[Mesa-dev] [PATCH 06/13] mesa: Replace sampler object locks with atomic inc/dec.

[Timothy Arceri]

On Fri, 2015-08-07 at 10:09 -0700, Ian Romanick wrote:
> I know we've talked about this about 100 times, but something in the
> back of my mind tells me that we have a pre-existing race.  What happens
> if the p_atomic_dec_zero happens on thread A while thread B is between
> the _mesa_lookup_renderbuffer call and the _mesa_reference_renderbuffer
> call on the same object?  Won't thread A free the memory out from under
> thread B?

Right I think I follow. It looks like the bind would silently fail right?


> 
> On 08/06/2015 05:10 PM, Matt Turner wrote:
> > ---
> >  src/mesa/main/mtypes.h     |  1 -
> >  src/mesa/main/samplerobj.c | 16 +++-------------
> >  2 files changed, 3 insertions(+), 14 deletions(-)
> > 
> > diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
> > index 2adfae2..fcc527f 100644
> > --- a/src/mesa/main/mtypes.h
> > +++ b/src/mesa/main/mtypes.h
> > @@ -1153,7 +1153,6 @@ typedef enum
> >   */
> >  struct gl_sampler_object
> >  {
> > -   mtx_t Mutex;
> >     GLuint Name;
> >     GLint RefCount;
> >     GLchar *Label;               /**< GL_KHR_debug */
> > diff --git a/src/mesa/main/samplerobj.c b/src/mesa/main/samplerobj.c
> > index c7b9666..dba2087 100644
> > --- a/src/mesa/main/samplerobj.c
> > +++ b/src/mesa/main/samplerobj.c
> > @@ -38,6 +38,7 @@
> >  #include "main/macros.h"
> >  #include "main/mtypes.h"
> >  #include "main/samplerobj.h"
> > +#include "util/u_atomic.h"
> >  
> >  
> >  struct gl_sampler_object *
> > @@ -85,16 +86,9 @@ _mesa_reference_sampler_object_(struct gl_context *ctx,
> >  
> >     if (*ptr) {
> >        /* Unreference the old sampler */
> > -      GLboolean deleteFlag = GL_FALSE;
> >        struct gl_sampler_object *oldSamp = *ptr;
> >  
> > -      mtx_lock(&oldSamp->Mutex);
> > -      assert(oldSamp->RefCount > 0);
> > -      oldSamp->RefCount--;
> > -      deleteFlag = (oldSamp->RefCount == 0);
> > -      mtx_unlock(&oldSamp->Mutex);
> > -
> > -      if (deleteFlag) {
> > +      if (p_atomic_dec_zero(&oldSamp->RefCount)) {
> >  	 assert(ctx->Driver.DeleteSamplerObject);
> >           ctx->Driver.DeleteSamplerObject(ctx, oldSamp);
> >        }
> > @@ -105,7 +99,6 @@ _mesa_reference_sampler_object_(struct gl_context *ctx,
> >  
> >     if (samp) {
> >        /* reference new sampler */
> > -      mtx_lock(&samp->Mutex);
> >        if (samp->RefCount == 0) {
> >           /* this sampler's being deleted (look just above) */
> >           /* Not sure this can every really happen.  Warn if it does. */
> > @@ -113,10 +106,9 @@ _mesa_reference_sampler_object_(struct gl_context 
> > *ctx,
> >           *ptr = NULL;
> >        }
> >        else {
> > -         samp->RefCount++;
> > +         p_atomic_inc(&samp->RefCount);
> >           *ptr = samp;
> >        }
> > -      mtx_unlock(&samp->Mutex);
> >     }
> >  }
> >  
> > @@ -127,7 +119,6 @@ _mesa_reference_sampler_object_(struct gl_context 
> > *ctx,
> >  static void
> >  _mesa_init_sampler_object(struct gl_sampler_object *sampObj, GLuint name)
> >  {
> > -   mtx_init(&sampObj->Mutex, mtx_plain);
> >     sampObj->Name = name;
> >     sampObj->RefCount = 1;
> >     sampObj->WrapS = GL_REPEAT;
> > @@ -170,7 +161,6 @@ static void
> >  _mesa_delete_sampler_object(struct gl_context *ctx,
> >                              struct gl_sampler_object *sampObj)
> >  {
> > -   mtx_destroy(&sampObj->Mutex);
> >     free(sampObj->Label);
> >     free(sampObj);
> >  }
> > 
> 
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