linux-kernel - Re: [PATCH] avr32: Don't acquire siglock when reading sighand action

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Message-ID: <20110402163722.GA2973@redhat.com>
Date:	Sat, 2 Apr 2011 18:37:22 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	linux-kernel@...r.kernel.org,
	Matt Fleming <matt.fleming@...ux.intel.com>
Subject: Re: [PATCH] avr32: Don't acquire siglock when reading sighand
	action

On 03/30, Matt Fleming wrote:
>
> From: Matt Fleming <matt.fleming@...ux.intel.com>
>
> There's no need to acquire the siglock when simply reading the action
> handler. We would need to lock it if we were modifying the handler or
> we were protecting it from modification across function calls, but if
> we're just reading it, there's no need to lock it

Agreed, ->siglock buys nothing.

But,

> --- a/arch/avr32/kernel/traps.c
> +++ b/arch/avr32/kernel/traps.c
> @@ -106,9 +106,7 @@ void _exception(long signr, struct pt_regs *regs, int code,
>  	if (is_global_init(current)) {

is_global_init() is not right in theory, /sbin/init can be multithreaded.
And, this doesn't cover the sub-namespace inits... I'd suggest to check
SIGNAL_UNKILLABLE, but looking closer I think you can simply remove this
code.

The comment says:

	/*
	 * Init gets no signals that it doesn't have a handler for.
	 * That's all very well, but if it has caused a synchronous
	 * exception and we ignore the resulting signal, it will just
	 * generate the same exception over and over again and we get
	 * nowhere.  Better to kill it and let the kernel panic.
	 */

This is correct, but please look at force_sig_info(), it already does
what you need:

	* We don't want to have recursive SIGSEGV's etc, for example,
	* that is why we also clear SIGNAL_UNKILLABLE.

Oleg.

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