vfs: dodge smp_mb in break_lease and break_deleg in the common case

These inlines show up in the fast path (e.g., in do_dentry_open()) and
induce said full barrier regarding i_flctx access when in most cases the
pointer is NULL.

The pointer can be safely checked before issuing the barrier, dodging it
in most cases as a result.

It is plausible the consume fence would be sufficient, but I don't want
to go audit all callers regarding what they before calling here.

Signed-off-by: Mateusz Guzik <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/filelock.h

···
 #ifdef CONFIG_FILE_LOCKING
 static inline int break_lease(struct inode *inode, unsigned int mode)
 {
+	struct file_lock_context *flctx;
+
 	/*
 	 * Since this check is lockless, we must ensure that any refcounts
 	 * taken are done before checking i_flctx->flc_lease. Otherwise, we
 	 * could end up racing with tasks trying to set a new lease on this
 	 * file.
 	 */
+	flctx = READ_ONCE(inode->i_flctx);
+	if (!flctx)
+		return 0;
 	smp_mb();
-	if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))
+	if (!list_empty_careful(&flctx->flc_lease))
 		return __break_lease(inode, mode, FL_LEASE);
 	return 0;
 }
 
 static inline int break_deleg(struct inode *inode, unsigned int mode)
 {
+	struct file_lock_context *flctx;
+
 	/*
 	 * Since this check is lockless, we must ensure that any refcounts
 	 * taken are done before checking i_flctx->flc_lease. Otherwise, we
 	 * could end up racing with tasks trying to set a new lease on this
 	 * file.
 	 */
+	flctx = READ_ONCE(inode->i_flctx);
+	if (!flctx)
+		return 0;
 	smp_mb();
-	if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))
+	if (!list_empty_careful(&flctx->flc_lease))
 		return __break_lease(inode, mode, FL_DELEG);
 	return 0;
 }