diff options
| -rw-r--r-- | mm/mempool.c | 61 |
1 files changed, 48 insertions, 13 deletions
diff --git a/mm/mempool.c b/mm/mempool.c index e73641b..11f0d0a 100644 --- a/mm/mempool.c +++ b/mm/mempool.c @@ -224,28 +224,31 @@ repeat_alloc: if (likely(pool->curr_nr)) { element = remove_element(pool); spin_unlock_irqrestore(&pool->lock, flags); + /* paired with rmb in mempool_free(), read comment there */ + smp_wmb(); return element; } - spin_unlock_irqrestore(&pool->lock, flags); /* We must not sleep in the GFP_ATOMIC case */ - if (!(gfp_mask & __GFP_WAIT)) + if (!(gfp_mask & __GFP_WAIT)) { + spin_unlock_irqrestore(&pool->lock, flags); return NULL; + } - /* Now start performing page reclaim */ + /* Let's wait for someone else to return an element to @pool */ gfp_temp = gfp_mask; init_wait(&wait); prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE); - smp_mb(); - if (!pool->curr_nr) { - /* - * FIXME: this should be io_schedule(). The timeout is there - * as a workaround for some DM problems in 2.6.18. - */ - io_schedule_timeout(5*HZ); - } - finish_wait(&pool->wait, &wait); + spin_unlock_irqrestore(&pool->lock, flags); + + /* + * FIXME: this should be io_schedule(). The timeout is there as a + * workaround for some DM problems in 2.6.18. + */ + io_schedule_timeout(5*HZ); + + finish_wait(&pool->wait, &wait); goto repeat_alloc; } EXPORT_SYMBOL(mempool_alloc); @@ -265,7 +268,39 @@ void mempool_free(void *element, mempool_t *pool) if (unlikely(element == NULL)) return; - smp_mb(); + /* + * Paired with the wmb in mempool_alloc(). The preceding read is + * for @element and the following @pool->curr_nr. This ensures + * that the visible value of @pool->curr_nr is from after the + * allocation of @element. This is necessary for fringe cases + * where @element was passed to this task without going through + * barriers. + * + * For example, assume @p is %NULL at the beginning and one task + * performs "p = mempool_alloc(...);" while another task is doing + * "while (!p) cpu_relax(); mempool_free(p, ...);". This function + * may end up using curr_nr value which is from before allocation + * of @p without the following rmb. + */ + smp_rmb(); + + /* + * For correctness, we need a test which is guaranteed to trigger + * if curr_nr + #allocated == min_nr. Testing curr_nr < min_nr + * without locking achieves that and refilling as soon as possible + * is desirable. + * + * Because curr_nr visible here is always a value after the + * allocation of @element, any task which decremented curr_nr below + * min_nr is guaranteed to see curr_nr < min_nr unless curr_nr gets + * incremented to min_nr afterwards. If curr_nr gets incremented + * to min_nr after the allocation of @element, the elements + * allocated after that are subject to the same guarantee. + * + * Waiters happen iff curr_nr is 0 and the above guarantee also + * ensures that there will be frees which return elements to the + * pool waking up the waiters. + */ if (pool->curr_nr < pool->min_nr) { spin_lock_irqsave(&pool->lock, flags); if (pool->curr_nr < pool->min_nr) { |
