linux-kernel - Re: [PATCH] docs: lockdep-design: correct the notation for writer

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Date:   Mon, 24 May 2021 18:32:30 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Xiongwei Song <sxwjean@...il.com>
Cc:     Waiman Long <llong@...hat.com>, Xiongwei Song <sxwjean@...com>,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        corbet@....net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] docs: lockdep-design: correct the notation for writer

On Mon, May 24, 2021 at 12:24:00PM +0800, Xiongwei Song wrote:
> On Fri, May 21, 2021 at 11:17 PM Waiman Long <llong@...hat.com> wrote:
> >
> > On 5/21/21 2:29 AM, Xiongwei Song wrote:
> > > From: Xiongwei Song <sxwjean@...il.com>
> > >
> > > The block condition matrix is using 'E' as the writer noation here, so it
> > > would be better to use 'E' as the reminder rather than 'W'.
> > >
> > > Signed-off-by: Xiongwei Song <sxwjean@...il.com>
> > > ---
> > >   Documentation/locking/lockdep-design.rst | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/locking/lockdep-design.rst b/Documentation/locking/lockdep-design.rst
> > > index 9f3cfca..c3b923a 100644
> > > --- a/Documentation/locking/lockdep-design.rst
> > > +++ b/Documentation/locking/lockdep-design.rst
> > > @@ -462,7 +462,7 @@ Block condition matrix, Y means the row blocks the column, and N means otherwise
> > >       | R | Y | Y | N |
> > >       +---+---+---+---+
> > >
> > > -     (W: writers, r: non-recursive readers, R: recursive readers)
> > > +     (E: writers, r: non-recursive readers, R: recursive readers)
> > >
> > >
> > >   acquired recursively. Unlike non-recursive read locks, recursive read locks
> >
> > I would say it should be the other way around. Both W and E refer to the
> > same type of lockers. W emphasizes writer aspect of it and E for
> > exclusive. I think we should change the block condition matrix to use W
> > instead of E.
> 
> The doc uses 'E'  to describe dependency egdes too. Should we change them
> to 'W'? Personally,  both 'W' and 'E' are fine.
> 

I also think Waiman's suggestion is solid, there are two ways to
classify locks:

1.	W (Writers), R (Recursive Readers), r (Non-recursive Readers)

2.	E (Exclusive locks), S (Shared locks), R (Recursive Readers),
	N (Non-recursive locks)

And the relations between them are as follow:

	E = W
	R = R
	N = W \/ r
	S = R \/ r 

, where "\/" is the set union.

The story is that I used the way #1 at first, and later on realized way
#2 is better for BFS implementation, also for reasoning, so here came
this leftover..

If you are interested, go ahead sending a patch fixing this, otherwise,
I will fix this.

Regards,
Boqun

> Thanks,
> Xiongwei
> >
> > Cheers,
> > Longman
> >