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Commit 89f39af1 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Al Viro
Browse files

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



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: default avatarNikolay Borisov <kernel@kyup.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 655042cc

fs/super.c

+3 −3
Original line number Original line Diff line number Diff line
@@ -1379,8 +1379,8 @@ int freeze_super(struct super_block *sb) 
		}

		}

	}

	}

	/*

	/*

	 * This is just for debugging purposes so that fs can warn if it

	 * For debugging purposes so that fs can warn if it sees write activity

	 * sees write activity when frozen is set to SB_FREEZE_COMPLETE.

	 * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().

	 */

	 */

	sb->s_writers.frozen = SB_FREEZE_COMPLETE;

	sb->s_writers.frozen = SB_FREEZE_COMPLETE;

	up_write(&sb->s_umount);

	up_write(&sb->s_umount);
@@ -1399,7 +1399,7 @@ int thaw_super(struct super_block *sb) 
	int error;

	int error;





	down_write(&sb->s_umount);

	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);

		up_write(&sb->s_umount);

		return -EINVAL;

		return -EINVAL;

	}

	}