linux-kernel - Re: raw_spinlock

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Message-ID: <1284914302.23787.2749.camel@gandalf.stny.rr.com>
Date:	Sun, 19 Sep 2010 12:38:22 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: raw_spinlock_t rules

On Sun, 2010-09-19 at 15:09 +0200, Avi Kivity wrote:
> Some time ago, the i8259 emulation code in kvm was changed to use 
> raw_spinlock_t, as it was called in a preempt_disable() and 
> local_irq_disable() context, which doesn't work with preemptible 
> spinlocks used with CONFIG_PREEMPT_RT.
> 
> In Linux 2.6.37, the spinlock will no longer be taken in these contexts, 
> so I'd like to change it to a normal spinlock_t.  However, it is still 
> taken in a spin_lock_irq() context.
> 
> Is it okay to do this change?  I figured since spin_lock_irq() is part 
> of the spinlock infrastructure it might to the right thing.
> 

In PREEMPT_RT all interrupts (besides the timer) is converted to a
thread. The spin_lock_irq() will not disable interrupts in PREEMPT_RT,
because it is not needed (irqs are threaded and wont cause a deadlock if
they share the same spin_locks).

Note, spin_locks used by the timer will must be raw_spin_lock(), and
threaded context must disable interrupts for real before using it.

-- Steve


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