linux-kernel - Re: [PATCH 3.17-rc4 v5 2/6] arm: fiq: Replace default FIQ handler

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Message-ID: <20140912170720.GM12361@n2100.arm.linux.org.uk>
Date:	Fri, 12 Sep 2014 18:07:20 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3.17-rc4 v5 2/6] arm: fiq: Replace default FIQ handler

On Fri, Sep 12, 2014 at 06:03:07PM +0100, Russell King - ARM Linux wrote:
> On Thu, Sep 11, 2014 at 12:31:14PM +0100, Daniel Thompson wrote:
> > -	.macro	svc_entry, stack_hole=0
> > +	.macro	svc_entry, stack_hole=0, call_trace=1
> >   UNWIND(.fnstart		)
> >   UNWIND(.save {r0 - pc}		)
> >  	sub	sp, sp, #(S_FRAME_SIZE + \stack_hole - 4)
> > @@ -183,7 +183,9 @@ ENDPROC(__und_invalid)
> >  	stmia	r7, {r2 - r6}
> >  
> >  #ifdef CONFIG_TRACE_IRQFLAGS
> > +	.if \call_trace
> >  	bl	trace_hardirqs_off
> > +	.endif
> >  #endif
> 
> Good, you picked this up from my patch.  But what about the call into
> lockdep from usr_entry?
> 
> Yes, it should be safe if we're entering from user mode, because by
> definition, the kernel can't be holding any locks at that point.
> However, I'd much prefer to keep to a set of simple rules here: avoid
> lockdep in FIQ code altogether.
> 
> That's much easier to understand than "we can call into lockdep provided
> we've been entered from user mode".
> 
> The other thing you miss is that /potentially/ call into the scheduler
> as well from a FIQ.  Do we /really/ want to do that kind of work here?
> 
> Not happy.

And you're also missing a .cantunwind for __fiq_usr, which means that the
Dwarf doesn't contain an explicit point to stop unwinding.

Lastly, don't thread your new patches to the old ones.  I utterly hate
that behaviour.  It makes subject lines totally pointless because all
I end up seeing is "[PAT" on the very right hand of the screen.  Far
from useful.

-- 
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according to speedtest.net.
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