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From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: psodagud@codeaurora.org, Kees Cook <keescook@chromium.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Will Drewry <wad@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rik van Riel <riel@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Eric Biggers <ebiggers@google.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	sherryy@android.com, Vegard Nossum <vegard.nossum@oracle.com>,
	Christoph Lameter <cl@linux.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Sasha Levin <alexander.levin@verizon.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	boqun.feng@gmail.com
Subject: Re: write_lock_irq(&tasklist_lock)
Date: Wed, 23 May 2018 16:36:07 +0100	[thread overview]
Message-ID: <20180523153607.GD2983@arm.com> (raw)
In-Reply-To: <CA+55aFwKLBKdf3gdWkJSdewQYYXYYfN29SPT+etPqpopxA0CMQ@mail.gmail.com>

[+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@arm.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

  reply	other threads:[~2018-05-23 15:35 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-22 19:40 write_lock_irq(&tasklist_lock) Sodagudi Prasad
2018-05-22 20:27 ` write_lock_irq(&tasklist_lock) Linus Torvalds
2018-05-22 21:17   ` write_lock_irq(&tasklist_lock) Peter Zijlstra
2018-05-22 21:31     ` write_lock_irq(&tasklist_lock) Linus Torvalds
2018-05-23  8:19       ` write_lock_irq(&tasklist_lock) Peter Zijlstra
2018-05-23 13:05 ` write_lock_irq(&tasklist_lock) Will Deacon
2018-05-23 15:25   ` write_lock_irq(&tasklist_lock) Linus Torvalds
2018-05-23 15:36     ` Will Deacon [this message]
2018-05-23 16:26       ` write_lock_irq(&tasklist_lock) Linus Torvalds
2018-05-24 12:49         ` write_lock_irq(&tasklist_lock) Will Deacon
2018-05-24 13:51           ` write_lock_irq(&tasklist_lock) Boqun Feng
2018-05-24 17:37             ` write_lock_irq(&tasklist_lock) Sodagudi Prasad
2018-05-24 18:28               ` write_lock_irq(&tasklist_lock) Peter Zijlstra
2018-05-24 21:14             ` write_lock_irq(&tasklist_lock) Andrea Parri

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