Re: [PATCH] drm/msm: use proper memory barriers for updating tail/head

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Oct 3, 2017 at 2:59 PM, Rob Clark <robdclark@gmail.com> wrote:
> On Tue, Oct 3, 2017 at 4:08 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Mon, Oct 02, 2017 at 10:37:11AM -0400, Rob Clark wrote:
>>> Fixes intermittent corruption of cmdstream dump.
>>>
>>> Signed-off-by: Rob Clark <robdclark@gmail.com>
>>
>> Rule of memory barriers: They always need to come in pairs, and you should
>> put a comment to each one explaining where the other side is. I.e. for all
>> the store_release ones you add here you need to put a few read_acquire
>> ones somewhere else, and then explain how they link.
>>
>> One side only doesn't really work with barriers - except when you're lucky
>> and on x86 :-)
>
> fwiw, this is straight out of Documentation/circular-buffers.txt
>
> I guess it could read_acquire when checking the avail/free space on
> the consumer/producer side.. otoh if we see an old value there it is
> harmless (ie. we just fall into a wait_event()).  I assume that is why
> the documentation didn't bother with a read_acquire.

smp_load_acquire is the right name, and it's right there. In the
example at least :-)
-Daniel

>
> BR,
> -R
>
>> -Daniel
>>> ---
>>>  drivers/gpu/drm/msm/msm_rd.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_rd.c b/drivers/gpu/drm/msm/msm_rd.c
>>> index 5c087a4228ff..918c65e19964 100644
>>> --- a/drivers/gpu/drm/msm/msm_rd.c
>>> +++ b/drivers/gpu/drm/msm/msm_rd.c
>>> @@ -120,7 +120,7 @@ static void rd_write(struct msm_rd_state *rd, const void *buf, int sz)
>>>               n = min(sz, circ_space_to_end(&rd->fifo));
>>>               memcpy(fptr, ptr, n);
>>>
>>> -             fifo->head = (fifo->head + n) & (BUF_SZ - 1);
>>> +             smp_store_release(&fifo->head, (fifo->head + n) & (BUF_SZ - 1));
>>>               sz  -= n;
>>>               ptr += n;
>>>
>>> @@ -157,7 +157,7 @@ static ssize_t rd_read(struct file *file, char __user *buf,
>>>               goto out;
>>>       }
>>>
>>> -     fifo->tail = (fifo->tail + n) & (BUF_SZ - 1);
>>> +     smp_store_release(&fifo->tail, (fifo->tail + n) & (BUF_SZ - 1));
>>>       *ppos += n;
>>>
>>>       wake_up_all(&rd->fifo_event);
>>> --
>>> 2.13.5
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel@lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>
>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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