linux-kernel - Re: [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length [ver #2]

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Date:   Wed, 30 Oct 2019 23:38:38 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-api@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring,
 not cursor and length [ver #2]

On 30/10/2019 23.16, Ilya Dryomov wrote:
> On Wed, Oct 30, 2019 at 9:35 PM Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>>
>> On 30/10/2019 17.19, Ilya Dryomov wrote:
>>> On Thu, Oct 24, 2019 at 11:49 AM David Howells <dhowells@...hat.com> wrote:
>>>>  /*
>>>> - * We use a start+len construction, which provides full use of the
>>>> - * allocated memory.
>>>> - * -- Florian Coosmann (FGC)
>>>> - *
>>>> + * We use head and tail indices that aren't masked off, except at the point of
>>>> + * dereference, but rather they're allowed to wrap naturally.  This means there
>>>> + * isn't a dead spot in the buffer, provided the ring size < INT_MAX.
>>>> + * -- David Howells 2019-09-23.
>>>
>>> Hi David,
>>>
>>> Is "ring size < INT_MAX" constraint correct?
>>
>> No. As long as one always uses a[idx % size] to access the array, the
>> only requirement is that size is representable in an unsigned int. Then
>> because one also wants to do the % using simple bitmasking, that further
>> restricts one to sizes that are a power of 2, so the end result is that
>> the max size is 2^31 (aka INT_MAX+1).
> 
> I think the fact that indices are free running and wrap at a power of
> two already restricts you to sizes the are a power of two,

Ah, yes, of course. When reducing indices mod n that may already have
been implicitly reduced mod N, N must be a multiple of n for the result
to be well-defined.

Rasmus