linux-kernel - Re: [PATCH RT] seqlock/rt: Prevent livelocks with seqlocks in RT

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Date:	Tue, 06 Mar 2012 07:47:28 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	John Kacur <jkacur@...hat.com>, Carsten Emde <cbe@...dl.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT] seqlock/rt: Prevent livelocks with seqlocks in RT

On Tue, 2012-03-06 at 09:18 +0100, Thomas Gleixner wrote:

> > Because read_seqlocks are used in the VDSO area, a raw_read_seqcount_begin()
> > was created to allow userspace tasks to access read_seqcount().
> > As the grabbing of the write_lock() is not allowed in VDSO, nor
> > is even referencing it.
> 
> This is completely bogus. The VDSO update write side runs with
> interrupts disabled, so it cannot be preempted at all.
> 
> > Note, a live lock can still happen if the userspace task that
> > does the read_seqlock is of higher priority than a user doing
> > the write_lock, so userspace needs to be careful.
> 
> What the hell are you smoking?

Cherries.


I forgot I still had that in my changelog. I wrote this code before I
had your changes. I noticed later that the vdso seqlocks were raw
spinlocks and shouldn't be an issue. As you told me that you had this
fixed, I never bothered to change the log.

Then last night when I hit this bug, I simply cherry picked this fixed
and posted it.

-- Steve


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