Message-ID: <a832ca37-b078-6f62-f99f-b2703674c1d8@redhat.com>
Date:   Fri, 17 Dec 2021 18:49:50 +0100
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Subject: Re: [PATCH v1 01/11] seqlock: provide lockdep-free raw_seqcount_t
 variant
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From:   David Hildenbrand <david@redhat.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>
References: <20211217113049.23850-1-david@redhat.com>
 <20211217113049.23850-2-david@redhat.com>
 <38BCB153-7E7C-4AAD-8657-E5C6F9E1EF9B@vmware.com>
 <058e97eb-1489-3d59-c6ee-94175dc13134@redhat.com>
Organization: Red Hat
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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@redhat.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,

David / dhildenb