Messages in this thread Patch in this message |  | | | Date | Tue, 03 Jan 2012 14:32:42 -0800 | | From | Greg KH <> | | Subject | [09/75] cfq-iosched: fix cfq_cic_link() race confition |
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3.1-stable review patch. If anyone has any objections, please let me know.
------------------ From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
commit 5eb46851de3904cd1be9192fdacb8d34deadc1fc upstream.
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> Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- block/cfq-iosched.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -3184,7 +3184,7 @@ static int cfq_cic_link(struct cfq_data } } - if (ret) + if (ret && ret != -EEXIST) printk(KERN_ERR "cfq: cic link failed!\n"); return ret; @@ -3200,6 +3200,7 @@ cfq_get_io_context(struct cfq_data *cfqd { struct io_context *ioc = NULL; struct cfq_io_context *cic; + int ret; might_sleep_if(gfp_mask & __GFP_WAIT); @@ -3207,6 +3208,7 @@ cfq_get_io_context(struct cfq_data *cfqd if (!ioc) return NULL; +retry: cic = cfq_cic_lookup(cfqd, ioc); if (cic) goto out; @@ -3215,7 +3217,12 @@ cfq_get_io_context(struct cfq_data *cfqd 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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