linux-kernel - Re: [PATCH] kernel/timer.c: using spin_lock_irqsave instead of spin_lock + local_irq_save, especially when CONFIG

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Date:	Wed, 19 Jun 2013 12:53:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chen Gang <gang.chen@...anux.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/timer.c: using spin_lock_irqsave instead of
 spin_lock + local_irq_save, especially when CONFIG_LOCKDEP not defined

On Wed, 19 Jun 2013, Chen Gang wrote:

> On 06/19/2013 05:59 PM, Thomas Gleixner wrote:
> > I'm well aware how that works. And there is no difference whether you
> > do:
> > 
> > 	local_irq_save(flags);
> > 	spin_lock(&lock);
> > or
> > 	spin_lock_irqsave(&lock, flags);
> 
> if CONFIG_LOCKDEP is not defined, they are not semantically the same.

Care to explain _your_ spinlock semantics to me?

The factual ones are:

    spin_lock_irqsave() returns with the lock held, interrupts and
    preemption disabled. 

    spin_lock() returns with the lock held, preemption disabled. It
    does not affect interrupt disabled/enabled state

So
	local_irq_save(flags);
	spin_lock(&lock);

is semantically the same as 

	spin_lock_irqsave(&lock, flags);

And this is completely independent of LOCKDEP.

Thanks,

	tglx

    
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