Date:   Wed, 10 Nov 2021 16:49:54 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
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
Subject: Re: [PATCH v3 00/15] Free user PTE page table pages
Message-ID: <YYv4Msg7zVLS3KE/@casper.infradead.org>
References: <20211110105428.32458-1-zhengqi.arch@bytedance.com>
 <20211110125601.GQ1740502@nvidia.com>
 <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com>
 <20211110143859.GS1740502@nvidia.com>
 <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com>
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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.