ipc/sem.c: update/correct memory barriers (3ed1f8a9) · Commits · 戴 / test

sem_lock() did not properly pair memory barriers: !spin_is_locked() and spin_unlock_wait() are both only control barriers. The code needs an acquire barrier, otherwise the cpu might perform read operations before the lock test. As no primitive exists inside <include/spinlock.h> and since it seems noone wants another primitive, the code creates a local primitive within ipc/sem.c. With regards to -stable: The change of sem_wait_array() is a bugfix, the change to sem_lock() is a nop (just a preprocessor redefinition to improve the readability). The bugfix is necessary for all kernels that use sem_wait_array() (i.e.: starting from 3.10). Signed-off-by:

Manfred Spraul <manfred@colorfullife.com> Reported-by:

Oleg Nesterov <oleg@redhat.com> Acked-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Kirill Tkhai <ktkhai@parallels.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: <stable@vger.kernel.org> [3.10+] Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

+14 −4

Original line number	Diff line number	Diff line
		@@ -252,6 +252,16 @@ static void sem_rcu_free(struct rcu_head *head)
		ipc_rcu_free(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.
		@@ -275,6 +285,7 @@ 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();
		}

		/*
		@@ -327,13 +338,12 @@ static inline int sem_lock(struct sem_array sma, struct sembuf sops,
		/* Then check that the global lock is free */
		if (!spin_is_locked(&sma->sem_perm.lock)) {
		/*
		* The ipc object lock check must be visible on all
		* cores before rechecking the complex count. Otherwise
		* we can race with another thread that does:
		* We need a memory barrier with acquire semantics,
		* otherwise we can race with another thread that does:
		* complex_count++;
		* spin_unlock(sem_perm.lock);
		*/
		smp_rmb();
		ipc_smp_acquire__after_spin_is_unlocked();

		/*
		* Now repeat the test of complex_count:

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