Messages in this thread Patch in this message |  | | Subject | Re: Kernel crash in icq_free_icq_rcu | | From | Shaohua Li <> | | Date | Wed, 18 Jan 2012 14:03:22 +0800 |
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On Wed, 2012-01-18 at 09:30 +0800, Shaohua Li wrote: > On Tue, 2012-01-17 at 17:11 -0800, Tejun Heo wrote: > > On Wed, Jan 18, 2012 at 09:05:26AM +0800, Shaohua Li wrote: > > > > Vivek is seeing the problem while switching elevators. Are you too? > > > > Or is it during normal operation? > > > same here. I had some problems when I debug my ioscheduler, but > > > eventually found even switching cfq and noop can trigger oops. > > > > Hmmm... maybe quiescing isn't working as expected and kmem cache is > > being destroyed with live icq's. I'll try to reproduce it. > this debug patch seems to fix for me. > > diff --git a/block/blk-core.c b/block/blk-core.c > index e6c05a9..c6a8ef5 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -872,11 +872,11 @@ retry: > spin_unlock_irq(q->queue_lock); > > /* create icq if missing */ > - if (unlikely(et->icq_cache && !icq)) > + if (unlikely(et->icq_cache && !icq && (rw_flags & REQ_ELVPRIV))) > icq = ioc_create_icq(q, gfp_mask); > > /* rqs are guaranteed to have icq on elv_set_request() if requested */ > - if (likely(!et->icq_cache || icq)) > + if (likely(!et->icq_cache || icq || !(rw_flags & REQ_ELVPRIV))) > rq = blk_alloc_request(q, icq, rw_flags, gfp_mask); > > if (unlikely(!rq)) { > Here is the formal patch. I hope it fixes all the problems related to icq_cache, but please open an eye :)
Subject: block: fix NULL icq_cache reference
CPU0: CPU1: switch from cfq to noop elv_quiesce_start C: get_request A: ioc_create_icq alloc icq with cfq B: do elevator switch ioc_clear_queue elv_quiesce_end insert icq to ioc switch from noop to cfq elv_quiesce_start do elevator switch ioc_clear_queue elv_quiesce_end CPU0 leaves some icq to ioc list after elevator is switching from cfq to noop. in the second ioc_clear_queue, the ioc has icq in its list, but current elevator is noop. so ioc_exit_icq will get a NULL et->icq_cache.
In above cases, if A runs after B, ioc_create_icq will have a NULL et->icq_cache, this will trigger another crash.
Note, get_request caches et without lock hold. Between C and A, a elevator switch can start. But we will have elvpriv++, elv_quiesce_start will drain all requests first. So this will not trigger crash.
Signed-off-by: Shaohua Li <shaohua.li@intel.com> --- block/blk-core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
Index: linux/block/blk-core.c =================================================================== --- linux.orig/block/blk-core.c 2012-01-18 12:44:13.000000000 +0800 +++ linux/block/blk-core.c 2012-01-18 12:45:28.000000000 +0800 @@ -872,11 +872,11 @@ retry: spin_unlock_irq(q->queue_lock); /* create icq if missing */ - if (unlikely(et->icq_cache && !icq)) + if (unlikely(et->icq_cache && !icq && (rw_flags & REQ_ELVPRIV))) icq = ioc_create_icq(q, gfp_mask); /* rqs are guaranteed to have icq on elv_set_request() if requested */ - if (likely(!et->icq_cache || icq)) + if (likely(!et->icq_cache || icq || !(rw_flags & REQ_ELVPRIV))) rq = blk_alloc_request(q, icq, rw_flags, gfp_mask); if (unlikely(!rq)) {
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