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fs/super.c: fix race between freeze_super() and thaw_super()

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commit 89f39af129382a40d7cd1f6914617282cfeee28e upstream.

Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than
frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which
drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering.

In this case thaw_super() will wrongly call s_op->unfreeze_fs() before
it was actually frozen, and call sb_freeze_unlock() which leads to the
unbalanced percpu_up_write(). Unfortunately lockdep can't detect this,
so this triggers misc BUG_ON()'s in kernel/rcu/sync.c.

Reported-and-tested-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>
This commit is contained in:
Oleg Nesterov
2016-09-26 18:07:48 +02:00
committed by Willy Tarreau
parent 3c15d16635
commit b24eac8630
1 changed files with 3 additions and 3 deletions
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fs/super.c
+3 -3
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@@ -1327,8 +1327,8 @@ int freeze_super(struct super_block *sb)
}
}
/*
* This is just for debugging purposes so that fs can warn if it
* sees write activity when frozen is set to SB_FREEZE_COMPLETE.
* For debugging purposes so that fs can warn if it sees write activity
* when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
*/
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
up_write(&sb->s_umount);
@@ -1347,7 +1347,7 @@ int thaw_super(struct super_block *sb)
int error;
down_write(&sb->s_umount);
if (sb->s_writers.frozen == SB_UNFROZEN) {
if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
up_write(&sb->s_umount);
return -EINVAL;
}