Messages in this thread | | | Date | Fri, 2 Dec 2011 08:49:13 -0500 | From | Vivek Goyal <> | Subject | Re: [patch]cfq-iosched: fix cfq_cic_link() race confition |
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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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