quota: Fix race between dqput() and dquot_scan_active()
authorJan Kara <jack@suse.cz>
Thu, 20 Feb 2014 16:02:27 +0000 (17:02 +0100)
committerBen Hutchings <ben@decadent.org.uk>
Tue, 1 Apr 2014 23:58:55 +0000 (00:58 +0100)
commit7149a5e5c1fecc22a838f87ed86bfa3cc0efc984
tree68a5c311e7d3eda70d0e07953518cf4eb6e18a79
parentcf5a6031f2bd8799ad551fb9b65f1b1e3104ea8a
quota: Fix race between dqput() and dquot_scan_active()

commit 1362f4ea20fa63688ba6026e586d9746ff13a846 upstream.

Currently last dqput() can race with dquot_scan_active() causing it to
call callback for an already deactivated dquot. The race is as follows:

CPU1 CPU2
  dqput()
    spin_lock(&dq_list_lock);
    if (atomic_read(&dquot->dq_count) > 1) {
     - not taken
    if (test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {
      spin_unlock(&dq_list_lock);
      ->release_dquot(dquot);
        if (atomic_read(&dquot->dq_count) > 1)
         - not taken
  dquot_scan_active()
    spin_lock(&dq_list_lock);
    if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
     - not taken
    atomic_inc(&dquot->dq_count);
    spin_unlock(&dq_list_lock);
        - proceeds to release dquot
    ret = fn(dquot, priv);
     - called for inactive dquot

Fix the problem by making sure possible ->release_dquot() is finished by
the time we call the callback and new calls to it will notice reference
dquot_scan_active() has taken and bail out.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
fs/quota/dquot.c