fs/super.c: fix race between freeze_super() and thaw_super()
commit 89f39af1 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>
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