From 1282cf037522a2fdbcc9438595a8d5db3543c3e4 Mon Sep 17 00:00:00 2001 From: Yasuaki Ishimatsu Date: Fri, 2 Dec 2011 10:07:07 +0100 Subject: cfq-iosched: fix cfq_cic_link() race confition 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 Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman --- block/cfq-iosched.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 1c9fba6b616d..e5c77d8ae2bc 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -1981,7 +1981,7 @@ static int cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc, } } - if (ret) + if (ret && ret != -EEXIST) printk(KERN_ERR "cfq: cic link failed!\n"); return ret; @@ -1997,6 +1997,7 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask) { struct io_context *ioc = NULL; struct cfq_io_context *cic; + int ret; might_sleep_if(gfp_mask & __GFP_WAIT); @@ -2004,6 +2005,7 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask) if (!ioc) return NULL; +retry: cic = cfq_cic_lookup(cfqd, ioc); if (cic) goto out; @@ -2012,7 +2014,12 @@ cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask) 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: -- cgit v1.2.3