linux-kernel - Re: [PATCH v1 01/11] seqlock: provide lockdep-free raw_seqcount

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Message-ID: <3F35C81D-34DB-4EA0-BC5C-A13EB71524A4@vmware.com>
Date:   Fri, 17 Dec 2021 18:01:17 +0000
From:   Nadav Amit <namit@...are.com>
To:     David Hildenbrand <david@...hat.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v1 01/11] seqlock: provide lockdep-free raw_seqcount_t
 variant



> On Dec 17, 2021, at 9:49 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> On 17.12.21 18:29, David Hildenbrand wrote:
>> On 17.12.21 18:02, Nadav Amit wrote:
>>> 
>>> 
>>>> On Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@...hat.com> wrote:
>>>> 
>>>> Sometimes it is required to have a seqcount implementation that uses
>>>> a structure with a fixed and minimal size -- just a bare unsigned int --
>>>> independent of the kernel configuration. This is especially valuable, when
>>>> the raw_ variants of the seqlock function will be used and the additional
>>>> lockdep part of the seqcount_t structure remains essentially unused.
>>>> 
>>>> Let's provide a lockdep-free raw_seqcount_t variant that can be used via
>>>> the raw functions to have a basic seqlock.
>>>> 
>>>> The target use case is embedding a raw_seqcount_t in the "struct page",
>>>> where we really want a minimal size and cannot tolerate a sudden grow of
>>>> the seqcount_t structure resulting in a significant "struct page"
>>>> increase or even a layout change.
>>>> 
>>>> Provide raw_read_seqcount_retry(), to make it easy to match to
>>>> raw_read_seqcount_begin() in the code.
>>>> 
>>>> Let's add a short documentation as well.
>>>> 
>>>> Note: There might be other possible users for raw_seqcount_t where the
>>>>     lockdep part might be completely unused and just wastes memory --
>>>>     essentially any users that only use the raw_ function variants.
>>>> 
>>> 
>>> Is it possible to force some policy when raw_seqcount_t is used to
>>> prevent its abuse? For instance not to allow to acquire other (certain?)
>>> locks when it is held?
>>> 
>> 
>> Good question ... in this series we won't be taking additional locks on
>> the reader or the writer side. Something like lockdep_forbid() /
>> lockdep_allow() to disallow any kind of locking. I haven't heard of
>> anything like that, maybe someone reading along has a clue?
>> 
>> The writer side might be easy to handle, but some seqcount operations
>> that don't do the full read()->retry() cycle are problematic
>> (->raw_read_seqcount).
> 
> Sorry, I forgot to mention an important point: the raw_seqcount_t
> doesn't give you any additional "power" to abuse.
> 
> You can just use the ordinary seqcount_t with the raw_ functions. One
> example is mm->write_protect_seq . So whatever we would want to "invent"
> should also apply to the raw_ functions in general --  which might be
> undesired or impossible (IIRC IRQ context).
> 

Thanks for the clarification. I was unfamiliar with
raw_read_seqcount_begin() (and friends). Indeed it is very very rarely
used.