On 09/02/2016 09:36 AM, Paul E. McKenney wrote:
On Fri, Sep 02, 2016 at 10:56:22AM -0400, Tejun Heo wrote:
(cc'ing Paul, hi!)
Hello,
On Thu, Sep 01, 2016 at 02:13:34PM -0600, Jens Axboe wrote:
On 09/01/2016 04:21 AM, kernel test robot wrote:
[ 7.323356] cdrom: Uniform CD-ROM driver Revision: 3.20
[ 7.334239]
[ 7.337256] ===============================
[ 7.340532] [ INFO: suspicious RCU usage. ]
[ 7.342419] 4.8.0-rc4-00008-g8bc4ad9 #1 Not tainted
[ 7.347065] -------------------------------
[ 7.350132] include/linux/cgroup.h:435 suspicious rcu_dereference_check()
usage!
Lockdep does not believe that any locks are held, correct?
Which is correct, the queue lock has been dropped at this point.
[ 7.410074] Call Trace:
[ 7.411328] [<ffffffff8178ed3b>] dump_stack+0x82/0xb8
[ 7.413982] [<ffffffff81123472>] lockdep_rcu_suspicious+0xf7/0x100
[ 7.415828] [<ffffffff817873f4>] bio_blkcg+0x89/0x93
[ 7.417336] [<ffffffff817891f0>] check_blkcg_changed+0x58/0x1b8
[ 7.428722] [<ffffffff81789b90>] cfq_set_request+0xd1/0x2a3
[ 7.439690] [<ffffffff81763fe3>] elv_set_request+0x1f/0x24
[ 7.442157] [<ffffffff8176856d>] get_request+0x38f/0x77f
[ 7.447449] [<ffffffff817689c2>] blk_get_request+0x65/0xa8
[ 7.449868] [<ffffffff81c692df>] ide_cd_queue_pc+0x76/0x19d
[ 7.453757] [<ffffffff81c695ee>] cdrom_check_status+0x51/0x53
[ 7.455372] [<ffffffff81c6a27e>] ide_cdrom_check_events_real+0x20/0x3f
[ 7.457294] [<ffffffff82579f94>] cdrom_update_events+0x18/0x21
[ 7.458987] [<ffffffff82579faf>] cdrom_check_events+0x12/0x1f
[ 7.460713] [<ffffffff81c68317>] idecd_check_events+0x1c/0x1e
[ 7.462393] [<ffffffff81778d12>] disk_check_events+0x47/0x103
[ 7.464129] [<ffffffff81778dea>] disk_events_workfn+0x1c/0x1e
[ 7.465844] [<ffffffff810fcafa>] process_one_work+0x272/0x4ee
[ 7.467462] [<ffffffff810fd247>] worker_thread+0x1eb/0x2c9
The warning is from
#define task_css_set_check(task, __c) \
rcu_dereference_check((task)->cgroups, \
lockdep_is_held(&cgroup_mutex) || \
lockdep_is_held(&css_set_lock) || \
((task)->flags & PF_EXITING) || (__c))
which is used by bio_blkcg() which is called by the following code in
check_blkcg_changed().
rcu_read_lock();
serial_nr = bio_blkcg(bio)->css.serial_nr;
rcu_read_unlock();
So, I have no idea. It looks like rcu_dereference_check() is being
called with rcu read locked but still triggering suspicious RCU usage
warning.
Perhaps there is an rcu_read_unlock() somewhere on the code path?
The code hasn't changed for quite a while now, so it's also really
weird that it's triggering now. Paul, does anything ring a bell?
I have not see something like this recently.
The question is if it's really new, or just re-triggering because the
writeback branch changes that function? It's further down though, so
can't impact the RCU section. And the writeback changes don't have any
RCU code in them...
--
Jens Axboe
