[PATCH] ipc/sem.c: Update/correct memory barriers

[Manfred Spraul]

3rd version of the patch:

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.

The patch:
- defines new barriers that defaults to smp_rmb().
- converts ipc/sem.c to the new barriers.

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@xxxxxxxxxxxxxxxx>
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 include/linux/spinlock.h | 15 +++++++++++++++
 ipc/sem.c                |  8 ++++----
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 3e18379..5049ff5 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -140,6 +140,21 @@ do {								\
 #define smp_mb__after_unlock_lock()	do { } while (0)
 #endif
 
+/*
+ * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
+ * are only control barriers, thus a memory barrier is required if the
+ * operation should act as an acquire memory barrier, i.e. if it should
+ * pair with the release memory barrier from the spin_unlock() that released
+ * the spinlock.
+ * smp_rmb() is sufficient, as writes cannot pass the implicit control barrier.
+ */
+#ifndef smp_acquire__after_spin_unlock_wait
+#define smp_acquire__after_spin_unlock_wait()	smp_rmb()
+#endif
+#ifndef smp_acquire__after_spin_is_unlocked
+#define smp_acquire__after_spin_is_unlocked()	smp_rmb()
+#endif
+
 /**
  * raw_spin_unlock_wait - wait until the spinlock gets unlocked
  * @lock: the spinlock in question.
diff --git a/ipc/sem.c b/ipc/sem.c
index 9284211..d580cfa 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -275,6 +275,7 @@ static void sem_wait_array(struct sem_array *sma)
 		sem = sma->sem_base + i;
 		spin_unlock_wait(&sem->lock);
 	}
+	smp_acquire__after_spin_unlock_wait();
 }
 
 /*
@@ -327,13 +328,12 @@ 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();
+			smp_acquire__after_spin_is_unlocked();
 
 			/*
 			 * Now repeat the test of complex_count:
-- 
2.1.0

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