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Date: Tue, 31 Jan 2006 16:55:25 -0800
From: Andrew Morton <akpm@osdl.org>
To: Francois Romieu <romieu@fr.zoreil.com>
Cc: jgarzik@pobox.com, herbert@gondor.apana.org.au, mingo@elte.hu,
       linux-kernel@vger.kernel.org, davem@redhat.com
Subject: Re: [PATCH] 8139too: fix a TX timeout watchdog thread against NAPI
 softirq race
Message-Id: <20060131165525.3e741285.akpm@osdl.org>
In-Reply-To: <20060131002418.GA917@electric-eye.fr.zoreil.com>
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	<20060131002418.GA917@electric-eye.fr.zoreil.com>
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Francois Romieu <romieu@fr.zoreil.com> wrote:
>
> +	unsigned int watchdog_fired : 1;
>  	unsigned int default_port : 4;	/* Last dev->if_port value. */
>  	unsigned int have_thread : 1;

Bear in mind that the compiler will put these three fields into the same
word and will use non-atomic RMWs when they are modified.

In this particualr case it's hard to see how an SMP or IRQ race can occur,
but it's a trap.   Which won't show up on x86.


struct x {
	unsigned int a:1;
	unsigned int b:1;
};

void foo(struct x *a)
{
	a->a = 1;
}

void bar(struct x *a)
{
	a->b = 1;
}


On ppc:


foo:
        lwz 0,0(3)
        oris 0,0,0x8000
        stw 0,0(3)
        blr
        .size   foo, .-foo
        .align 2
        .globl bar
        .type   bar, @function
bar:
        lwz 0,0(3)
        oris 0,0,0x4000
        stw 0,0(3)
        blr