From d6de8be711b28049a5cb93c954722c311c7d3f7f Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Wed, 28 May 2008 14:46:59 +0200 Subject: [PATCH] cfq-iosched: fix RCU problem in cfq_cic_lookup() cfq_cic_lookup() needs to properly protect ioc->ioc_data before dereferencing it and also exclude updaters of ioc->ioc_data as well. Also add a number of comments documenting why the existing RCU usage is OK. Thanks a lot to "Paul E. McKenney" for review and comments! Signed-off-by: Jens Axboe --- block/cfq-iosched.c | 28 ++++++++++++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 4df3f05224..d01b411c72 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1142,6 +1142,9 @@ static void cfq_put_queue(struct cfq_queue *cfqq) kmem_cache_free(cfq_pool, cfqq); } +/* + * Must always be called with the rcu_read_lock() held + */ static void __call_for_each_cic(struct io_context *ioc, void (*func)(struct io_context *, struct cfq_io_context *)) @@ -1197,6 +1200,11 @@ static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic) cfq_cic_free(cic); } +/* + * Must be called with rcu_read_lock() held or preemption otherwise disabled. + * Only two callers of this - ->dtor() which is called with the rcu_read_lock(), + * and ->trim() which is called with the task lock held + */ static void cfq_free_io_context(struct io_context *ioc) { /* @@ -1502,20 +1510,24 @@ static struct cfq_io_context * cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc) { struct cfq_io_context *cic; + unsigned long flags; void *k; if (unlikely(!ioc)) return NULL; + rcu_read_lock(); + /* * we maintain a last-hit cache, to avoid browsing over the tree */ cic = rcu_dereference(ioc->ioc_data); - if (cic && cic->key == cfqd) + if (cic && cic->key == cfqd) { + rcu_read_unlock(); return cic; + } do { - rcu_read_lock(); cic = radix_tree_lookup(&ioc->radix_root, (unsigned long) cfqd); rcu_read_unlock(); if (!cic) @@ -1524,10 +1536,13 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc) k = cic->key; if (unlikely(!k)) { cfq_drop_dead_cic(cfqd, ioc, cic); + rcu_read_lock(); continue; } + spin_lock_irqsave(&ioc->lock, flags); rcu_assign_pointer(ioc->ioc_data, cic); + spin_unlock_irqrestore(&ioc->lock, flags); break; } while (1); @@ -2134,6 +2149,10 @@ static void *cfq_init_queue(struct request_queue *q) static void cfq_slab_kill(void) { + /* + * Caller already ensured that pending RCU callbacks are completed, + * so we should have no busy allocations at this point. + */ if (cfq_pool) kmem_cache_destroy(cfq_pool); if (cfq_ioc_pool) @@ -2292,6 +2311,11 @@ static void __exit cfq_exit(void) ioc_gone = &all_gone; /* ioc_gone's update must be visible before reading ioc_count */ smp_wmb(); + + /* + * this also protects us from entering cfq_slab_kill() with + * pending RCU callbacks + */ if (elv_ioc_count_read(ioc_count)) wait_for_completion(ioc_gone); cfq_slab_kill(); -- 2.39.5