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    Subject[34-longterm 088/179] cfq-iosched: fix locking around ioc->ioc_data assignment
    From: Jens Axboe <>

    This is a commit scheduled for the next v2.6.34 longterm release.
    If you see a problem with using this for longterm, please comment.

    commit ab4bd22d3cce6977dc039664cc2d052e3147d662 upstream.

    Since we are modifying this RCU pointer, we need to hold
    the lock protecting it around it.

    This fixes a potential reuse and double free of a cfq
    io_context structure. The bug has been in CFQ for a long
    time, it hit very few people but those it did hit seemed
    to see it a lot.

    Tracked in RH bugzilla here:

    Credit goes to Paul Bolle for figuring out that the issue
    was around the one-hit ioc->ioc_data cache. Thanks to his
    hard work the issue is now fixed.

    Signed-off-by: Jens Axboe <>
    Signed-off-by: Paul Gortmaker <>
    block/cfq-iosched.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 4fab7c9..510552a 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -2559,8 +2559,11 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
    cic->dead_key = (unsigned long) cic->key;
    cic->key = NULL;

    - if (ioc->ioc_data == cic)
    + if (rcu_dereference(ioc->ioc_data) == cic) {
    + spin_lock(&ioc->lock);
    rcu_assign_pointer(ioc->ioc_data, NULL);
    + spin_unlock(&ioc->lock);
    + }

    if (cic->cfqq[BLK_RW_ASYNC]) {
    cfq_exit_cfqq(cfqd, cic->cfqq[BLK_RW_ASYNC]);

     \ /
      Last update: 2012-05-15 10:22    [W:2.579 / U:0.428 seconds]
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