Merge branch 'for-mingo' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/rcu
Pull RCU cleanup from Paul E. McKenney: "Privatize smp_mb__after_unlock_lock(). This commit moves the definition of smp_mb__after_unlock_lock() to kernel/rcu/tree.h, in recognition of the fact that RCU is the only thing using this, that nothing else is likely to use it, and that it is likely to go away completely." Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -1854,16 +1854,10 @@ RELEASE are to the same lock variable, but only from the perspective of
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another CPU not holding that lock. In short, a ACQUIRE followed by an
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RELEASE may -not- be assumed to be a full memory barrier.
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Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not
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imply a full memory barrier. If it is necessary for a RELEASE-ACQUIRE
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pair to produce a full barrier, the ACQUIRE can be followed by an
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smp_mb__after_unlock_lock() invocation. This will produce a full barrier
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(including transitivity) if either (a) the RELEASE and the ACQUIRE are
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executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on
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the same variable. The smp_mb__after_unlock_lock() primitive is free
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on many architectures. Without smp_mb__after_unlock_lock(), the CPU's
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execution of the critical sections corresponding to the RELEASE and the
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ACQUIRE can cross, so that:
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Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
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not imply a full memory barrier. Therefore, the CPU's execution of the
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critical sections corresponding to the RELEASE and the ACQUIRE can cross,
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so that:
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*A = a;
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RELEASE M
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@ -1901,29 +1895,6 @@ the RELEASE would simply complete, thereby avoiding the deadlock.
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a sleep-unlock race, but the locking primitive needs to resolve
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such races properly in any case.
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With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
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For example, with the following code, the store to *A will always be
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seen by other CPUs before the store to *B:
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*A = a;
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RELEASE M
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ACQUIRE N
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smp_mb__after_unlock_lock();
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*B = b;
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The operations will always occur in one of the following orders:
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STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
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STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
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ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
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If the RELEASE and ACQUIRE were instead both operating on the same lock
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variable, only the first of these alternatives can occur. In addition,
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the more strongly ordered systems may rule out some of the above orders.
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But in any case, as noted earlier, the smp_mb__after_unlock_lock()
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ensures that the store to *A will always be seen as happening before
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the store to *B.
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Locks and semaphores may not provide any guarantee of ordering on UP compiled
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systems, and so cannot be counted on in such a situation to actually achieve
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anything at all - especially with respect to I/O accesses - unless combined
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@ -2154,40 +2125,6 @@ But it won't see any of:
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*E, *F or *G following RELEASE Q
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However, if the following occurs:
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CPU 1 CPU 2
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=============================== ===============================
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WRITE_ONCE(*A, a);
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ACQUIRE M [1]
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WRITE_ONCE(*B, b);
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WRITE_ONCE(*C, c);
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RELEASE M [1]
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WRITE_ONCE(*D, d); WRITE_ONCE(*E, e);
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ACQUIRE M [2]
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smp_mb__after_unlock_lock();
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WRITE_ONCE(*F, f);
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WRITE_ONCE(*G, g);
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RELEASE M [2]
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WRITE_ONCE(*H, h);
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CPU 3 might see:
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*E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
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ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
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But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
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*B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
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*A, *B or *C following RELEASE M [1]
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*F, *G or *H preceding ACQUIRE M [2]
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*A, *B, *C, *E, *F or *G following RELEASE M [2]
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Note that the smp_mb__after_unlock_lock() is critically important
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here: Without it CPU 3 might see some of the above orderings.
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Without smp_mb__after_unlock_lock(), the accesses are not guaranteed
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to be seen in order unless CPU 3 holds lock M.
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ACQUIRES VS I/O ACCESSES
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------------------------
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@ -28,8 +28,6 @@
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#include <asm/synch.h>
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#include <asm/ppc-opcode.h>
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#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
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#ifdef CONFIG_PPC64
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/* use 0x800000yy when locked, where yy == CPU number */
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#ifdef __BIG_ENDIAN__
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@ -130,16 +130,6 @@ do { \
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#define smp_mb__before_spinlock() smp_wmb()
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#endif
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/*
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* Place this after a lock-acquisition primitive to guarantee that
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* an UNLOCK+LOCK pair act as a full barrier. This guarantee applies
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* if the UNLOCK and LOCK are executed by the same CPU or if the
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* UNLOCK and LOCK operate on the same lock variable.
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*/
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#ifndef smp_mb__after_unlock_lock
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#define smp_mb__after_unlock_lock() do { } while (0)
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#endif
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/**
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* raw_spin_unlock_wait - wait until the spinlock gets unlocked
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* @lock: the spinlock in question.
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@ -653,3 +653,15 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
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#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
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}
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#endif /* #ifdef CONFIG_RCU_TRACE */
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/*
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* Place this after a lock-acquisition primitive to guarantee that
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* an UNLOCK+LOCK pair act as a full barrier. This guarantee applies
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* if the UNLOCK and LOCK are executed by the same CPU or if the
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* UNLOCK and LOCK operate on the same lock variable.
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*/
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#ifdef CONFIG_PPC
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#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */
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#else /* #ifdef CONFIG_PPC */
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#define smp_mb__after_unlock_lock() do { } while (0)
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#endif /* #else #ifdef CONFIG_PPC */
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