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author | Manfred Spraul <manfred@colorfullife.com> | 2015-08-14 15:35:10 -0700 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2015-09-13 09:10:48 -0700 |
commit | 502b83be8213615aa9cec0b24b71035a36b4f017 (patch) | |
tree | 2d15819b8179a6c6a3b75ad1ea419587d082599b | |
parent | 9ab6ec258094da5f24e149243cf1464d610ce985 (diff) | |
download | lwn-502b83be8213615aa9cec0b24b71035a36b4f017.tar.gz lwn-502b83be8213615aa9cec0b24b71035a36b4f017.zip |
ipc/sem.c: update/correct memory barriers
commit 3ed1f8a99d70ea1cd1508910eb107d0edcae5009 upstream.
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r-- | ipc/sem.c | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/ipc/sem.c b/ipc/sem.c index 45e1de73f4f8..e53c96f7db42 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -253,6 +253,16 @@ static void sem_rcu_free(struct rcu_head *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_array *sma) sem = sma->sem_base + i; spin_unlock_wait(&sem->lock); } + ipc_smp_acquire__after_spin_is_unlocked(); } /* @@ -326,8 +337,13 @@ 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)) { - /* spin_is_locked() is not a memory barrier */ - smp_mb(); + /* + * We need a memory barrier with acquire semantics, + * otherwise we can race with another thread that does: + * complex_count++; + * spin_unlock(sem_perm.lock); + */ + ipc_smp_acquire__after_spin_is_unlocked(); /* Now repeat the test of complex_count: * It can't change anymore until we drop sem->lock. |