Re: [PATCH] locks: ignore same lock in blocked_lock_hash

[NeilBrown <neilb@suse.com>]

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On Thu, Mar 21 2019, 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.
>
> If posix_locks_deadlock() fails to find a deadlock, the caller_fl
> will be passed to __locks_insert_block(), and this wakes up all
> locks that are blocked on caller_fl, clearing the fl_blocker link.
>
> So if posix_locks_deadlock() finds caller_fl while searching for
> a deadlock, it can be sure that link in the cycle is about to be
> broken and it need not treat it as the cause of a deadlock.
>
> More details here: https://bugzilla.kernel.org/show_bug.cgi?id=202975

There is a weak pattern of using

URL: https://bugzilla.kernel.org/show_bug.cgi?id=202975

>
> Cc: stable@vger.kernel.org
> Fixes: 5946c4319ebb ("fs/locks: allow a lock request to block other requests.")
> 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 | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/fs/locks.c b/fs/locks.c
> index eaa1cfaf73b0..b074f6d7fd2d 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1023,6 +1023,10 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
>  	while ((block_fl = what_owner_is_waiting_for(block_fl))) {
>  		if (i++ > MAX_DEADLK_ITERATIONS)
>  			return 0;
> +
> +		if (caller_fl == block_fl)
> +			return 0;
> +

I would always write this with the value being tested first, and the
value it is being tested against second (and yes, I know that equality
is mathematically symmetric).

 if (block_fl == caller_fl)
    return 0;

But .... whatever.  At least your ordering matches the
posix_same_owner() below, and consistency is a good thing too.

Thanks,
NeilBrown


>  		if (posix_same_owner(caller_fl, block_fl))
>  			return 1;
>  	}
> -- 
> 2.20.1

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