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

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Message-ID: <54143210.1070704@linaro.org>
Date:	Sat, 13 Sep 2014 13:01:20 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 3.17-rc4 v5 2/6] arm: fiq: Replace default FIQ handler

On 12/09/14 18:03, 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?

That was writen from your review comment rather than taken from your patch.

> 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.

Ok. You're right that I followed the "can't be holding any locks" logic
when I didn't update usr_entry in reaction to the original review comment.

I'm also happy with the "avoid lockdep in FIQ code altogether" approach.
I'll do this.

> 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.

Sorry. I will fix these.


Daniel.


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