From 6262db7c088bbfc26480d10144cde70bbf077be3 Mon Sep 17 00:00:00 2001 From: Boqun Feng Date: Fri, 10 Jun 2016 11:51:28 +0800 Subject: [PATCH] powerpc/spinlock: Fix spin_unlock_wait() There is an ordering issue with spin_unlock_wait() on powerpc, because the spin_lock primitive is an ACQUIRE and an ACQUIRE is only ordering the load part of the operation with memory operations following it. Therefore the following event sequence can happen: CPU 1 CPU 2 CPU 3 ================== ==================== ============== spin_unlock(&lock); spin_lock(&lock): r1 = *lock; // r1 == 0; o = object; o = READ_ONCE(object); // reordered here object = NULL; smp_mb(); spin_unlock_wait(&lock); *lock = 1; smp_mb(); o->dead = true; < o = READ_ONCE(object); > // reordered upwards if (o) // true BUG_ON(o->dead); // true!! To fix this, we add a "nop" ll/sc loop in arch_spin_unlock_wait() on ppc, the "nop" ll/sc loop reads the lock value and writes it back atomically, in this way it will synchronize the view of the lock on CPU1 with that on CPU2. Therefore in the scenario above, either CPU2 will fail to get the lock at first or CPU1 will see the lock acquired by CPU2, both cases will eliminate this bug. This is a similar idea as what Will Deacon did for ARM64 in: d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers") Furthermore, if the "nop" ll/sc figures out the lock is locked, we actually don't need to do the "nop" ll/sc trick again, we can just do a normal load+check loop for the lock to be released, because in that case, spin_unlock_wait() is called when someone is holding the lock, and the store part of the "nop" ll/sc happens before the lock release of the current lock holder: "nop" ll/sc -> spin_unlock() and the lock release happens before the next lock acquisition: spin_unlock() -> spin_lock() which means the "nop" ll/sc happens before the next lock acquisition: "nop" ll/sc -> spin_unlock() -> spin_lock() With a smp_mb() preceding spin_unlock_wait(), the store of object is guaranteed to be observed by the next lock holder: STORE -> smp_mb() -> "nop" ll/sc -> spin_unlock() -> spin_lock() This patch therefore fixes the issue and also cleans the arch_spin_unlock_wait() a little bit by removing superfluous memory barriers in loops and consolidating the implementations for PPC32 and PPC64 into one. Suggested-by: "Paul E. McKenney" Signed-off-by: Boqun Feng Reviewed-by: "Paul E. McKenney" [mpe: Inline the "nop" ll/sc loop and set EH=0, munge change log] Signed-off-by: Michael Ellerman --- arch/powerpc/include/asm/spinlock.h | 38 ++++++++++++++++++++++++----- arch/powerpc/lib/locks.c | 16 ------------ 2 files changed, 32 insertions(+), 22 deletions(-) diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h index 523673d7583c..fa37fe93bc02 100644 --- a/arch/powerpc/include/asm/spinlock.h +++ b/arch/powerpc/include/asm/spinlock.h @@ -162,12 +162,38 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock) lock->slock = 0; } -#ifdef CONFIG_PPC64 -extern void arch_spin_unlock_wait(arch_spinlock_t *lock); -#else -#define arch_spin_unlock_wait(lock) \ - do { while (arch_spin_is_locked(lock)) cpu_relax(); } while (0) -#endif +static inline void arch_spin_unlock_wait(arch_spinlock_t *lock) +{ + arch_spinlock_t lock_val; + + smp_mb(); + + /* + * Atomically load and store back the lock value (unchanged). This + * ensures that our observation of the lock value is ordered with + * respect to other lock operations. + */ + __asm__ __volatile__( +"1: " PPC_LWARX(%0, 0, %2, 0) "\n" +" stwcx. %0, 0, %2\n" +" bne- 1b\n" + : "=&r" (lock_val), "+m" (*lock) + : "r" (lock) + : "cr0", "xer"); + + if (arch_spin_value_unlocked(lock_val)) + goto out; + + while (lock->slock) { + HMT_low(); + if (SHARED_PROCESSOR) + __spin_yield(lock); + } + HMT_medium(); + +out: + smp_mb(); +} /* * Read-write spinlocks, allowing multiple readers diff --git a/arch/powerpc/lib/locks.c b/arch/powerpc/lib/locks.c index f7deebdf3365..b7b1237d4aa6 100644 --- a/arch/powerpc/lib/locks.c +++ b/arch/powerpc/lib/locks.c @@ -68,19 +68,3 @@ void __rw_yield(arch_rwlock_t *rw) get_hard_smp_processor_id(holder_cpu), yield_count); } #endif - -void arch_spin_unlock_wait(arch_spinlock_t *lock) -{ - smp_mb(); - - while (lock->slock) { - HMT_low(); - if (SHARED_PROCESSOR) - __spin_yield(lock); - } - HMT_medium(); - - smp_mb(); -} - -EXPORT_SYMBOL(arch_spin_unlock_wait); -- 2.39.5