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    Subject[patch]cfq-iosched: fix cfq_cic_link() race confition
    cfq_cic_link() has race condition. When some processes which shared ioc
    issue I/O to same block device simultaneously, cfq_cic_link() returns -EEXIST
    sometimes. The race condition might stop I/O by following steps:

    step 1: Process A: Issue an I/O to /dev/sda
    step 2: Process A: Get an ioc (iocA here) in get_io_context() which does not
    linked with a cic for the device
    step 3: Process A: Get a new cic for the device (cicA here) in
    cfq_alloc_io_context()

    step 4: Process B: Issue an I/O to /dev/sda
    step 5: Process B: Get iocA in get_io_context() since process A and B share the
    same ioc
    step 6: Process B: Get a new cic for the device (cicB here) in
    cfq_alloc_io_context() since iocA has not been linked with a
    cic for the device yet

    step 7: Process A: Link cicA to iocA in cfq_cic_link()
    step 8: Process A: Dispatch I/O to driver and finish it

    step 9: Process B: Try to link cicB to iocA in cfq_cic_link()
    But it fails with showing "cfq: cic link failed!" kernel
    message, since iocA has already linked with cicA at step 7.
    step 10: Process B: Wait for finishig I/O in get_request_wait()
    The function does not wake up, when there is no I/O to the
    device.

    When cfq_cic_link() returns -EEXIST, it means ioc has already linked with cic.
    So when cfq_cic_link() return -EEXIST, retry cfq_cic_lookup().

    Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
    ---
    block/cfq-iosched.c | 10 ++++++++--
    1 file changed, 8 insertions(+), 2 deletions(-)

    Index: linux-3.2-rc3-fix/block/cfq-iosched.c
    ===================================================================
    --- linux-3.2-rc3-fix.orig/block/cfq-iosched.c 2011-11-30 18:32:28.000000000 +0900
    +++ linux-3.2-rc3-fix/block/cfq-iosched.c 2011-11-30 18:24:02.000000000 +0900
    @@ -3184,7 +3184,7 @@
    }
    }

    - if (ret)
    + if (ret && ret != -EEXIST)
    printk(KERN_ERR "cfq: cic link failed!\n");

    return ret;
    @@ -3200,6 +3200,7 @@
    {
    struct io_context *ioc = NULL;
    struct cfq_io_context *cic;
    + int ret;

    might_sleep_if(gfp_mask & __GFP_WAIT);

    @@ -3207,6 +3208,7 @@
    if (!ioc)
    return NULL;

    +retry:
    cic = cfq_cic_lookup(cfqd, ioc);
    if (cic)
    goto out;
    @@ -3215,7 +3217,12 @@
    if (cic == NULL)
    goto err;

    - if (cfq_cic_link(cfqd, ioc, cic, gfp_mask))
    + ret = cfq_cic_link(cfqd, ioc, cic, gfp_mask);
    + if (ret == -EEXIST) {
    + /* someone has linked cic to ioc already */
    + cfq_cic_free(cic);
    + goto retry;
    + } else if (ret)
    goto err_free;

    out:


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