Commit-ID: 64d816cba06c67eeee455b8c78ebcda349d49c24 Gitweb: http://git.kernel.org/tip/64d816cba06c67eeee455b8c78ebcda349d49c24 Author: Waiman Long <waiman.l...@hpe.com> AuthorDate: Mon, 9 Nov 2015 19:09:21 -0500 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 23 Nov 2015 10:01:58 +0100
locking/qspinlock: Use _acquire/_release() versions of cmpxchg() & xchg() This patch replaces the cmpxchg() and xchg() calls in the native qspinlock code with the more relaxed _acquire or _release versions of those calls to enable other architectures to adopt queued spinlocks with less memory barrier performance overhead. Signed-off-by: Waiman Long <waiman.l...@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Davidlohr Bueso <d...@stgolabs.net> Cc: Douglas Hatch <doug.ha...@hpe.com> Cc: H. Peter Anvin <h...@zytor.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Scott J Norton <scott.nor...@hpe.com> Cc: Thomas Gleixner <t...@linutronix.de> Link: http://lkml.kernel.org/r/1447114167-47185-2-git-send-email-waiman.l...@hpe.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- include/asm-generic/qspinlock.h | 9 +++++---- kernel/locking/qspinlock.c | 29 ++++++++++++++++++++++++----- 2 files changed, 29 insertions(+), 9 deletions(-) diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h index e2aadbc..39e1cb2 100644 --- a/include/asm-generic/qspinlock.h +++ b/include/asm-generic/qspinlock.h @@ -12,8 +12,9 @@ * GNU General Public License for more details. * * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P. + * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP * - * Authors: Waiman Long <waiman.l...@hp.com> + * Authors: Waiman Long <waiman.l...@hpe.com> */ #ifndef __ASM_GENERIC_QSPINLOCK_H #define __ASM_GENERIC_QSPINLOCK_H @@ -62,7 +63,7 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock) static __always_inline int queued_spin_trylock(struct qspinlock *lock) { if (!atomic_read(&lock->val) && - (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0)) + (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0)) return 1; return 0; } @@ -77,7 +78,7 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock) { u32 val; - val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL); + val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL); if (likely(val == 0)) return; queued_spin_lock_slowpath(lock, val); @@ -93,7 +94,7 @@ static __always_inline void queued_spin_unlock(struct qspinlock *lock) /* * smp_mb__before_atomic() in order to guarantee release semantics */ - smp_mb__before_atomic_dec(); + smp_mb__before_atomic(); atomic_sub(_Q_LOCKED_VAL, &lock->val); } #endif diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 87e9ce6a..7868418 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -14,8 +14,9 @@ * (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P. * (C) Copyright 2013-2014 Red Hat, Inc. * (C) Copyright 2015 Intel Corp. + * (C) Copyright 2015 Hewlett-Packard Enterprise Development LP * - * Authors: Waiman Long <waiman.l...@hp.com> + * Authors: Waiman Long <waiman.l...@hpe.com> * Peter Zijlstra <pet...@infradead.org> */ @@ -176,7 +177,12 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) { struct __qspinlock *l = (void *)lock; - return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET; + /* + * Use release semantics to make sure that the MCS node is properly + * initialized before changing the tail code. + */ + return (u32)xchg_release(&l->tail, + tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET; } #else /* _Q_PENDING_BITS == 8 */ @@ -208,7 +214,11 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) for (;;) { new = (val & _Q_LOCKED_PENDING_MASK) | tail; - old = atomic_cmpxchg(&lock->val, val, new); + /* + * Use release semantics to make sure that the MCS node is + * properly initialized before changing the tail code. + */ + old = atomic_cmpxchg_release(&lock->val, val, new); if (old == val) break; @@ -319,7 +329,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) if (val == new) new |= _Q_PENDING_VAL; - old = atomic_cmpxchg(&lock->val, val, new); + /* + * Acquire semantic is required here as the function may + * return immediately if the lock was free. + */ + old = atomic_cmpxchg_acquire(&lock->val, val, new); if (old == val) break; @@ -426,7 +440,12 @@ queue: set_locked(lock); break; } - old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL); + /* + * The smp_load_acquire() call above has provided the necessary + * acquire semantics required for locking. At most two + * iterations of this loop may be ran. + */ + old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); if (old == val) goto release; /* No contention */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
