linux-kernel - Re: [PATCH] ksmbd: force "fail immediately" flag on fs with its own ->lock

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Date:   Wed, 22 Dec 2021 18:01:18 +0900
From:   Namjae Jeon <linkinjeon@...nel.org>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steve French <sfrench@...ba.org>,
        Hyunchul Lee <hyc.lee@...il.com>,
        Bruce Fields <bfields@...hat.com>,
        Jeff Layton <jlayton@...nel.org>, kernel@...nvz.org,
        linux-cifs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ksmbd: force "fail immediately" flag on fs with its own ->lock

2021-12-17 20:23 GMT+09:00, Vasily Averin <vvs@...tuozzo.com>:
> Like nfsd and lockd, ksmbd can cause deadlock if the exported
> file system does not support asynchronous processing of blocking locks:
> if all ksmbd worker threads handles such requests,
> they can never finish and the server will not be able to handle
> any other incoming requests.
>
> Any filesystem that leaves ->lock NULL will use posix_lock_file(), which
> does the right thing. Simplest is just to assume that any filesystem
> that defines its own ->lock is not safe to request a blocking lock from.
>
> To work around the problem we need to drop fl->fl_flag FL_SLEEP before
> vfs_lock_file() execution, it forces affected functions to avoid blocking.
>
> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
Acked-by: Namjae Jeon <linkinjeon@...nel.org>

Thanks!
> ---
>  fs/ksmbd/smb2pdu.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fs/ksmbd/smb2pdu.c b/fs/ksmbd/smb2pdu.c
> index 121f8e8c70ac..f2225491af02 100644
> --- a/fs/ksmbd/smb2pdu.c
> +++ b/fs/ksmbd/smb2pdu.c
> @@ -6634,6 +6634,12 @@ static int smb2_set_flock_flags(struct file_lock
> *flock, int flags)
>  {
>  	int cmd = -EINVAL;
>
> +	if ((flock->fl_file->f_op->lock) &&
> +	    ((flags == SMB2_LOCKFLAG_SHARED) ||
> +	     (flags == SMB2_LOCKFLAG_EXCLUSIVE))) {
> +		ksmbd_debug(SMB, "force fail immediately request\n");
> +		flags |= SMB2_LOCKFLAG_FAIL_IMMEDIATELY;
> +	}
>  	/* Checking for wrong flag combination during lock request*/
>  	switch (flags) {
>  	case SMB2_LOCKFLAG_SHARED:
> --
> 2.25.1
>
>