From: Yasuaki Ishimatsu Date: Fri, 2 Dec 2011 09:07:07 +0000 (+0100) Subject: cfq-iosched: fix cfq_cic_link() race confition X-Git-Tag: v3.2-rc6~9^2~3 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5eb46851de3904cd1be9192fdacb8d34deadc1fc;p=pandora-kernel.git 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 Cc: stable@kernel.org Signed-off-by: Jens Axboe --- Reading git-diff-tree failed