linux-kernel - Re: [PATCH 01/17] locking/lockdep: Add lock type enum to explicitly specify read or write locks

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Date:   Tue, 14 May 2019 09:31:48 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will.deacon@....com, Ingo Molnar <mingo@...nel.org>,
        Bart Van Assche <bvanassche@....org>, ming.lei@...hat.com,
        Frederic Weisbecker <frederic@...nel.org>, tglx@...utronix.de,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/17] locking/lockdep: Add lock type enum to explicitly
 specify read or write locks

Thanks for review.

On Mon, 13 May 2019 at 19:45, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, May 13, 2019 at 05:11:47PM +0800, Yuyang Du wrote:
> > + * Note that we have an assumption that a lock class cannot ever be both
> > + * read and recursive-read.
>
> We have such locks in the kernel... see:
>
>   kernel/qrwlock.c:queued_read_lock_slowpath()
>
> And yes, that is somewhat unfortunate, but hard to get rid of due to
> hysterical raisins.

That is ok, then LOCK_TYPE_RECURSIVE has to be 3 such that
LOCK_TYPE_RECURSIVE & LOCK_TYPE_READ != 0. I thought to do this in the
first place without assuming. Anyway, it is better to know.

And I guess in a task:

(1) read(X);
    recursive_read(x);      /* this is ok ? */

(2) recursive_read(x);
    read(x)      /* not ok ? */

Either way, very small change may need to be made.