Re: [PATCH v4] drm/scheduler: Avoid accessing freed bad job.

From: Lucas Stach <l.stach@pengutronix.de>
To: "Andrey Grodzovsky" <Andrey.Grodzovsky@amd.com>,
	"Luben Tuikov" <luben.tuikov@amd.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Alex Deucher" <alexdeucher@gmail.com>
Cc: Emily Deng <Emily.Deng@amd.com>,
	amd-gfx list <amd-gfx@lists.freedesktop.org>,
	Maling list - DRI developers <dri-devel@lists.freedesktop.org>,
	steven.price@arm.com
Subject: Re: [PATCH v4] drm/scheduler: Avoid accessing freed bad job.
Date: Tue, 21 Jul 2020 13:03:04 +0200	[thread overview]
Message-ID: <e38c494a544d7e0607ee89abcc6a85419e700011.camel@pengutronix.de> (raw)
In-Reply-To: <2cef1ca3-115c-44ee-9caf-6cb8d6404796@amd.com>

Hi Andrey,

Am Mittwoch, den 12.02.2020, 11:33 -0500 schrieb Andrey Grodzovsky:
> On 2/11/20 7:53 PM, Luben Tuikov wrote:
> > On 2020-02-11 4:27 p.m., Andrey Grodzovsky wrote:
> > > On 2/11/20 10:55 AM, Andrey Grodzovsky wrote:
> > > > On 2/10/20 4:50 PM, Luben Tuikov wrote:
> > > > > Hi Lucas,
> > > > > 
> > > > > Thank you for bringing awareness of this issue, publicly.
> > > > > 
> > > > > As soon as this patch showed up back in November of 2019,
> > > > > I objected to it, privately.
> > > > 
> > > > I didn't find this objection in my mail actually
> > Yes, I didn't send it to you.
> > 
> > > > > I suggested to instead use a _list_ to store the "state" of
> > > > > all jobs of the same state. Then, at any time, timeout interrupt
> > > > > or whatever, we can atomically (irq spinlock) move the timeout/bad
> > > > > job to the timedout/cleanup/bad job list, and wake someone up
> > > > > to deal with that list asynchronously, and return from the
> > > > > interrupt/etc.
> > > > > immediately.
> > > > 
> > > > Sounds a good idea to me, i think enough for us to have 2 lists,
> > > > timeout list for jobs scheduled to HW and not yet completed
> > > > (completion fence signaled) and cleanup list for those that did
> > > > complete. This should give alternative solution to the race condition
> > > > this patch was addressing without causing the break the Lucas
> > > > reported. If no one objects I think i can try implement it.
> > > > 
> > > > Andrey
> > > 
> > > Thinking more i realize Luben is right about having also bad job list as
> > > this is needed for normal job competition (by fence callback from
> > > amdgpu_fence_process)  and you need to decide if you move it to cleanup
> > > list from timeout list or not. If it's already in bad job list - meaning
> > > that it's being processed by GPU recovery code you don't touch it,
> > > otherwise you move it to cleanup list where it will be freed eventually
> > > by invocation of drm_sched_get_cleanup_job.
> > Yep...
> > 
> > Perhaps fewer lists, than "timeout", "bad" and "cleanup" could be had.
> > I'd also name the "bad" list as "recovery" list, as that is what would
> > be done to commands on that list.
> > 
> > "Timeout" is a status "timed-out", so perhaps just set the timeout
> > flag and move it to a "done" list. (Note that the command can still
> > complete asynchronously while on that list and while it has status
> > "timed-out'.)
> > 
> > The idea is that,
> > 1) it avoid contention and races when more than one context
> >     can update the job at the same time, and
> > 2) easy to process all jobs of a certain state and/or
> >     move them around, etc.
> > 
> > Let's discuss it and come up with a plan. :-)
> > 
> > Regards,
> > Luben
> 
> Sure, let me maybe come up with a draft patch so we have more concrete 
> stuff to discuss and review.

It seems we all dropped the ball on this one. I believe this is still
an open issue. Has there been any progress from your side on fixing
this?

Regards,
Lucas

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