From: Jeremy Fitzhardinge <jeremy@goop.org>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: William Lee Irwin III <wli@holomorphy.com>,
"Van Maren, Kevin" <kevin.vanmaren@unisys.com>,
linux-ia64@linuxia64.org,
Linux Kernel List <linux-kernel@vger.kernel.org>,
rusty@rustcorp.com.au, dhowells@redhat.com, mingo@elte.hu
Subject: Re: [Linux-ia64] reader-writer livelock problem
Date: 08 Nov 2002 09:38:25 -0800 [thread overview]
Message-ID: <1036777105.13021.13.camel@ixodes.goop.org> (raw)
In-Reply-To: <Pine.LNX.4.44.0211080918220.4298-100000@home.transmeta.com>
On Fri, 2002-11-08 at 09:25, Linus Torvalds wrote:
> There's another reason for not doing it that way: allowing readers to keep
> interrupts on even in the presense of interrupt uses of readers.
>
> If you do the "pending writes stop readers" approach, you get
>
> cpu1 cpu2
>
> read_lock() - get
>
> write_lock_irq() - pending
>
> irq happens
> - read_lock() - deadlock
>
> and that means that you need to make readers protect against interrupts
> even if the interrupts only read themselves.
Even without interrupts that would be a bug. It isn't ever safe to
attempt to retake a read lock if you already hold it, because you may
deadlock with a pending writer. Fair multi-reader locks aren't
recursive locks.
> NOTE! I'm not saying the existing practice is necessarily a good tradeoff,
> and maybe we should just make sure to find all such cases and turn the
> read_lock() calls into read_lock_irqsave() and then make the rw-locks
> block readers on pending writers. But it's certainly more work and cause
> for subtler problems than just naively changing the rw implementation.
Yes, I'd agree. It would definitely be a behavioural change with
respect to the legality of retaking a lock for reading, which would
probably be quite irritating to find (since they'd only cause a problem
if they actually coincide with an attempted write lock).
> Actually, giving this som emore thought, I really suspect that the
> simplest solution is to alloc a separate "fair_read_lock()", and paths
> that need to care about fairness (and know they don't have the irq
> issue)
> can use that, slowly porting users over one by one...
Do you mean have a separate lock type, or have two different read_lock
operations on the current type?
J
next prev parent reply other threads:[~2002-11-08 17:32 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <3FAD1088D4556046AEC48D80B47B478C0101F4E7@usslc-exch-4.slc.unisys.com>
2002-11-08 3:51 ` [Linux-ia64] reader-writer livelock problem William Lee Irwin III
2002-11-08 17:13 ` Jeremy Fitzhardinge
2002-11-08 17:25 ` Linus Torvalds
2002-11-08 17:28 ` Linus Torvalds
2002-11-08 17:38 ` Jeremy Fitzhardinge [this message]
2002-11-08 17:43 ` David Howells
2002-11-08 17:57 ` Linus Torvalds
2002-11-09 2:48 ` Rusty Russell
2002-11-09 4:36 ` William Lee Irwin III
[not found] ` <3DCFDAE9.6D359448@email.mot.com>
2002-11-11 19:22 ` David Mosberger
2002-11-12 1:39 ` your mail Rik van Riel
2002-11-08 17:34 ` [Linux-ia64] reader-writer livelock problem David Howells
2002-11-08 17:54 ` David Howells
2002-11-08 17:55 ` Stephen Hemminger
2002-11-08 17:41 Van Maren, Kevin
2002-11-08 17:52 ` Matthew Wilcox
2002-11-08 18:05 Van Maren, Kevin
2002-11-08 19:19 ` Matthew Wilcox
2002-11-08 19:26 ` David Mosberger
2002-11-08 20:17 Van Maren, Kevin
2002-11-08 20:39 ` Matthew Wilcox
2002-11-08 20:24 Van Maren, Kevin
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