linux-kernel - Re: [PATCH v2 11/17] locking/lockdep: Adjust lockdep selftest cases

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Message-ID: <CAHttsrZ962Gw_6OA6J6GEhAx06yV70B5PEzqGEaYGDNSy57-3A@mail.gmail.com>
Date:   Thu, 30 May 2019 15:37:50 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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,
        paulmck@...ux.ibm.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 11/17] locking/lockdep: Adjust lockdep selftest cases

Thanks for review.

On Wed, 29 May 2019 at 19:44, Boqun Feng <boqun.feng@...il.com> wrote:
>
> > @@ -424,7 +424,7 @@ static void rwsem_ABBA2(void)
> >       ML(Y1);
> >       RSL(X1);
> >       RSU(X1);
> > -     MU(Y1); // should fail
> > +     MU(Y1); // should NOT fail
>
> I'm afraid you get this wrong ;-) reader of rwsem is non-recursive if I
> understand correctly, so case like:
>
>         Task 0                  Task 1
>
>         down_read(A);
>                                 mutex_lock(B);
>
>                                 down_read(A);
>         mutex_lock(B);
>
> can be a deadlock, if we consider a third independent task:
>
>         Task 0                  Task 1                  Task 2
>
>         down_read(A);
>                                 mutex_lock(B);
>                                                         down_write(A);
>                                 down_read(A);
>         mutex_lock(B);
>
> in this case, Task 1 can not get it's lock for A, therefore, deadlock.

Well, yes. This situation is damn counterintuitive and looks
suboptimal, but I guess I can understand why this is done so. It is a
shame read locks are not 100% concurrent. I wish I were bright enough
to have figured this out on my own.

Ok, now this perhaps can be easily remedied. it is merely a matter
that finally I can set straight the lock exclusiveness table, and then
from there the only change seems to be now only recursive-read locks
are no deadlock.

Thanks,
Yuyang