linux-kernel - Re: [PATCH 5/5] iommu/iova: Avoid double-negatives in magazine helpers

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Date:   Mon, 4 Oct 2021 14:33:31 +0100
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will@...nel.org>
CC:     <joro@...tes.org>, <mst@...hat.com>, <jasowang@...hat.com>,
        <robin.murphy@....com>, <xieyongji@...edance.com>,
        <linux-kernel@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
        <virtualization@...ts.linux-foundation.org>, <linuxarm@...wei.com>,
        <thunder.leizhen@...wei.com>, <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH 5/5] iommu/iova: Avoid double-negatives in magazine
 helpers

On 04/10/2021 12:38, Will Deacon wrote:

Hi Will,

>> To avoid this, stop using double-negatives, like !iova_magazine_full() and
>> !iova_magazine_empty(), and use positive tests, like
>> iova_magazine_has_space() and iova_magazine_has_pfns(), respectively; these
>> can safely deal with cpu_rcache->{loaded, prev} = NULL.
> I don't understand why you're saying that things like !iova_magazine_empty()
> are double-negatives. What about e.g. !list_empty() elsewhre in the kernel?

IMO, a check for an empty magazine is a negative check, as opposed to a 
check for availability.

But I'm not saying that patterns like !list_empty() are a bad practice. 
I'm just saying that they are not NULL safe, and that matters in this 
case as we can potentially pass a NULL pointer.

> 
> The crux of the fix seems to be:
> 
>> @@ -783,8 +787,9 @@ static bool __iova_rcache_insert(struct iova_caching_domain *rcached,
>>   		if (new_mag) {
>>   			spin_lock(&rcache->lock);
>>   			if (rcache->depot_size < MAX_GLOBAL_MAGS) {
>> -				rcache->depot[rcache->depot_size++] =
>> -						cpu_rcache->loaded;
>> +				if (cpu_rcache->loaded)
>> +					rcache->depot[rcache->depot_size++] =
>> +							cpu_rcache->loaded;
> Which could be independent of the renaming?

If cpu_rcache->loaded was NULL, then we crash before we reach this code.

Anyway, since I earlier reworked init_iova_rcaches() to properly handle 
failed mem allocations for rcache->cpu_rcaches, I can rework further to 
fail the init for failed mem allocations for cpu_rcaches->loaded, so we 
don't need this change.

Thanks,
John