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> Cc: stable@kernel.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -3184,7 +3184,7 @@ static int cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
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}
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}
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if (ret)
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if (ret && ret != -EEXIST)
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printk(KERN_ERR "cfq: cic link failed!\n");
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return ret;
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@ -3200,6 +3200,7 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
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{
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struct io_context *ioc = NULL;
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struct cfq_io_context *cic;
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int ret;
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might_sleep_if(gfp_mask & __GFP_WAIT);
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@ -3207,6 +3208,7 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
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if (!ioc)
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return NULL;
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retry:
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cic = cfq_cic_lookup(cfqd, ioc);
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if (cic)
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goto out;
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@ -3215,7 +3217,12 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
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if (cic == NULL)
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goto err;
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if (cfq_cic_link(cfqd, ioc, cic, gfp_mask))
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ret = cfq_cic_link(cfqd, ioc, cic, gfp_mask);
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if (ret == -EEXIST) {
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/* someone has linked cic to ioc already */
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cfq_cic_free(cic);
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goto retry;
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} else if (ret)
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goto err_free;
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out:
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