linux-kernel - Re: write_lock_irq(&tasklist

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Date:   Wed, 23 May 2018 16:36:07 +0100
From:   Will Deacon <will.deacon@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     psodagud@...eaurora.org, Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>, sherryy@...roid.com,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Christoph Lameter <cl@...ux.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sasha Levin <alexander.levin@...izon.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        boqun.feng@...il.com
Subject: Re: write_lock_irq(&tasklist_lock)

[+Boqun]

On Wed, May 23, 2018 at 08:25:06AM -0700, Linus Torvalds wrote:
> On Wed, May 23, 2018 at 6:05 AM Will Deacon <will.deacon@....com> wrote:
> 
> > Please use a newer kernel. We've addressed this in mainline by moving
> > arm64 over to the qrwlock implementation which (after some other changes)
> > guarantees forward progress for well-behaved readers and writers.
> 
> Oh, I didn't even realize that this wasn't x86, and that there was still
> the very unfair rwlock issue on 4.14 on arm.
> 
> Yeah, the queuing rwlocks shouldn't show the really pathological problems
> we used to have long ago.

Yup, although they do reveal new issues that Boqun has been running into
recently with his lockdep improvements. The general pattern is if you
have:

P0:			P1:			P2:

spin_lock(&slock)	read_lock(&rwlock)	write_lock(&rwlock)
read_lock(&rwlock)	spin_lock(&slock)

then the CPUs can be queued on the rwlock such that P1 has the lock, then
P2 is queued and then P0. If P0 has taken the spinlock, we have a deadlock
which wouldn't arise with the non-queued version.

In other words, qrwlock requires consistent locking order wrt spinlocks.

Will