Re: Kernel crash in icq_free_icq_rcu

[Vivek Goyal]

On Tue, Jan 17, 2012 at 09:19:28PM +0100, Jens Axboe wrote:
> On 2012-01-17 21:18, Vivek Goyal wrote:
> > Hi Tejun,
> > 
> > With latest linus kernel, I see following crash. I was running some
> > scripts which create cgroups and launch fio jobs in those groups. In
> > a separate window I wrote a script to change the IO scheduler on the
> > device in a loop while IO was happening on the device. After few
> > seconds I see following. So far I tried it twice and reproduced it
> > both the times in first few seconds.
> > 
> > Thanks
> > Vivek
> > 
> > 
> > [   94.217015] BUG: unable to handle kernel NULL pointer dereference at 000000000000001c
> > [   94.218004] IP: [<ffffffff81142fae>] kmem_cache_free+0x5e/0x200
> > [   94.218004] PGD 13abda067 PUD 137d52067 PMD 0 
> > [   94.218004] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
> > [   94.218004] CPU 0 
> > [   94.218004] Modules linked in: [last unloaded: scsi_wait_scan]
> > [   94.218004] 
> > [   94.218004] Pid: 0, comm: swapper/0 Not tainted 3.2.0+ #16 Hewlett-Packard HP xw6600 Workstation/0A9Ch
> > [   94.218004] RIP: 0010:[<ffffffff81142fae>]  [<ffffffff81142fae>] kmem_cache_free+0x5e/0x200
> > [   94.218004] RSP: 0018:ffff88013fc03de0  EFLAGS: 00010006
> > [   94.218004] RAX: ffffffff81e0d020 RBX: ffff880138b3c680 RCX: 00000001801c001b
> > [   94.218004] RDX: 00000000003aac1d RSI: ffff880138b3c680 RDI: ffffffff81142fae
> > [   94.218004] RBP: ffff88013fc03e10 R08: ffff880137830238 R09: 0000000000000001
> > [   94.218004] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > [   94.218004] R13: ffffea0004e2cf00 R14: ffffffff812f6eb6 R15: 0000000000000246
> > [   94.218004] FS:  0000000000000000(0000) GS:ffff88013fc00000(0000) knlGS:0000000000000000
> > [   94.218004] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> > [   94.218004] CR2: 000000000000001c CR3: 00000001395ab000 CR4: 00000000000006f0
> > [   94.218004] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > [   94.218004] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > [   94.218004] Process swapper/0 (pid: 0, threadinfo ffffffff81e00000, task ffffffff81e0d020)
> > [   94.218004] Stack:
> > [   94.218004]  0000000000000102 ffff88013fc0db20 ffffffff81e22700 ffff880139500f00
> > [   94.218004]  0000000000000001 000000000000000a ffff88013fc03e20 ffffffff812f6eb6
> > [   94.218004]  ffff88013fc03e90 ffffffff810c8da2 ffffffff81e01fd8 ffff880137830240
> > [   94.218004] Call Trace:
> > [   94.218004]  <IRQ> 
> > [   94.218004]  [<ffffffff812f6eb6>] icq_free_icq_rcu+0x16/0x20
> > [   94.218004]  [<ffffffff810c8da2>] __rcu_process_callbacks+0x1c2/0x420
> > [   94.218004]  [<ffffffff810c9038>] rcu_process_callbacks+0x38/0x250
> > [   94.218004]  [<ffffffff810405ee>] __do_softirq+0xce/0x3e0
> > [   94.218004]  [<ffffffff8108ed04>] ? clockevents_program_event+0x74/0x100
> > [   94.218004]  [<ffffffff81090104>] ? tick_program_event+0x24/0x30
> > [   94.218004]  [<ffffffff8183ed1c>] call_softirq+0x1c/0x30
> > [   94.218004]  [<ffffffff8100422d>] do_softirq+0x8d/0xc0
> > [   94.218004]  [<ffffffff81040c3e>] irq_exit+0xae/0xe0
> > [   94.218004]  [<ffffffff8183f4be>] smp_apic_timer_interrupt+0x6e/0x99
> > [   94.218004]  [<ffffffff8183e330>] apic_timer_interrupt+0x70/0x80
> > [   94.218004]  <EOI> 
> > [   94.218004]  [<ffffffff8100a806>] ? mwait_idle+0xb6/0x4c0
> > [   94.218004]  [<ffffffff8100a7fd>] ? mwait_idle+0xad/0x4c0
> > [   94.218004]  [<ffffffff810011e6>] cpu_idle+0x96/0xe0
> > [   94.218004]  [<ffffffff818064df>] rest_init+0x133/0x144
> > [   94.218004]  [<ffffffff81806425>] ? rest_init+0x79/0x144
> > [   94.218004]  [<ffffffff81ed4b51>] start_kernel+0x35b/0x366
> > [   94.218004]  [<ffffffff81ed4321>] x86_64_start_reservations+0x131/0x135
> > [   94.218004]  [<ffffffff81ed4415>] x86_64_start_kernel+0xf0/0xf7
> > [   94.218004] Code: f3 e8 97 cb ee ff 48 c1 e8 0c 48 c1 e0 06 49 01 c5 49 8b 45 00 f6 c4 80 0f 85 99 00 00 00 4c 8b 75 08 9c 41 5f fa e8 12 f8 f4 ff <49> 63 74 24 1c 48 89 df e8 e5 4b f5 ff 41 f7 c7 00 02 00 00 74 
> > [   94.218004] RIP  [<ffffffff81142fae>] kmem_cache_free+0x5e/0x200
> > [   94.218004]  RSP <ffff88013fc03de0>
> > [   94.218004] CR2: 000000000000001c
> 
> Can you try this?

Nope, this does not help either. Can reproduce the issue with below
patch applied.

> 
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 163263d..ee55019 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3117,18 +3117,17 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
>   */
>  static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  {
> -	struct cfq_queue *old_cfqq = cfqd->active_queue;
> -
>  	cfq_log_cfqq(cfqd, cfqq, "preempt");
> -	cfq_slice_expired(cfqd, 1);
>  
>  	/*
>  	 * workload type is changed, don't save slice, otherwise preempt
>  	 * doesn't happen
>  	 */
> -	if (cfqq_type(old_cfqq) != cfqq_type(cfqq))
> +	if (cfqq_type(cfqd->active_queue) != cfqq_type(cfqq))
>  		cfqq->cfqg->saved_workload_slice = 0;
>  
> +	cfq_slice_expired(cfqd, 1);
> +

cfq_slice_expired() will overwrite the value of
cfqq->cfqg->saved_workload_slice. So we need to set it to zero after
cfq_slice_expired.

I was thinking of just saving the workload type of cfqq before
cfq_slice_expired() so that we don't access old cfqq after
cfq_slice_expired(). But then I noticed that we don't drop a cfqq
reference in cfq_slice_expired(). So not sure how cfq_slice_expired()
can lead to freeing up of queue. It should happen only when process
has exited and last request on the queue if finished. 

Thanks
Vivek
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