From 33ac279677dcc2441cb93d8cb9cf7a74df62814d Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 24 May 2016 13:17:12 +0200 Subject: [PATCH] locking/barriers: Introduce smp_acquire__after_ctrl_dep() Introduce smp_acquire__after_ctrl_dep(), this construct is not uncommon, but the lack of this barrier is. Use it to better express smp_rmb() uses in WRITE_ONCE(), the IPC semaphore code and the qspinlock code. Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar --- include/linux/compiler.h | 17 ++++++++++++----- ipc/sem.c | 14 ++------------ kernel/locking/qspinlock.c | 2 +- 3 files changed, 15 insertions(+), 18 deletions(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 2bcaedc0..59a7004 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -305,6 +305,17 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s }) /** + * smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency + * + * A control dependency provides a LOAD->STORE order, the additional RMB + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, + * aka. (load)-ACQUIRE. + * + * Architectures that do not do load speculation can have this be barrier(). + */ +#define smp_acquire__after_ctrl_dep() smp_rmb() + +/** * smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering * @ptr: pointer to the variable to wait on * @cond: boolean expression to wait for @@ -314,10 +325,6 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s * * Due to C lacking lambda expressions we load the value of *ptr into a * pre-named variable @VAL to be used in @cond. - * - * The control dependency provides a LOAD->STORE order, the additional RMB - * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, - * aka. ACQUIRE. */ #ifndef smp_cond_load_acquire #define smp_cond_load_acquire(ptr, cond_expr) ({ \ @@ -329,7 +336,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s break; \ cpu_relax(); \ } \ - smp_rmb(); /* ctrl + rmb := acquire */ \ + smp_acquire__after_ctrl_dep(); \ VAL; \ }) #endif diff --git a/ipc/sem.c b/ipc/sem.c index b3757ea..84dff3d 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head *head) } /* - * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they - * are only control barriers. - * The code must pair with spin_unlock(&sem->lock) or - * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient. - * - * smp_rmb() is sufficient, as writes cannot pass the control barrier. - */ -#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb() - -/* * Wait until all currently ongoing simple ops have completed. * Caller must own sem_perm.lock. * New simple ops cannot start, because simple ops first check @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_array *sma) sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); } /* @@ -350,7 +340,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, * complex_count++; * spin_unlock(sem_perm.lock); */ - ipc_smp_acquire__after_spin_is_unlocked(); + smp_acquire__after_ctrl_dep(); /* * Now repeat the test of complex_count: diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 1b8dda9..7306555 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -379,7 +379,7 @@ void queued_spin_unlock_wait(struct qspinlock *lock) cpu_relax(); done: - smp_rmb(); /* CTRL + RMB -> ACQUIRE */ + smp_acquire__after_ctrl_dep(); } EXPORT_SYMBOL(queued_spin_unlock_wait); #endif -- 2.7.4