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

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Date:   Fri, 17 Dec 2021 18:29:03 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <namit@...are.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 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).

> [ snip ]
> 
>> +/**
>> + * raw_seqcount_init() - runtime initializer for raw_seqcount_t
>> + * @s: Pointer to the raw_seqcount_t instance
>> + */
>> +# define raw_seqcount_init(s) __raw_seqcount_init(s)
>> +
>> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>
>> # define SEQCOUNT_DEP_MAP_INIT(lockname)				\
>> @@ -111,11 +129,16 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
>> # define seqcount_lockdep_reader_access(x)
>> #endif
>>
>> +/**
>> + * RAW_SEQCNT_ZERO() - static initializer for raw_seqcount_t
>> + */
>> +#define RAW_SEQCNT_ZERO() 0
> 
> I am not sure why RAW_SWQCNT_ZERO() should be a function-like macro.
> 

I think I just went for consistency with SEQCNT_ZERO() -- but I agree,
that can just be simplified!

Thanks!


-- 
Thanks,

David / dhildenb