blk-cgroup: Fix RCU correctness warning in cfq_init_queue()

rhvgoyal · Jens Axboe · commit dcf097b247af · 2010-05-06T08:54:00.000+02:00
It is necessary to be in an RCU read-side critical section when invoking css_id(), so this patch adds one to blkiocg_add_blkio_group(). This is actually a false positive, because this is called at initialization time and hence always refers to the root cgroup, which cannot go away. [ 103.790505] =================================================== [ 103.790509] [ INFO: suspicious rcu_dereference_check() usage. ] [ 103.790511] --------------------------------------------------- [ 103.790514] kernel/cgroup.c:4432 invoked rcu_dereference_check() without protection! [ 103.790517] [ 103.790517] other info that might help us debug this: [ 103.790519] [ 103.790521] [ 103.790521] rcu_scheduler_active = 1, debug_locks = 1 [ 103.790524] 4 locks held by bash/4422: [ 103.790526] #0: (&buffer->mutex){+.+.+.}, at: [<ffffffff8114befa>] sysfs_write_file+0x3c/0x144 [ 103.790537] #1: (s_active#102){.+.+.+}, at: [<ffffffff8114bfa5>] sysfs_write_file+0xe7/0x144 [ 103.790544] #2: (&q->sysfs_lock){+.+.+.}, at: [<ffffffff812263b1>] queue_attr_store+0x49/0x8f [ 103.790552] #3: (&(&blkcg->lock)->rlock){......}, at: [<ffffffff8122e4db>] blkiocg_add_blkio_group+0x2b/0xad [ 103.790560] [ 103.790561] stack backtrace: [ 103.790564] Pid: 4422, comm: bash Not tainted 2.6.34-rc4-blkio-second-crash torvalds#81 [ 103.790567] Call Trace: [ 103.790572] [<ffffffff81068f57>] lockdep_rcu_dereference+0x9d/0xa5 [ 103.790577] [<ffffffff8107fac1>] css_id+0x44/0x57 [ 103.790581] [<ffffffff8122e503>] blkiocg_add_blkio_group+0x53/0xad [ 103.790586] [<ffffffff81231936>] cfq_init_queue+0x139/0x32c [ 103.790591] [<ffffffff8121f2d0>] elv_iosched_store+0xbf/0x1bf [ 103.790595] [<ffffffff812263d8>] queue_attr_store+0x70/0x8f [ 103.790599] [<ffffffff8114bfa5>] ? sysfs_write_file+0xe7/0x144 [ 103.790603] [<ffffffff8114bfc6>] sysfs_write_file+0x108/0x144 [ 103.790609] [<ffffffff810f527f>] vfs_write+0xae/0x10b [ 103.790612] [<ffffffff81069863>] ? trace_hardirqs_on_caller+0x10c/0x130 [ 103.790616] [<ffffffff810f539c>] sys_write+0x4a/0x6e [ 103.790622] [<ffffffff81002b5b>] system_call_fastpath+0x16/0x1b [ 103.790625] Located-by: Miles Lane <miles.lane@gmail.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
@@ -3694,8 +3694,10 @@ static void *cfq_init_queue(struct request_queue *q)
 	 * to make sure that cfq_put_cfqg() does not try to kfree root group
 	 */
 	atomic_set(&cfqg->ref, 1);
+	rcu_read_lock();
 	blkiocg_add_blkio_group(&blkio_root_cgroup, &cfqg->blkg, (void *)cfqd,
 					0);
+	rcu_read_unlock();
 #endif
 	/*
 	 * Not strictly needed (since RB_ROOT just clears the node and we
