ipc/sem.c: update/correct memory barriers
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>
diff --git a/ipc/sem.c b/ipc/sem.c
index 178f303..b471e5a 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -253,6 +253,16 @@
}
/*
+ * 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
@@ -275,6 +285,7 @@
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
}
+ ipc_smp_acquire__after_spin_is_unlocked();
}
/*
@@ -327,13 +338,12 @@
/* 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:
