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authorAl Viro <viro@zeniv.linux.org.uk>2013-07-20 03:13:55 +0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-08-04 16:51:15 +0800
commitd0f1a6e5be8d8ada73891832af1bd3ecb3f52bb7 (patch)
tree3de8e255c9fb90a5eef99a75c16f9ae63907b782
parentba9a3c3ce30b8c399d5bfeef1d48a26170dc499e (diff)
downloadlinux-bcache-d0f1a6e5be8d8ada73891832af1bd3ecb3f52bb7.tar.zst
livelock avoidance in sget()
commit acfec9a5a892f98461f52ed5770de99a3e571ae2 upstream. Eric Sandeen has found a nasty livelock in sget() - take a mount(2) about to fail. The superblock is on ->fs_supers, ->s_umount is held exclusive, ->s_active is 1. Along comes two more processes, trying to mount the same thing; sget() in each is picking that superblock, bumping ->s_count and trying to grab ->s_umount. ->s_active is 3 now. Original mount(2) finally gets to deactivate_locked_super() on failure; ->s_active is 2, superblock is still ->fs_supers because shutdown will *not* happen until ->s_active hits 0. ->s_umount is dropped and now we have two processes chasing each other: s_active = 2, A acquired ->s_umount, B blocked A sees that the damn thing is stillborn, does deactivate_locked_super() s_active = 1, A drops ->s_umount, B gets it A restarts the search and finds the same superblock. And bumps it ->s_active. s_active = 2, B holds ->s_umount, A blocked on trying to get it ... and we are in the earlier situation with A and B switched places. The root cause, of course, is that ->s_active should not grow until we'd got MS_BORN. Then failing ->mount() will have deactivate_locked_super() shut the damn thing down. Fortunately, it's easy to do - the key point is that grab_super() is called only for superblocks currently on ->fs_supers, so it can bump ->s_count and grab ->s_umount first, then check MS_BORN and bump ->s_active; we must never increment ->s_count for superblocks past ->kill_sb(), but grab_super() is never called for those. The bug is pretty old; we would've caught it by now, if not for accidental exclusion between sget() for block filesystems; the things like cgroup or e.g. mtd-based filesystems don't have anything of that sort, so they get bitten. The right way to deal with that is obviously to fix sget()... Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--fs/super.c25
1 files changed, 10 insertions, 15 deletions
diff --git a/fs/super.c b/fs/super.c
index 7465d4364208..68307c029228 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -336,19 +336,19 @@ EXPORT_SYMBOL(deactivate_super);
* and want to turn it into a full-blown active reference. grab_super()
* is called with sb_lock held and drops it. Returns 1 in case of
* success, 0 if we had failed (superblock contents was already dead or
- * dying when grab_super() had been called).
+ * dying when grab_super() had been called). Note that this is only
+ * called for superblocks not in rundown mode (== ones still on ->fs_supers
+ * of their type), so increment of ->s_count is OK here.
*/
static int grab_super(struct super_block *s) __releases(sb_lock)
{
- if (atomic_inc_not_zero(&s->s_active)) {
- spin_unlock(&sb_lock);
- return 1;
- }
- /* it's going away */
s->s_count++;
spin_unlock(&sb_lock);
- /* wait for it to die */
down_write(&s->s_umount);
+ if ((s->s_flags & MS_BORN) && atomic_inc_not_zero(&s->s_active)) {
+ put_super(s);
+ return 1;
+ }
up_write(&s->s_umount);
put_super(s);
return 0;
@@ -463,11 +463,6 @@ retry:
destroy_super(s);
s = NULL;
}
- down_write(&old->s_umount);
- if (unlikely(!(old->s_flags & MS_BORN))) {
- deactivate_locked_super(old);
- goto retry;
- }
return old;
}
}
@@ -660,10 +655,10 @@ restart:
if (hlist_unhashed(&sb->s_instances))
continue;
if (sb->s_bdev == bdev) {
- if (grab_super(sb)) /* drops sb_lock */
- return sb;
- else
+ if (!grab_super(sb))
goto restart;
+ up_write(&sb->s_umount);
+ return sb;
}
}
spin_unlock(&sb_lock);