Subject: Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

From: Boqun Feng <[email-masked]>
Date: Wed, 23 Aug 2017 13:01:56 +0800

On Wed, Aug 23, 2017 at 01:46:17PM +0900, Byungchul Park wrote:
> On Wed, Aug 23, 2017 at 11:49:51AM +0800, Boqun Feng wrote:
> > Hi Byungchul,
> > 
> > On Wed, Aug 23, 2017 at 09:03:04AM +0900, Byungchul Park wrote:
> > > On Tue, Aug 22, 2017 at 09:43:56PM +0000, Bart Van Assche wrote:
> > > > On Tue, 2017-08-22 at 19:47 +0900, Sergey Senozhatsky wrote:
> > > > > ======================================================
> > > > > WARNING: possible circular locking dependency detected
> > > > > 4.13.0-rc6-next-20170822-dbg-00020-g39758ed8aae0-dirty #1746 Not tainted
> > > > > ------------------------------------------------------
> > > > > fsck.ext4/148 is trying to acquire lock:
> > > > >  (&bdev->bd_mutex){+.+.}, at: [<ffffffff8116e73e>] __blkdev_put+0x33/0x190
> > > > > 
> > > > >  but now in release context of a crosslock acquired at the following:
> > > > >  ((complete)&wait#2){+.+.}, at: [<ffffffff812159e0>] blk_execute_rq+0xbb/0xda
> > > > > 
> > > > >  which lock already depends on the new lock.
> > > > > 
> > 
> > I felt this message really misleading, because the deadlock is detected
> > at the commit time of "((complete)&wait#2)" rather than the acquisition
> > time of "(&bdev->bd_mutex)", so I made the following improvement.
> > 
> > Thoughts?
> > 
> > Regards,
> > Boqun
> > 
> > ----------------------->8
> > From: Boqun Feng <[email-masked]>
> > Date: Wed, 23 Aug 2017 10:18:30 +0800
> > Subject: [PATCH] lockdep: Improve the readibility of crossrelease related
> >  splats
> > 
> > When a crossrelease related deadlock is detected in a commit, the
> > current implemention makes splats like:
> > 
> > > fsck.ext4/148 is trying to acquire lock:
> > >  (&bdev->bd_mutex){+.+.}, at: [<ffffffff8116e73e>] __blkdev_put+0x33/0x190
> > >
> > >  but now in release context of a crosslock acquired at the following:
> > >  ((complete)&wait#2){+.+.}, at: [<ffffffff812159e0>] blk_execute_rq+0xbb/0xda
> > >
> > >  which lock already depends on the new lock.
> > > ...
> > 
> > However, it could be misleading because the current task has got the
> > lock already, and in fact the deadlock is detected when it is doing the
> > commit of the crossrelease lock. So make the splats more accurate to
> > describe the deadlock case.
> > 
> > Signed-off-by: Boqun Feng <[email-masked]>
> > ---
> >  kernel/locking/lockdep.c | 22 ++++++++++++++--------
> >  1 file changed, 14 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 66011c9f5df3..642fb5362507 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -1195,17 +1195,23 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth,
> >  	pr_warn("WARNING: possible circular locking dependency detected\n");
> >  	print_kernel_ident();
> >  	pr_warn("------------------------------------------------------\n");
> > -	pr_warn("%s/%d is trying to acquire lock:\n",
> > -		curr->comm, task_pid_nr(curr));
> > -	print_lock(check_src);
> >  
> > -	if (cross_lock(check_tgt->instance))
> > -		pr_warn("\nbut now in release context of a crosslock acquired at the following:\n");
> > -	else
> > +	if (cross_lock(check_tgt->instance)) {
> > +		pr_warn("%s/%d is committing a crossrelease lock:\n",
> > +			curr->comm, task_pid_nr(curr));
> 
> I think it would be better to print something in term of acquisition,
> since the following print_lock() will print infromation of acquisition.
> 

Well, that print_lock() will print the cross lock acquisition
information at other contexts, but the current thread is doing the
commit. So I think the information would be a little misleading. I will
add "aacquired at" to indicate the lock information is for acquisition.

> > +		print_lock(check_tgt);
> > +		pr_warn("\n, with the following lock held:\n");
> 
> The lock does not have to be held at the commit.
> 

Ah.. right.

How about this:

		pr_warn("%s/%d is committing a crossrelease lock acquired at:\n",
			curr->comm, task_pid_nr(curr));
		print_lock(check_tgt);
		pr_warn("\n, after having the following lock held at least once:\n");

Regards,
Boqun

> > +		print_lock(check_src);
> > +		pr_warn("\non which lock the crossrelease lock already depends.\n\n");
> > +	} else {
> > +		pr_warn("%s/%d is trying to acquire lock:\n",
> > +			curr->comm, task_pid_nr(curr));
> > +		print_lock(check_src);
> >  		pr_warn("\nbut task is already holding lock:\n");
> > +		print_lock(check_tgt);
> > +		pr_warn("\nwhich lock already depends on the new lock.\n\n");
> > +	}
> >  
> > -	print_lock(check_tgt);
> > -	pr_warn("\nwhich lock already depends on the new lock.\n\n");
> >  	pr_warn("\nthe existing dependency chain (in reverse order) is:\n");
> >  
> >  	print_circular_bug_entry(entry, depth);
> > -- 
> > 2.14.1