Commit 0a30288d authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe

blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe

blk-mq uses percpu_ref for its usage counter which tracks the number
of in-flight commands and used to synchronously drain the queue on
freeze.  percpu_ref shutdown takes measureable wallclock time as it
involves a sched RCU grace period.  This means that draining a blk-mq
takes measureable wallclock time.  One would think that this shouldn't
matter as queue shutdown should be a rare event which takes place
asynchronously w.r.t. userland.

Unfortunately, SCSI probing involves synchronously setting up and then
tearing down a lot of request_queues back-to-back for non-existent
LUNs.  This means that SCSI probing may take more than ten seconds
when scsi-mq is used.

This will be properly fixed by implementing a mechanism to keep
q->mq_usage_counter in atomic mode till genhd registration; however,
that involves rather big updates to percpu_ref which is difficult to
apply late in the devel cycle (v3.17-rc6 at the moment).  As a
stop-gap measure till the proper fix can be implemented in the next
cycle, this patch introduces __percpu_ref_kill_expedited() and makes
blk_mq_freeze_queue() use it.  This is heavy-handed but should work
for testing the experimental SCSI blk-mq implementation.
Signed-off-by: default avatarTejun Heo <>
Reported-by: default avatarChristoph Hellwig <>
Fixes: add703fd ("blk-mq: use percpu_ref for mq usage count")
Cc: Kent Overstreet <>
Cc: Jens Axboe <>
Tested-by: default avatarChristoph Hellwig <>
Signed-off-by: default avatarJens Axboe <>
parent 452b6361
......@@ -119,7 +119,16 @@ void blk_mq_freeze_queue(struct request_queue *q)
if (freeze) {
* XXX: Temporary kludge to work around SCSI blk-mq stall.
* SCSI synchronously creates and destroys many queues
* back-to-back during probe leading to lengthy stalls.
* This will be fixed by keeping ->mq_usage_counter in
* atomic mode until genhd registration, but, for now,
* let's work around using expedited synchronization.
blk_mq_run_queues(q, false);
wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->mq_usage_counter));
......@@ -71,6 +71,7 @@ void percpu_ref_reinit(struct percpu_ref *ref);
void percpu_ref_exit(struct percpu_ref *ref);
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
percpu_ref_func_t *confirm_kill);
void __percpu_ref_kill_expedited(struct percpu_ref *ref);
* percpu_ref_kill - drop the initial ref
......@@ -184,3 +184,19 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
* XXX: Temporary kludge to work around SCSI blk-mq stall. Used only by
* block/blk-mq.c::blk_mq_freeze_queue(). Will be removed during v3.18
* devel cycle. Do not use anywhere else.
void __percpu_ref_kill_expedited(struct percpu_ref *ref)
WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,
"percpu_ref_kill() called more than once on %pf!",
ref->pcpu_count_ptr |= PCPU_REF_DEAD;
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