nfs: block notification on fs with its own ->lock

NFSv4.1 supports an optional lock notification feature which notifies
the client when a lock comes available. (Normally NFSv4 clients just
poll for locks if necessary.) To make that work, we need to request a
blocking lock from the filesystem.

We turned that off for NFS in commit f657f8eef3ff ("nfs: don't atempt
blocking locks on nfs reexports") [sic] because it actually blocks the
nfsd thread while waiting for the lock.

Thanks to Vasily Averin for pointing out that NFS isn't the only
filesystem with that problem.

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.

So, this patch mostly reverts commit f657f8eef3ff ("nfs: don't atempt
blocking locks on nfs reexports") [sic] and commit b840be2f00c0 ("lockd:
don't attempt blocking locks on nfs reexports"), and instead uses a
check of ->lock (Vasily's suggestion) to decide whether to support
blocking lock notifications on a given filesystem. Also add a little
documentation.

Perhaps someday we could add back an export flag later to allow
filesystems with "good" ->lock methods to support blocking lock
notifications.

Reported-by: Vasily Averin <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>
[ cel: Description rewritten to address checkpatch nits ]
[ cel: Fixed warning when SUNRPC debugging is disabled ]
[ cel: Fixed NULL check ]
Signed-off-by: Chuck Lever <[e-mail]>
Reviewed-by: Vasily Averin <[e-mail]>

fs/lockd/svclock.c

···
 	    struct nlm_host *host, struct nlm_lock *lock, int wait,
 	    struct nlm_cookie *cookie, int reclaim)
 {
-	struct nlm_block	*block = NULL;
+#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
 	struct inode		*inode = nlmsvc_file_inode(file);
+#endif
+	struct nlm_block	*block = NULL;
 	int			error;
 	int			mode;
 	int			async_block = 0;
···
 				(long long)lock->fl.fl_end,
 				wait);
 
-	if (inode->i_sb->s_export_op->flags & EXPORT_OP_SYNC_LOCKS) {
+	if (nlmsvc_file_file(file)->f_op->lock) {
 		async_block = wait;
 		wait = 0;
 	}

fs/nfs/export.c

···
 	.fetch_iversion = nfs_fetch_iversion,
 	.flags = EXPORT_OP_NOWCC|EXPORT_OP_NOSUBTREECHK|
 		EXPORT_OP_CLOSE_BEFORE_UNLINK|EXPORT_OP_REMOTE_FS|
-		EXPORT_OP_NOATOMIC_ATTR|EXPORT_OP_SYNC_LOCKS,
+		EXPORT_OP_NOATOMIC_ATTR,
 };

fs/nfsd/nfs4state.c

···
 	struct nfsd4_blocked_lock *nbl = NULL;
 	struct file_lock *file_lock = NULL;
 	struct file_lock *conflock = NULL;
-	struct super_block *sb;
 	__be32 status = 0;
 	int lkflg;
 	int err;
···
 		dprintk("NFSD: nfsd4_lock: permission denied!\n");
 		return status;
 	}
-	sb = cstate->current_fh.fh_dentry->d_sb;
 
 	if (lock->lk_is_new) {
 		if (nfsd4_has_session(cstate))
···
 	fp = lock_stp->st_stid.sc_file;
 	switch (lock->lk_type) {
 		case NFS4_READW_LT:
-			if (nfsd4_has_session(cstate) &&
-			    !(sb->s_export_op->flags & EXPORT_OP_SYNC_LOCKS))
+			if (nfsd4_has_session(cstate))
 				fl_flags |= FL_SLEEP;
 			fallthrough;
 		case NFS4_READ_LT:
···
 			fl_type = F_RDLCK;
 			break;
 		case NFS4_WRITEW_LT:
-			if (nfsd4_has_session(cstate) &&
-			    !(sb->s_export_op->flags & EXPORT_OP_SYNC_LOCKS))
+			if (nfsd4_has_session(cstate))
 				fl_flags |= FL_SLEEP;
 			fallthrough;
 		case NFS4_WRITE_LT:
···
 		status = nfserr_openmode;
 		goto out;
 	}
+
+	/*
+	 * Most filesystems with their own ->lock operations will block
+	 * the nfsd thread waiting to acquire the lock.  That leads to
+	 * deadlocks (we don't want every nfsd thread tied up waiting
+	 * for file locks), so don't attempt blocking lock notifications
+	 * on those filesystems:
+	 */
+	if (nf->nf_file->f_op->lock)
+		fl_flags &= ~FL_SLEEP;
 
 	nbl = find_or_allocate_block(lock_sop, &fp->fi_fhandle, nn);
 	if (!nbl) {

include/linux/exportfs.h

···
 #define EXPORT_OP_NOATOMIC_ATTR		(0x10) /* Filesystem cannot supply
 						  atomic attribute updates
 						*/
-#define EXPORT_OP_SYNC_LOCKS		(0x20) /* Filesystem can't do
-						  asychronous blocking locks */
 	unsigned long	flags;
 };
 

include/linux/lockd/lockd.h

···
 int           nlmsvc_unlock_all_by_sb(struct super_block *sb);
 int           nlmsvc_unlock_all_by_ip(struct sockaddr *server_addr);
 
+static inline struct file *nlmsvc_file_file(struct nlm_file *file)
+{
+	return file->f_file[O_RDONLY] ?
+	       file->f_file[O_RDONLY] : file->f_file[O_WRONLY];
+}
+
 static inline struct inode *nlmsvc_file_inode(struct nlm_file *file)
 {
-	return locks_inode(file->f_file[O_RDONLY] ?
-			   file->f_file[O_RDONLY] : file->f_file[O_WRONLY]);
+	return locks_inode(nlmsvc_file_file(file));
 }
 
 static inline int __nlm_privileged_request4(const struct sockaddr *sap)