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

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Date:   Tue, 8 Dec 2020 16:23:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Waiman Long' <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "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

From: Waiman Long
> Sent: 08 December 2020 15:34
> 
> On 12/8/20 4:12 AM, 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.
> >
> > The code used to use down_timeout() (it was written for 2.4).
> > When I changed to mutex_(to get optimistic spinning) I lost
> > the ability to do the timeouts.
> 
> The primary reason for sending out that patchset was to work around some
> circular locking problem in existing code even though these circular
> locking scenarios are not likely to happen. Your case is certainly
> another potential circular locking problem as well.

If you've got lock-ordering problems they need fixing.
Neither signals nor timeouts are real solutions.
Either may help diagnose the problem, but they aren't fixes.

OTOH if it reasonable to have a request interrupted by a signal
it must also be reasonable to implement a timeout.
Of course, one might wonder whether 'correct' code should ever
be waiting on a mutex for any length of time.
So is there even a justification for interruptible waits for mutex.

FWIW I could implement my timeouts using SIGALARM - but it is a lot
of work :-)

	David

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