linux-kernel - Re[2]: [PATCH 10/10] ARM: FIQ: Get rid of init

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Message-ID: <1353656161.911816605@f287.mail.ru>
Date:	Fri, 23 Nov 2012 11:36:01 +0400
From:	Alexander Shiyan <shc_work@...l.ru>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linaro-kernel@...ts.linaro.org,
	Russell King <linux@....linux.org.uk>,
	patches@...aro.org, linux-kernel@...r.kernel.org,
	John Stultz <john.stultz@...aro.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	kernel-team@...roid.com, linux-arm-kernel@...ts.infradead.org
Subject: Re[2]: [PATCH 10/10] ARM: FIQ: Get rid of init_FIQ()

> On Fri, Nov 23, 2012 at 10:27:51AM +0400, Alexander Shiyan wrote:
> > > On Fri, Nov 23, 2012 at 07:40:30AM +0400, Alexander Shiyan wrote:
> > > [...]
> > > > >  static unsigned long no_fiq_insn;
> > > > > +static int got_no_fiq_insn;
> > > > > @@ -78,11 +79,14 @@ void show_fiq_list(struct seq_file *p, int prec)
> > > > >  
> > > > >  void set_fiq_handler(void *start, unsigned int length)
> > > > >  {
> > > > > -#if defined(CONFIG_CPU_USE_DOMAINS)
> > > > > -	memcpy((void *)0xffff001c, start, length);
> > > > > -#else
> > > > > -	memcpy(vectors_page + 0x1c, start, length);
> > > > > +	unsigned long *addr = (void *)0xffff001c;
> > > > > +
> > > > > +#ifndef CONFIG_CPU_USE_DOMAINS
> > > > > +	addr = vectors_page + 0x1c;
> > > > >  #endif
> > > > > +	if (!cmpxchg(&got_no_fiq_insn, 0, 1))
> > > > > +		no_fiq_insn = *addr;
> > > > > +	memcpy(addr, start, length);
> > > > >  	flush_icache_range(0xffff001c, 0xffff001c + length);
> > > > >  	if (!vectors_high())
> > > > >  		flush_icache_range(0x1c, 0x1c + length);
> > > > > @@ -126,8 +130,3 @@ EXPORT_SYMBOL(__set_fiq_regs);	/* defined in fiqasm.S */
> > > > > -
> > > > > -void __init init_FIQ(void)
> > > > > -{
> > > > > -	no_fiq_insn = *(unsigned long *)0xffff001c;
> > > > 
> > > > it seems that this is wrong. In this case we have an uninitialized variable and
> > > > sequential call claim_fiq and release_fiq could be fatal. FIXME please.
> > > 
> > > Um... I don't think I understand, can you please elaborate?
> > 
> > OK, I'll try to explain it.
> > At the end of the release_fiq function we have a call fiq_op. For the default
> > handler - is a fiq_def_op function, and we call this function with the option
> > "relinquish = 0", i.e. we want to restore old fiq_handler. But if we do not call
> > set_fiq_handler never before, we will have an uninitialized no_fiq_insn variable.
> 
> It should not matter when or in what order anyone calls the
> set_fiq_handler(), since it stores "no FIQ instruction" into no_fiq_insn
> at its first invocation:
> 
> 	if (!cmpxchg(&got_no_fiq_insn, 0, 1))
> 		no_fiq_insn = *addr;
> 
> If we never called set_fiq_handler() before, during release_fiq() we'll:
> 
> 1. Copy the initial instruction from the vector page to 'no_fiq_insn';
> 2. Copy the initial instruction from 'no_fiq_insn' to the vector page;
> 
> So no_fiq_insn gets initialized, then we just instantly write the same
> value back.

got_no_fiq_insn also not initialized.
I think as a whole on this issue requires additional comments from Russell
as the author of implementation.

PS: In any case, I can reproduce it and check it out if you send me a patchset
to my email as an attachment.


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