this small time, hence this can be a transitional state.
|
|
|
|
(**) There is a small time when bit 0 is set but there are no
|
|
waiters. This can happen when grabbing the lock in the slow path.
|
|
To prevent a cmpxchg of the owner releasing the lock, we need to
|
|
set this bit before looking at the lock.
|
|
|
|
BTW, there is still technically a "Pending Owner", it's just not called
|
|
that anymore. The pending owner happens to be the top_waiter of a lock
|
|
that has no owner and has been woken up to grab the lock.
|