From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch 4/5] xtime_lock: Convert to raw_seqlock
Date: Thu, 18 Feb 2010 12:02:14 +0100 (CET)
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To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Linux-Arch <linux-arch@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>, John Stultz <johnstul@us.ibm.com>

On Thu, 18 Feb 2010, Peter Zijlstra wrote:

> On Wed, 2010-02-17 at 18:47 +0000, Thomas Gleixner wrote:
> > 
> > xtime_lock needs a raw_spinlock in preempt-rt. Convert it to
> > raw_seqlock and fix up all users.
> > 
> s/raw_spinlock/raw_seqlock/ ?
> 
> Maybe add an explanation on _why_ -rt needs this for the uninformed
> amongst us.
> 
> -rt switches to sleeping spinlocks, but since the vdso is basically
> userspace it cannot schedule, hence we need to keep using actual
> spinlocks (this is also the reason the vdso things must not call into
> lockdep)

No, the read_seq side is not taking the lock. It's just the write side
which is taking the spinlock to serialize against other writers.

xtime_lock is write locked in the timer interrupt context and therefor
cannot take a sleeping spinlock.

Thanks,

	tglx