Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH 5/5] iommu/iova: Avoid double-negatives in magazine
 helpers
To:     Will Deacon <will@kernel.org>
CC:     <joro@8bytes.org>, <mst@redhat.com>, <jasowang@redhat.com>,
        <robin.murphy@arm.com>, <xieyongji@bytedance.com>,
        <linux-kernel@vger.kernel.org>, <iommu@lists.linux-foundation.org>,
        <virtualization@lists.linux-foundation.org>, <linuxarm@huawei.com>,
        <thunder.leizhen@huawei.com>, <baolu.lu@linux.intel.com>
References: <1632477717-5254-1-git-send-email-john.garry@huawei.com>
 <1632477717-5254-6-git-send-email-john.garry@huawei.com>
 <20211004113836.GB27373@willie-the-truck>
From:   John Garry <john.garry@huawei.com>
Message-ID: <743a089c-6b5f-ed3f-d0bd-23b891c03a66@huawei.com>
Date:   Mon, 4 Oct 2021 14:33:31 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.12.1
MIME-Version: 1.0
In-Reply-To: <20211004113836.GB27373@willie-the-truck>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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