linux-kernel - Re: [PATCH] locking/rwlocks: do not starve writers

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Date:   Tue, 21 Jun 2022 11:55:33 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>, Roman Penyaev <rpenyaev@...e.de>
Subject: Re: [PATCH] locking/rwlocks: do not starve writers

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Fri, Jun 17, 2022 at 2:39 PM Eric Dumazet <edumazet@...gle.com> wrote:
>>
>> I am converting RAW sockets to RCU.
>
> RCU is usually absolutely the best approach. Use regular spinlocks for
> writers, and RCU for readers.
>
> I'd love to see the tasklist_lock be converted to RCU too.  But that
> locks predates RCU (and probably 99% of all kernel code), and it's
> messy, so nobody sane has ever willingly tried to do that afaik.

Well sort of.  I converted proc many many years ago.

Like Peter mentioned the big obvious challenge for converting
signal delivery to something else is the atomic delivery aspect.

I am playing with it, and I think I see how to convert signal delivery.
Something like a quick grab of lock that updates struct pid and creates
a list of signals are pending to be delivered.  Plus code that forces
clone to deliver the pending signal before clone creates a new task.

Plus something like a generation counter so I can see when pulling the
signal in clone if the signal has already been delivered.

I think tasks exiting before getting a signal is ok, and does not need
any code.

I have some patches that are almost working that can use siglock to
protect the parent/child/ptrace relation ship for SIGCHLD processing.
Which will remove the pressure on tasklist_lock when I get them sorted.

Not that any of this will kill tasklist_lock but with a little luck we
can get to short deterministic hold times.

Eric