[Intel-gfx] [RFC 01/11] drm/i915: Early exit from semaphore_waits_for for execlist mode.

[Chris Wilson]

On Tue, Jun 16, 2015 at 04:46:05PM +0100, Tomas Elf wrote:
> On 16/06/2015 14:44, Daniel Vetter wrote:
> >On Mon, Jun 08, 2015 at 06:03:19PM +0100, Tomas Elf wrote:
> >>When submitting semaphores in execlist mode the hang checker crashes in this
> >>function because it is only runnable in ring submission mode. The reason this
> >>is of particular interest to the TDR patch series is because we use semaphores
> >>as a mean to induce hangs during testing (which is the recommended way to
> >>induce hangs for gen8+). It's not clear how this is supposed to work in
> >>execlist mode since:
> >>
> >>1. This function requires a ring buffer.
> >>
> >>2. Retrieving a ring buffer in execlist mode requires us to retrieve the
> >>corresponding context, which we get from a request.
> >>
> >>3. Retieving a request from the hang checker is not straight-forward since that
> >>requires us to grab the struct_mutex in order to synchronize against the
> >>request retirement thread.
> >>
> >>4. Grabbing the struct_mutex from the hang checker is nothing that we will do
> >>since that puts us at risk of deadlock since a hung thread might be holding the
> >>struct_mutex already.
> >>
> >>Therefore it's not obvious how we're supposed to deal with this. For now, we're
> >>doing an early exit from this function, which avoids any kernel panic situation
> >>when running our own internal TDR ULT.
> >>
> >>Signed-off-by: Tomas Elf <tomas.elf at intel.com>
> >
> >We should have a Testcase: line here which mentions the igt testcase which
> >provoke this bug. Or we need to fill this gap asap.
> >-Daniel
> 
> You know this better than I do: Is there an IGT test that submits a
> semaphore in execlist mode? Because that's all you need to do to
> reproduce this. We could certainly add one if there is none like
> that already.

No, we don't have anything submitting a hanging semaphore from
userspace or igt specifically.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre