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SubjectRe: [patch]cfq-iosched: fix cfq_cic_link() race confition
On Fri, Dec 02, 2011 at 05:05:48PM +0900, Yasuaki Ishimatsu wrote:
> 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>

Looks good to me. Thanks.

Acked-by: Vivek Goyal <vgoyal@redhat.com>

Vivek



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