Messages in this thread Patch in this message | | | Date | Thu, 18 Jul 2019 15:49:56 +0200 | From | Peter Zijlstra <> | Subject | [PATCH 2/4] rwsem: Add missing ACQUIRE to read_slowpath sleep loop |
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While reviewing another read_slowpath patch, both Will and I noticed another missing ACQUIRE, namely:
X = 0;
CPU0 CPU1
rwsem_down_read() for (;;) { set_current_state(TASK_UNINTERRUPTIBLE);
X = 1; rwsem_up_write(); rwsem_mark_wake() atomic_long_add(adjustment, &sem->count); smp_store_release(&waiter->task, NULL);
if (!waiter.task) break;
... }
r = X;
Allows 'r == 0'.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reported-by: Will Deacon <will@kernel.org> Acked-by: Will Deacon <will@kernel.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- kernel/locking/rwsem.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
--- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -1069,8 +1069,10 @@ rwsem_down_read_slowpath(struct rw_semap /* wait to be given the lock */ while (true) { set_current_state(state); - if (!waiter.task) + if (!smp_load_acquire(&waiter.task)) { + /* Orders against rwsem_mark_wake()'s smp_store_release() */ break; + } if (signal_pending_state(state, current)) { raw_spin_lock_irq(&sem->wait_lock); if (waiter.task)
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