Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 22, 2010 at 11:54:52AM -0400, Vivek Goyal wrote:
> With RCU correctness on, We see following warning. This patch fixes it.

This is in initialization code, so that there cannot be any concurrent
updates, correct?  If so, looks good.

(Just wanting to make sure that we are not papering over a real error!)

							Thanx, Paul

> [  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 #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]
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  block/cfq-iosched.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 002a5b6..9386bf8 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3741,8 +3741,10 @@ 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
> -- 
> 1.6.2.5
>