Re: [PATCH] drm/i915/gt: Handle errors for i915_gem_object_trylock

[Hellstrom, Thomas]

On Wed, 2022-03-02 at 10:37 +0000, Tvrtko Ursulin wrote:
> 
> + Thomas, Matt
> 
> On 02/03/2022 06:19, Jiasheng Jiang wrote:
> > As the potential failure of the i915_gem_object_trylock(),
> > it should be better to check it and return error if fails.
> > 
> > Fixes: 94ce0d65076c ("drm/i915/gt: Setup a default migration
> > context on the GT")
> > Signed-off-by: Jiasheng Jiang <jiasheng@xxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/gt/selftest_migrate.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/selftest_migrate.c
> > b/drivers/gpu/drm/i915/gt/selftest_migrate.c
> > index fa4293d2944f..79c6c68f7316 100644
> > --- a/drivers/gpu/drm/i915/gt/selftest_migrate.c
> > +++ b/drivers/gpu/drm/i915/gt/selftest_migrate.c
> > @@ -465,7 +465,11 @@ create_init_lmem_internal(struct intel_gt *gt,
> > size_t sz, bool try_lmem)
> >                         return obj;
> >         }
> >   
> > -       i915_gem_object_trylock(obj, NULL);
> 
> Guys why is this a trylock to start with? (Since being added in 
> 94ce0d65076c ("drm/i915/gt: Setup a default migration context on the
> GT").
> 
> Surely it can't ever fail since the object has just been created.

Typically in some situations, we want to create locked objects when
we're already in a ww transaction and might not have access to the ww
context, in which case an ordinary sleeping lock would trigger a
lockdep splat, so a trylock is used instead since it will never fail.

I once introduced a i915_gem_object_lock_isolated() wrapper around
trylock to annotate this situation, but that was removed during the
obj->mm.lock removal IIRC. Similarly TTM has an option to create an
object locked.

So here we should probably BUG on a trylock failure, if anything.
 In the long run we should probably mimic TTM and introduce an
interface to create an object locked.

/Thomas




> 
> Regards,

> 
> Tvrtko





> 
> > +       if (!i915_gem_object_trylock(obj, NULL)) {
> > +               i915_gem_object_put(obj);
> > +               return ERR_PTR(-EBUSY);
> > +       }
> > +
> >         err = i915_gem_object_pin_pages(obj);
> >         if (err) {
> >                 i915_gem_object_unlock(obj);

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