linux-kernel - Re: [PATCH] Use test_and_clear_bit() instead atomic_dec_and_test() for stop

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Message-ID: <87mx4zr0l9.fsf@devron.myhome.or.jp>
Date:	Wed, 23 May 2012 13:19:14 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Use test_and_clear_bit() instead atomic_dec_and_test() for stop_machine

OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> writes:

>> The down side to this is that it adds 4 more bytes on a 64bit
>> machine. (sizeof(unsigned log) == 8 and sizeof(atomic_t) == 4)

Another patch without additional 4bytes. This simply change
atomic_dec_and_test() to atomic_xchg().

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>


stop_machine_first is just to see if it is first one or not.  In this
usage, atomic_dec_and_test() makes value less than 0.

I think it is not desirable, because it only triggers
atomic_dec_and_test() less than 0 debug patch. (the patch tests result
of atomic_dec_and_test() is < 0)

So, this uses atomic_xchg() instead.

Signed-off-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
---

 arch/x86/kernel/alternative.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN arch/x86/kernel/alternative.c~stop_machine-use-atomic_xchg arch/x86/kernel/alternative.c
--- linux/arch/x86/kernel/alternative.c~stop_machine-use-atomic_xchg	2012-05-23 13:10:03.000000000 +0900
+++ linux-hirofumi/arch/x86/kernel/alternative.c	2012-05-23 13:10:03.000000000 +0900
@@ -664,7 +664,7 @@ static int __kprobes stop_machine_text_p
 	struct text_poke_param *p;
 	int i;
 
-	if (atomic_dec_and_test(&stop_machine_first)) {
+	if (atomic_xchg(&stop_machine_first, 0)) {
 		for (i = 0; i < tpp->nparams; i++) {
 			p = &tpp->params[i];
 			text_poke(p->addr, p->opcode, p->len);
_
--
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