Messages in this thread | | | Date | Tue, 19 Oct 2010 08:33:20 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH] Fix array overflow in CFQ |
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On Tue, Oct 19, 2010 at 07:49:33AM -0400, Vivek Goyal wrote: > On Tue, Oct 19, 2010 at 12:01:40PM +0200, Jens Axboe wrote: > > On 2010-10-19 11:10, Andi Kleen wrote: > > > From: Andi Kleen <ak@linux.intel.com> > > > > > > gcc 4.5 complains when compiling a recent rc with > > > > > > linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’: > > > linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array bounds > > > > > > and it is right: > > > > > > slice = group_slice * count / > > > max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio], > > > cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg)); > > > > > > busy_queues_avg can be indexed by this enum > > > > > > enum wl_prio_t { > > > BE_WORKLOAD = 0, > > > RT_WORKLOAD = 1, > > > IDLE_WORKLOAD = 2, > > > }; > > > > > > in cfqd->serving_prio, but is only declared as > > > > > > unsigned int busy_queues_avg[2]; > > > > > > which is clearly off by one. Fix this here. > > > > Indeed, that is definitely buggy. ->service_trees[][] looks buggy, too. > > WTF?! > > Hi Jens, > > busy_queues_avg[] definitely looks buggy. Looks like I introduced this bug > while converting corrado's logic to group logic. I will fix it in a while. > Sorry for the goof up here.
Jens,
Staring at the code for some more time, it looks like that busy_queues_avg[] is also not buggy (at least at run time).
We maintain busy_queues_avg() only for RT and BE class. For IDLE class, we expire the workload immediately after a jiffy.
/* Choose next priority. RT > BE > IDLE */ if (cfq_group_busy_queues_wl(RT_WORKLOAD, cfqd, cfqg)) cfqd->serving_prio = RT_WORKLOAD; else if (cfq_group_busy_queues_wl(BE_WORKLOAD, cfqd, cfqg)) cfqd->serving_prio = BE_WORKLOAD; else { cfqd->serving_prio = IDLE_WORKLOAD; cfqd->workload_expires = jiffies + 1; return; }
... ... ...
slice = group_slice * count / max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio], cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
So for IDLE class, we return immediately from the function and never execute cfqg->busy_queues_avg[IDLE].
Now to remove the gcc warning we can increase the size of busy_queues_avg[] array but third field should always remain unused.
Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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