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>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 3beed83437a3268242e1bb33a10076644dd7b338..4c12869fcf77e044f75dfd3f4728c3343d9ef1f3 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3184,7 +3184,7 @@ 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;
@@ -3200,6 +3200,7 @@ 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);
 
@@ -3207,6 +3208,7 @@ 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;
@@ -3215,7 +3217,12 @@ 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: