linux-kernel - Re: [PATCH] kernel/softirq.c: delete 'while' looping to improve a little performance and beautify code

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Message-ID: <20130610122543.GG5146@linux.vnet.ibm.com>
Date:	Mon, 10 Jun 2013 05:25:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/softirq.c: delete 'while' looping to improve a
 little performance and beautify code

On Sun, Jun 09, 2013 at 08:30:19PM +0800, Chen Gang wrote:
> 
> After finish the internal 'while', need not test TASKLET_STATE_SCHED
> again, so looping back to outside 'while' is only for set_bit().
> 
> When use 'if' and set_bit() instead of 'while', it will save at least
> one running conditional instruction, and also will be clearer for readers
> (although the binary size will be a little bigger).
> 
> The related patch is "1da177e Linux-2.6.12-rc2"
> 
> 
> Signed-off-by: Chen Gang <gang.chen@...anux.com>
> ---
>  kernel/softirq.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index a5f8836..52da25f 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -540,10 +540,11 @@ void tasklet_kill(struct tasklet_struct *t)
>  	if (in_interrupt())
>  		printk("Attempt to kill tasklet from interrupt\n");
> 
> -	while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
> +	if (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
>  		do {
>  			yield();
>  		} while (test_bit(TASKLET_STATE_SCHED, &t->state));
> +		set_bit(TASKLET_STATE_SCHED, &t->state);

This replaces an atomic test-and-set with two operations, a test and
a set.  Is this safe?

							Thanx, Paul

>  	}
>  	tasklet_unlock_wait(t);
>  	clear_bit(TASKLET_STATE_SCHED, &t->state);
> -- 
> 1.7.7.6
> 

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