Re: [PATCH v2 hmm 05/11] mm/hmm: Remove duplicate condition test before wait_event_timeout

[Ralph Campbell <rcampbell@nvidia.com>]

On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> The wait_event_timeout macro already tests the condition as its first
> action, so there is no reason to open code another version of this, all
> that does is skip the might_sleep() debugging in common cases, which is
> not helpful.
> 
> Further, based on prior patches, we can no simplify the required condition
> test:
>   - If range is valid memory then so is range->hmm
>   - If hmm_release() has run then range->valid is set to false
>     at the same time as dead, so no reason to check both.
>   - A valid hmm has a valid hmm->mm.
> 
> Also, add the READ_ONCE for range->valid as there is no lock held here.
> 
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> ---
>   include/linux/hmm.h | 12 ++----------
>   1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 4ee3acabe5ed22..2ab35b40992b24 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -218,17 +218,9 @@ static inline unsigned long hmm_range_page_size(const struct hmm_range *range)
>   static inline bool hmm_range_wait_until_valid(struct hmm_range *range,
>   					      unsigned long timeout)
>   {
> -	/* Check if mm is dead ? */
> -	if (range->hmm == NULL || range->hmm->dead || range->hmm->mm == NULL) {
> -		range->valid = false;
> -		return false;
> -	}
> -	if (range->valid)
> -		return true;
> -	wait_event_timeout(range->hmm->wq, range->valid || range->hmm->dead,
> +	wait_event_timeout(range->hmm->wq, range->valid,
>   			   msecs_to_jiffies(timeout));
> -	/* Return current valid status just in case we get lucky */
> -	return range->valid;
> +	return READ_ONCE(range->valid);
>   }
>   
>   /*
> 

Since we are simplifying things, perhaps we should consider merging
hmm_range_wait_until_valid() info hmm_range_register() and
removing hmm_range_wait_until_valid() since the pattern
is to always call the two together.

In any case, this looks OK to me so you can add
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>