Re: [PATCH 3/7] lockd: fix race in async lock request handling

[Chuck Lever <chuck.lever@oracle.com>]

On Wed, Aug 23, 2023 at 05:33:48PM -0400, Alexander Aring wrote:
> This patch fixes a race in async lock request handling between adding
> the relevant struct nlm_block to nlm_blocked list after the request was
> sent by vfs_lock_file() and nlmsvc_grant_deferred() does a lookup of the
> nlm_block in the nlm_blocked list. It could be that the async request is
> completed before the nlm_block was added to the list. This would end
> in a -ENOENT and a kernel log message of "lockd: grant for unknown
> block".
> 
> To solve this issue we add the nlm_block before the vfs_lock_file() call
> to be sure it has been added when a possible nlmsvc_grant_deferred() is
> called. If the vfs_lock_file() results in an case when it wouldn't be
> added to nlm_blocked list, the nlm_block struct will be removed from
> this list again.
> 
> The introducing of the new B_PENDING_CALLBACK nlm_block flag will handle
> async lock requests on a pending lock requests as a retry on the caller
> level to hit the -EAGAIN case.

This last paragraph might be obsolete? I don't see a
B_PENDING_CALLBACK flag in the kernel or in this patch series.


> Signed-off-by: Alexander Aring <aahringo@redhat.com>
> ---
>  fs/lockd/svclock.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/lockd/svclock.c b/fs/lockd/svclock.c
> index aa4174fbaf5b..3b158446203b 100644
> --- a/fs/lockd/svclock.c
> +++ b/fs/lockd/svclock.c
> @@ -546,6 +546,9 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
>  		ret = nlm_lck_blocked;
>  		goto out;
>  	}
> +
> +	/* Append to list of blocked */
> +	nlmsvc_insert_block_locked(block, NLM_NEVER);
>  	spin_unlock(&nlm_blocked_lock);
>  
>  	if (!wait)
> @@ -557,9 +560,12 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
>  	dprintk("lockd: vfs_lock_file returned %d\n", error);
>  	switch (error) {
>  		case 0:
> +			nlmsvc_remove_block(block);
>  			ret = nlm_granted;
>  			goto out;
>  		case -EAGAIN:
> +			if (!wait)
> +				nlmsvc_remove_block(block);
>  			ret = async_block ? nlm_lck_blocked : nlm_lck_denied;
>  			goto out;
>  		case FILE_LOCK_DEFERRED:
> @@ -570,17 +576,16 @@ nlmsvc_lock(struct svc_rqst *rqstp, struct nlm_file *file,
>  			ret = nlmsvc_defer_lock_rqst(rqstp, block);
>  			goto out;
>  		case -EDEADLK:
> +			nlmsvc_remove_block(block);
>  			ret = nlm_deadlock;
>  			goto out;
>  		default:			/* includes ENOLCK */
> +			nlmsvc_remove_block(block);
>  			ret = nlm_lck_denied_nolocks;
>  			goto out;
>  	}
>  
>  	ret = nlm_lck_blocked;
> -
> -	/* Append to list of blocked */
> -	nlmsvc_insert_block(block, NLM_NEVER);
>  out:
>  	mutex_unlock(&file->f_mutex);
>  	nlmsvc_release_block(block);
> -- 
> 2.31.1
> 

-- 
Chuck Lever