linux-kernel - Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpHpGSpix8+mB76Virb+HAMrOqB3wG8E4EXPrRCnBoBGeA@mail.gmail.com>
Date: Tue, 12 Nov 2024 07:18:45 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, minchan@...gle.com, 
	jannh@...gle.com, shakeel.butt@...ux.dev, souravpanda@...gle.com, 
	pasha.tatashin@...een.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com
Subject: Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

On Mon, Nov 11, 2024 at 8:58 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Nov 11, 2024 at 12:55:05PM -0800, Suren Baghdasaryan wrote:
> > When a reader takes read lock, it increments the atomic, unless the
> > top two bits are set indicating a writer is present.
> > When writer takes write lock, it sets VMA_LOCK_WR_LOCKED bit if there
> > are no readers or VMA_LOCK_WR_WAIT bit if readers are holding the lock
> > and puts itself onto newly introduced mm.vma_writer_wait. Since all
> > writers take mmap_lock in write mode first, there can be only one writer
> > at a time. The last reader to release the lock will signal the writer
> > to wake up.
>
> I don't think you need two bits.  You can do it this way:
>
> 0x8000'0000 - No readers, no writers
> 0x1-7fff'ffff - Some number of readers
> 0x0 - Writer held
> 0x8000'0001-0xffff'ffff - Reader held, writer waiting
>
> A prospective writer subtracts 0x8000'0000.  If the result is 0, it got
> the lock, otherwise it sleeps until it is 0.
>
> A writer unlocks by adding 0x8000'0000 (not by setting the value to
> 0x8000'0000).
>
> A reader unlocks by adding 1.  If the result is 0, it wakes the writer.
>
> A prospective reader subtracts 1.  If the result is positive, it got the
> lock, otherwise it does the unlock above (this might be the one which
> wakes the writer).
>
> And ... that's it.  See how we use the CPU arithmetic flags to tell us
> everything we need to know without doing arithmetic separately?

Yes, this is neat! You are using the fact that write-locked == no
readers to eliminate unnecessary state. I'll give that a try. Thanks!

>