linux-kernel - Re: [PATCH] mutex: make mutex_lock

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Message-ID: <20151022174400.GW32532@n2100.arm.linux.org.uk>
Date:	Thu, 22 Oct 2015 18:44:00 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Liam Girdwood <lgirdwood@...il.com>,
	linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...nel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] mutex: make mutex_lock_nested an inline function

On Thu, Oct 22, 2015 at 05:09:59PM +0200, Peter Zijlstra wrote:
> Hmm, I was sure I send a reply, but I cannot even find it in my own sent
> folder so who knows.
> 
> My current preference is to keep the thing a macro and work around it in
> the usage site because while these warns are annoying, they're at least
> visible. Whereas, with an inline, code bloat is entirely silent. Even if
> the sites you found are harmless, there's no saying what the future will
> bring etc..

I agree - we've got way too many inline functions already.  My biggest
annoyance in that respect is the asm-generic dma_map_single()
implementation that we're now forced to use on ARM, which results in
quite a large chunk of code at every callsite.

The problem there is that when you have drivers which do something like:

	dma = dma_map_single(dev, page_address(page), size, dir);

you end up with code which converts the struct page to a virtual address,
and then you end up with code in the dma_map_single() inline function
which then converts it back to a struct page + offset - none of which,
with modern ARM kernels, the compiler has a hope in hell of optimising.

So we end up with all that junk at every single dma_map_single() callsite.
If dma_map_single() were a library function, it would be a lot smaller
since we'd only have one copy of the complex virt->struct page conversion.

-- 
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