linux-kernel - Re: REGRESSION/TEST FAILURE caused by DEBUG

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Date:   Thu, 5 Apr 2018 07:40:33 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Waiman Long <longman@...hat.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: REGRESSION/TEST FAILURE caused by DEBUG_RWSEMS

On Wed, Apr 04, 2018 at 09:55:18AM -0400, Waiman Long wrote:
> On 04/04/2018 09:23 AM, Theodore Y. Ts'o wrote:
> > The commit 5149cbac4235: "locking/rwsem: Add DEBUG_RWSEMS to look for
> > lock/unlock emismatches" (newly added in this merge window) will cause
> > xfstests tests failures in generic/068 for all file systems.
> >
> > This is because the freeze and thaw ioctls, by design, are run in
> > different processes.  It looks like the concern for why
> > CONFIG_DEBUG_RWSEMS was added is because read/write semaphores can
> > spin --- which if the case, we need to make sure that (a) the spinlock
> > code path is not used for the freeze/thaw ioctls, and (b) we don't
> > trigger the debug WARN_ON.
> >
> > Cheers,
> >
> The rwsem code has an up_read_non_owner() function for this kind of
> cross task lock/unlock.  I think we should add the corresponding
> up_write_non_owner() for that.

Keep in mind this is a percpu_rwsem, not a rwsem. It's the
sb_freeze_unlock() call in thaw_super() that is triggering it, and
it would appear that percpu_rwsems have no concept on "non_owner"
operation ATM.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com