linux-kernel - Re: [PATCH] drm/msm/a5xx: Fix barrier usage in set_preempt

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Message-ID: <20191015162144.fuyc25tdwvc6ddu3@linux-p48b>
Date:   Tue, 15 Oct 2019 09:21:44 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     robdclark@...il.com
Cc:     sean@...rly.run, dri-devel@...ts.freedesktop.org,
        Davidlohr Bueso <dbueso@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/a5xx: Fix barrier usage in set_preempt_state()

Forgot to Cc lkml.

On Mon, 14 Oct 2019, Davidlohr Bueso wrote:

>Because it is not a Rmw operation, atomic_set() is never serialized,
>and therefore the 'upgradable' smp_mb__{before,after}_atomic() calls
>that order the write to preempt_state are completely bogus.
>
>This patch replaces these with smp_mb(), which seems like the
>original intent of when the code was written.
>
>Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
>---
> drivers/gpu/drm/msm/adreno/a5xx_preempt.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/gpu/drm/msm/adreno/a5xx_preempt.c b/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
>index 9cf9353a7ff1..d27d8d3208c6 100644
>--- a/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
>+++ b/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
>@@ -30,10 +30,10 @@ static inline void set_preempt_state(struct a5xx_gpu *gpu,
> 	 * preemption or in the interrupt handler so barriers are needed
> 	 * before...
> 	 */
>-	smp_mb__before_atomic();
>+	smp_mb();
> 	atomic_set(&gpu->preempt_state, new);
> 	/* ... and after*/
>-	smp_mb__after_atomic();
>+	smp_mb();
> }
>
> /* Write the most recent wptr for the given ring into the hardware */
>-- 
>2.16.4
>