Re: [PATCH v3] locks: wake any locks blocked on request before deadlock check

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Mar 25, 2019 at 08:32:52AM -0400, Jeff Layton wrote:
> Andreas reported that he was seeing the tdbtorture test fail in some
> cases with -EDEADLCK when it wasn't before. Some debugging showed that
> deadlock detection was sometimes discovering the caller's lock request
> itself in a dependency chain.
> 
> While we remove the request from the blocked_lock_hash prior to
> reattempting to acquire it, any locks that are blocked on that request
> will still be present in the hash and will still have their fl_blocker
> pointer set to the current request.

This description is a lot easier for me to follow, thanks!

> This causes posix_locks_deadlock to find a deadlock dependency chain
> when it shouldn't, as a lock request cannot block itself.
> 
> We are going to end up waking all of those blocked locks anyway when we
> go to reinsert the request back into the blocked_lock_hash, so just do
> it prior to checking for deadlocks. This ensures that any lock blocked
> on the current request will no longer be part of any blocked request
> chain.

Looks right to me.

--b.

> 
> URL: https://bugzilla.kernel.org/show_bug.cgi?id=202975
> Fixes: 5946c4319ebb ("fs/locks: allow a lock request to block other requests.")
> Cc: stable@vger.kernel.org
> Reported-by: Andreas Schneider <asn@redhat.com>
> Signed-off-by: Neil Brown <neilb@suse.com>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/locks.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index eaa1cfaf73b0..71d0c6c2aac5 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1160,6 +1160,11 @@ static int posix_lock_inode(struct inode *inode, struct file_lock *request,
>  			 */
>  			error = -EDEADLK;
>  			spin_lock(&blocked_lock_lock);
> +			/*
> +			 * Ensure that we don't find any locks blocked on this
> +			 * request during deadlock detection.
> +			 */
> +			__locks_wake_up_blocks(request);
>  			if (likely(!posix_locks_deadlock(request, fl))) {
>  				error = FILE_LOCK_DEFERRED;
>  				__locks_insert_block(fl, request,
> -- 
> 2.20.1