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

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Date:   Fri, 17 Dec 2021 17:02:22 +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 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?

[ 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.

Moreover, the documentation showed:

+	/* static */
+	static raw_seqcount_t foo_seqcount = RAW_SEQCNT_ZERO(foo_seqcount);
+

But RAW_SEQCNT_ZERO does not have an argument?