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    Subject[074/107] cfq-iosched: fix locking around ioc->ioc_data assignment
    2.6.39-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Jens Axboe <jaxboe@fusionio.com>

    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:

    https://bugzilla.redhat.com/show_bug.cgi?id=577968

    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 <jaxboe@fusionio.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    block/cfq-iosched.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -2704,8 +2704,11 @@ static void __cfq_exit_single_io_context
    smp_wmb();
    cic->key = cfqd_dead_key(cfqd);

    - 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]);



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