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

Boqun Feng
2017-08-23T05:01:56Z

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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 <boqun.feng@gmail.com>
> > 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 <boqun.feng@gmail.com>
> > ---
> > 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

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