[Intel-gfx] [PATCH 2/2] drm/i915: Avoid unguarded reads from the request pointer

[Chris Wilson]

On Mon, Feb 06, 2017 at 03:57:47PM +0200, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > In commit 86aa7e760a67 ("drm/i915: Assert that the context-switch
> > completion matches our context") I added a read to the irq tasklet
> > handler that compared the on-chip status with that of our sw tracking,
> > using an unguarded read of the request pointer to get the context and
> > beyond. Whilst we hold a reference to the request, we do not hold
> > anything on the context and if we are unlucky it may be reaped from a
> > second thread retiring the request (since it may retire the request as
> > soon as the breadcrumb is complete, even before we finish processing the
> > context switch) as we try to read from the context pointer.
> >
> 
> Please add warning of the possibility of context vanishing beneath
> our feet. Perhaps a good spot is when we store a bug on variable
> context_id.

I prefer it in ctx-switch, since that is where we are in a softirq
section trying to operate under the minimal of locking.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre