The patch titled
Subject: ipc/sem.c: update/correct memory barriers
has been added to the -mm tree. Its filename is
ipc-semc-update-correct-memory-barriers.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/ipc-semc-update-correct-memory-barriers.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/ipc-semc-update-correct-memory-barriers.patch
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------------------------------------------------------
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Subject: 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@xxxxxxxxxxxxxxxx>
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.10+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
ipc/sem.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
diff -puN ipc/sem.c~ipc-semc-update-correct-memory-barriers ipc/sem.c
--- a/ipc/sem.c~ipc-semc-update-correct-memory-barriers
+++ a/ipc/sem.c
@@ -253,6 +253,16 @@ static void sem_rcu_free(struct rcu_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
@@ -275,6 +285,7 @@ static void sem_wait_array(struct sem_ar
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_ar
/* 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:
_
Patches currently in -mm which might be from manfred@xxxxxxxxxxxxxxxx are
ipcsem-fix-use-after-free-on-ipc_rmid-after-a-task-using-same-semaphore-set-exits.patch
ipcsem-remove-uneeded-sem_undo_list-lock-usage-in-exit_sem.patch
ipc-semc-update-correct-memory-barriers.patch
ipc-convert-invalid-scenarios-to-use-warn_on.patch
slab-leaks3-default-y.patch
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