Re: [PATCH 2/2] drm/i915: fix up _wait_for macro

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Mar 28, 2013 at 12:03:25AM +0100, Daniel Vetter wrote:
> As Thomas Gleixner spotted, it's rather horrible racy:
> - We can miss almost a full tick, so need to compensate by 1 jiffy.

I have a feeling this is a rather common pattern, so I wonder
if [mu]secs_to_jiffies() should do the +1 already for everyone.
Or maybe there should be some other macros for specifying
timeouts that would do the +1.

> - We need to re-check the condition when having timed-out, since a
>   the last check could have been before the timeout expired. E.g. when
>   we've been preempted or a long irq happened.
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Reported-by: Jack Winter <jbh@alchemy.lu>
> Cc: Jack Winter <jbh@alchemy.lu>
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/intel_drv.h | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index c8c1979..9dcae4e 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -33,12 +33,21 @@
>  #include <drm/drm_fb_helper.h>
>  #include <drm/drm_dp_helper.h>
>  
> +/**
> + * _wait_for - magic (register) wait macro
> + *
> + * Does the right thing for modeset paths when run under kdgb or similar atomic
> + * contexts. Note that it's important that we check the condition again after
> + * having timed out, since the timeout could be due to preemption or similar and
> + * we've never had a chance to check the condition before the timeout.
> + */
>  #define _wait_for(COND, MS, W) ({ \
> -	unsigned long timeout__ = jiffies + msecs_to_jiffies(MS);	\
> +	unsigned long timeout__ = jiffies + msecs_to_jiffies(MS) + 1;	\
>  	int ret__ = 0;							\
>  	while (!(COND)) {						\
>  		if (time_after(jiffies, timeout__)) {			\
> -			ret__ = -ETIMEDOUT;				\
> +			if (!(COND))					\
> +				ret__ = -ETIMEDOUT;			\
>  			break;						\
>  		}							\
>  		if (W && drm_can_sleep())  {				\
> -- 
> 1.7.11.7
> 
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-- 
Ville Syrjälä
Intel OTC