Re: perf fuzzer crash [PATCH] perf: Get group events reference before moving the group

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jan 19, 2015 at 02:40:28PM +0000, Mark Rutland wrote:
> On Fri, Jan 16, 2015 at 02:11:04PM +0000, Peter Zijlstra wrote:
> > On Fri, Jan 16, 2015 at 11:46:44AM +0100, Peter Zijlstra wrote:
> > > Its a bandaid at best :/ The problem is (again) that we changes
> > > event->ctx without any kind of serialization.
> > >
> > > The issue came up before:
> > >
> > >   https://lkml.org/lkml/2014/9/5/397
> 
> In the end neither the CCI or CCN perf drivers migrate events on
> hotplug, so ARM is currently safe from the perf_pmu_migrate_context
> case, but I see that you fix the move_group handling too.
> 
> I had a go at testing this by hacking migration back into the CCI PMU
> driver (atop of v3.19-rc5), but I'm seeing lockups after a few minutes
> with my original test case (https://lkml.org/lkml/2014/9/1/569 with
> PMU_TYPE and PMU_EVENT fixed up).
> 
> I unfortunately don't have a suitable x86 box spare to run that on.
> Would someone be able to give it a spin on something with an uncore PMU?
> 
> I'll go and dig a bit further. I may just be hitting another latent
> issue on my board.

I'm able to trigger the lockups even without both your patch and the
call to perf_pmu_migrate_context, so there is a latent issue.

On vanilla v3.19-rc5 and vanilla v3.18, I'm able to get my hotplug
script hung when run concurrently with the test case against the CCI PMU
driver (without migration). The v3.18 and v3.19-rc5 lockups are
identical:

INFO: task hpall.sh:1506 blocked for more than 120 seconds.
      Not tainted 3.19.0-rc5 #9
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
hpall.sh        D 804a6ffc     0  1506   1497 0x00000000
[<804a6ffc>] (__schedule) from [<80022308>] (cpu_hotplug_begin+0xa0/0xac)
[<80022308>] (cpu_hotplug_begin) from [<8002236c>] (_cpu_up+0x24/0x180)
[<8002236c>] (_cpu_up) from [<8002253c>] (cpu_up+0x74/0x98)
[<8002253c>] (cpu_up) from [<802bce60>] (device_online+0x64/0x90)
[<802bce60>] (device_online) from [<802bcef4>] (online_store+0x68/0x74)
[<802bcef4>] (online_store) from [<8014059c>] (kernfs_fop_write+0xbc/0x1a0)
[<8014059c>] (kernfs_fop_write) from [<800e71b0>] (vfs_write+0xa0/0x1ac)
[<800e71b0>] (vfs_write) from [<800e7808>] (SyS_write+0x44/0x9c)
[<800e7808>] (SyS_write) from [<8000e560>] (ret_fast_syscall+0x0/0x48)
7 locks held by hpall.sh/1506:
 #0:  (sb_writers#6){.+.+.+}, at: [<800e729c>] vfs_write+0x18c/0x1ac
 #1:  (&of->mutex){+.+.+.}, at: [<8014052c>] kernfs_fop_write+0x4c/0x1a0
 #2:  (s_active#15){.+.+.+}, at: [<80140534>] kernfs_fop_write+0x54/0x1a0
 #3:  (device_hotplug_lock){+.+.+.}, at: [<802bbe44>] lock_device_hotplug_sysfs+0xc/0x4c
 #4:  (&dev->mutex){......}, at: [<802bce14>] device_online+0x18/0x90
 #5:  (cpu_add_remove_lock){+.+.+.}, at: [<80022508>] cpu_up+0x40/0x98
 #6:  (cpu_hotplug.lock){++++++}, at: [<80022268>] cpu_hotplug_begin+0x0/0xac

I guess that lockup is my fundamental issue, and with your patch the
perf_rwsem manages to spread a transitive dependency on one of those
locks all over the perf subsystem. I haven't considered that in great
detail, however.

Thanks,
Mark.