Re: [PATCH] filelock: fix deadlock detection in POSIX locking

[NeilBrown]

On Mon, 19 Feb 2024, Jeff Layton wrote:
> The FL_POSIX check in __locks_insert_block was inadvertantly broken
> recently and is now inserting only OFD locks instead of only legacy
> POSIX locks.
> 
> This breaks deadlock detection in POSIX locks, and may also be the root
> cause of a performance regression noted by the kernel test robot.
> Restore the proper sense of the test.
> 
> Fixes: b6be3714005c ("filelock: convert __locks_insert_block, conflict and deadlock checks to use file_lock_core")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202402181229.f8147f40-oliver.sang@xxxxxxxxx
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> Disregard what I said earlier about this bug being harmless. It broke
> deadlock detection in POSIX locks (LTP fcntl17 shows the bug). This
> patch fixes it. It may be best to squash this into the patch that
> introduced the regression.
> 
> I'm not certain if this fixes the performance regression that the KTR
> noticed recently in this patch, but that's what got me looking more
> closely, so I'll give it credit for reporting this. Hopefully it'll
> confirm that result for us.
> ---
>  fs/locks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 26d52ef5314a..90c8746874de 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -812,7 +812,7 @@ static void __locks_insert_block(struct file_lock_core *blocker,
>  	list_add_tail(&waiter->flc_blocked_member,
>  		      &blocker->flc_blocked_requests);
>  
> -	if ((blocker->flc_flags & (FL_POSIX|FL_OFDLCK)) == (FL_POSIX|FL_OFDLCK))
> +	if ((blocker->flc_flags & (FL_POSIX|FL_OFDLCK)) == FL_POSIX)
>  		locks_insert_global_blocked(waiter);

I wonder how that happened... sorry I didn't notice it in my review.

Reviewed-by: NeilBrown <neilb@xxxxxxx>

Thanks,
NeilBrown


>  
>  	/* The requests in waiter->flc_blocked are known to conflict with
> 
> ---
> base-commit: 292fcaa1f937345cb65f3af82a1ee6692c8df9eb
> change-id: 20240218-flsplit4-e843536f4c11
> 
> Best regards,
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>
> 
>