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Date:   Wed, 10 Nov 2021 17:53:57 +0100
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Subject: Re: [PATCH v3 00/15] Free user PTE page table pages
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To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        akpm@linux-foundation.org, tglx@linutronix.de,
        kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, songmuchun@bytedance.com,
        zhouchengming@bytedance.com
References: <20211110105428.32458-1-zhengqi.arch@bytedance.com>
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 <YYv4Msg7zVLS3KE/@casper.infradead.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 10.11.21 17:49, Matthew Wilcox wrote:
> On Wed, Nov 10, 2021 at 04:37:14PM +0100, David Hildenbrand wrote:
>>> I'd suggest to make this new lock a special rwsem which allows either
>>> concurrent read access OR concurrent PTL access, but not both. This
>>
>> I looked into such a lock recently in similar context and something like
>> that does not exist yet (and fairness will be challenging). You either
>> have a single reader or multiple writer. I'd be interested if someone
>> knows of something like that.
> 
> We've talked about having such a lock before for filesystems which want
> to permit either many direct-IO accesses or many buffered-IO accesses, but
> want to exclude a mixture of direct-IO and buffered-IO.  The name we came
> up with for such a lock was the red-blue lock.  Either Team Red has the
> lock, or Team Blue has the lock (or it's free).  Indicate free with velue
> zero, Team Red with positive numbers and Team Blue with negative numbers.
> If we need to indicate an opposing team is waiting for the semaphore,
> we can use a high bit (1 << 30) to indicate no new team members can
> acquire the lock.  Not sure whether anybody ever coded it up.

Interesting, thanks for sharing!

My excessive google search didn't reveal anything back then (~3 months
ago) -- only questions on popular coding websites asking essentially for
the same thing without any helpful replies. But maybe I used the wrong
keywords (e.g., "multiple reader, multiple writer", I certainly didn't
search for "red-blue lock", but I do like the name :) ).

Fairness might still be the biggest issue, but I am certainly no locking
expert.

-- 
Thanks,

David / dhildenb