linux-kernel - Re: [PATCH v2] blktrace: Fix potentail deadlock between delete & sysfs ops

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Date:   Thu, 17 Aug 2017 20:41:10 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "longman@...hat.com" <longman@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
CC:     "bfields@...ldses.org" <bfields@...ldses.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "jlayton@...chiereds.net" <jlayton@...chiereds.net>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2] blktrace: Fix potentail deadlock between delete &
 sysfs ops

On Thu, 2017-08-17 at 16:30 -0400, Steven Rostedt wrote:
> On Thu, 17 Aug 2017 12:24:39 -0400
> Waiman Long <longman@...hat.com> wrote:
> 
> > On 08/17/2017 09:34 AM, Steven Rostedt wrote:
> > > On Wed, 16 Aug 2017 16:40:40 -0400
> > > Waiman Long <longman@...hat.com> wrote:
> > >  
> > > > The lockdep code had reported the following unsafe locking scenario:
> > > > 
> > > >        CPU0                    CPU1
> > > >        ----                    ----
> > > >   lock(s_active#228);
> > > >                                lock(&bdev->bd_mutex/1);
> > > >                                lock(s_active#228);
> > > >   lock(&bdev->bd_mutex);  
> > > 
> > > Can you show the exact locations of these locks. I have no idea where
> > > this "s_active" is.  
> > 
> > The s_active isn't an actual lock. It is a reference count (kn->count)
> > on the sysfs (kernfs) file. Removal of a sysfs file, however, require
> > a wait until all the references are gone. The reference count is
> > treated like a rwsem using lockdep instrumentation code.
> 
> Which kernel is this? I don't see any lockdep annotation around
> kn->count (doing a git grep, I find it referenced in fs/kernfs/dir.c)

As far as I know the s_active lockdep annotations were introduced in 2007
through commit 0ab66088c855 ("sysfs: implement sysfs_dirent active reference
and immediate disconnect"). Today these annotations exist in kernfs:
$ git grep -nHw dep_map fs/kernfs
fs/kernfs/dir.c:421:		rwsem_acquire_read(&kn->dep_map, 0, 1, _RET_IP_);
fs/kernfs/dir.c:441:		rwsem_release(&kn->dep_map, 1, _RET_IP_);
fs/kernfs/dir.c:468:		rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_);
fs/kernfs/dir.c:470:			lock_contended(&kn->dep_map, _RET_IP_);
fs/kernfs/dir.c:478:		lock_acquired(&kn->dep_map, _RET_IP_);
fs/kernfs/dir.c:479:		rwsem_release(&kn->dep_map, 1, _RET_IP_);
fs/kernfs/dir.c:1385:		rwsem_acquire(&kn->dep_map, 0, 1, _RET_IP_);
fs/kernfs/file.c:1000:		lockdep_init_map(&kn->dep_map, "s_active", key, 0);

Bart.