linux-kernel - Re: [PATCH 2/3] rwsem: Implement down_read

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Date:   Tue, 8 Dec 2020 13:32:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Waiman Long' <longman@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Christopher Yeoh <cyeoh@....ibm.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Arnd Bergmann <arnd@...db.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 2/3] rwsem: Implement down_read_interruptible

On Tue, Dec 08, 2020 at 09:12:36AM +0000, David Laight wrote:
> From: Waiman Long
> > Sent: 07 December 2020 19:02
> ...
> > > How much more difficult would it be to also add a timeout option?
> > > I looked at adding one to the mutex code - and fell into a big pile
> > > of replicated code.
> > >
> > > ISTM that one the initial locked exchange (and spin) fails a few
> > > extra instructions when heading for the sleep don't really matter
> > >
> > Actually, I had tried that before. See
> > 
> > https://lore.kernel.org/lkml/20190911150537.19527-1-longman@redhat.com/
> > 
> > That is for rwsem, but the same can be done for mutex. However, Peter
> > didn't seem to like the idea of a timeout parameter. Anyway, it is
> > certainly doable if there is a good use case for it.
> 
> 'Unfortunately' my use-case if for an out-of-tree driver.
> 
> The problem I was solving is a status call blocking because
> some other code is 'stuck' (probably an oops) with a mutex held.

Working around oopses is not a sane use-case. If you oops, you get to
keep all the pieces.