Re: [PATCH 12/37] drm/i915: Add fetch_and_zero() macro

[Chris Wilson <chris@chris-wilson.co.uk>]

On Fri, Aug 12, 2016 at 10:55:46AM +0300, Joonas Lahtinen wrote:
> On pe, 2016-08-12 at 07:54 +0100, Chris Wilson wrote:
> > A simple little macro to clear a pointer and return the old value. This
> > is useful for writing
> > 
> > 	value = *ptr;
> > 	if (!value)
> > 		return;
> > 
> > 	*ptr = 0;
> > 	...
> > 	free(value);
> > 
> > in a slightly more concise form:
> > 
> > 	value = fetch_and_zero(ptr);
> > 	if (!value)
> > 		return;
> > 
> > 	...
> > 	free(value);
> > 
> > with the idea that this establishes a pattern that may be extended for
> > atomic use (using xchg or cmpxchg) i.e. atomic_fetch_and_zero() and
> > similar to llist.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > ---
> >  drivers/gpu/drm/i915/i915_drv.h | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index a4203ff0cf7f..731c67fc9796 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -3903,4 +3903,10 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
> >  	return false;
> >  }
> >  
> > +#define fetch_and_zero(ptr) ({					\
> > +	typeof(*ptr) T__ = *(ptr);					\
> 
> Looking at container_of etc. it's always __ as prefix.

The advice to use the suffix is from gcc's manual iirc, and there are a
few macros that do dotted around the place, but it looks overwhelmingly
in favour of the prefix.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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