> On Tue, Mar 21, 2017 at 5:00 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:

> Hi Dave,

> Today's linux-next merge of the drm tree got a conflict in:

> drivers/gpu/drm/i915/i915_gem_shrinker.c

> between commit:

> 3d3d18f086cd ("drm/i915: Avoid rcu_barrier() from reclaim paths (shrinker)")

> from the drm-intel-fixes tree and commit:

> 519d52498156 ("drm/i915: i915_gem_shrink_all() needs an awake device")

> from the drm tree.

> I fixed it up (see below) and can carry the fix as necessary. This

> is now fixed as far as linux-next is concerned, but any non trivial

> conflicts should be mentioned to your upstream maintainer when your tree

> is submitted for merging. You may also want to consider cooperating

> with the maintainer of the conflicting tree to minimise any particularly

> complex conflicts.

> --

> Cheers,

> Stephen Rothwell

> diff --cc drivers/gpu/drm/i915/i915_gem_shrinker.c

> index d5d2b4c6ed38,006a8b908f77..000000000000

> --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c

> +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c

> @@@ -263,7 -264,9 +264,9 @@@ unsigned long i915_gem_shrink_all(struc

> I915_SHRINK_BOUND |

> I915_SHRINK_UNBOUND |

> I915_SHRINK_ACTIVE);

> + intel_runtime_pm_put(dev_priv);

> +

> - rcu_barrier(); /* wait until our RCU delayed slab frees are completed */

> + synchronize_rcu(); /* wait for our earlier RCU delayed slab frees */

Unless I am really confused, the RCU delayed slab frees are via call_rcu().

This means that synchronize_rcu() is -not- guaranteed to wait on them.

For example, CPU 0 might be making its way through a pile of callbacks,

including some RCU delayed slab free callbacks. If there are enough

callbacks, it could take CPU 0 several grace periods to work through the

backlog. CPU 1 might have no callbacks queued, and might execute the

above code. Now synchronize_rcu() will wait for a grace period, but only

one, and not necessarily for CPU 0's backlog to drain.

I do not believe that the above fix is safe.

> return freed;

> }