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Message-ID: <c2f150f4a73739b42c3f70002073c2a5de11ef0e.camel@intel.com>
Date:   Thu, 3 Mar 2022 10:22:49 +0000
From:   "Hellstrom, Thomas" <thomas.hellstrom@...el.com>
To:     "Souza, Jose" <jose.souza@...el.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "jiasheng@...as.ac.cn" <jiasheng@...as.ac.cn>,
        "Vivi, Rodrigo" <rodrigo.vivi@...el.com>,
        "Sripada, Radhakrishna" <radhakrishna.sripada@...el.com>,
        "joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
        "ville.syrjala@...ux.intel.com" <ville.syrjala@...ux.intel.com>,
        "Roper, Matthew D" <matthew.d.roper@...el.com>,
        "tvrtko.ursulin@...ux.intel.com" <tvrtko.ursulin@...ux.intel.com>,
        "Auld, Matthew" <matthew.auld@...el.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>,
        "De Marchi, Lucas" <lucas.demarchi@...el.com>
CC:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/i915/gt: Handle errors for i915_gem_object_trylock

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@...as.ac.cn>
> > ---
> >   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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