linux-kernel - Re: [PATCH] Fix print out of function which called WARN

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Date:	Sun, 17 May 2009 15:18:19 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hugh@...itas.com>
cc:	Andi Kleen <ak@...ux.intel.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Fix print out of function which called WARN_ON()

On Sun, 17 May 2009, Hugh Dickins wrote:

> On Sat, 16 May 2009, Linus Torvalds wrote:
> > 
> > This patch not only avoids the warnings and gets the right caller 
> > information, it cleans up the code too:
> > 
> >  - it uses '%pS' instead of of sprint_symbol
> 
> > -	char function[KSYM_SYMBOL_LEN];
> 
> This should be a big improvement, because that buffer on the stack
> was netting lots of stale return addresses, printed out with ?s in
> the warning's dump_stack().  I had been wanting to add a memset,
> but your %pS should circumvent the need for that nicely.

Are you sure it was that function[] array?

The thing is, on at least x86-64, any function using va_start() will 
allocate something like 64 bytes of stack space for the reg-save area. I'm 
not quite sure _why_ it does that, but it's very irritating, and it showed 
up quite clearly in some of the stackspace usage things.

I even sent the gcc people a patch to fix the worst of it (gcc used to 
allocate about twice as much space because it also had a XMM save area 
even if you compiled without XMM support or something like that), but my 
point is, I'm afraid there is still a noticeable gap on the stack due to 
this, at least for the _fmt() case.

But yes, for the _null() case we now have neither that function buffer 
_nor_ the stupid va_list save area, so that case should be much better.

Of course, we could avoid that entirely if we were to just pass in the 
right stack pointer to dump_stack(), and the "warn_slowpath_xyz()" 
functions could just pass in the stack of their caller. Sadly, we 
currently have no way to do that :(

We could change the dump_stack() calling convention to give a stack 
pointer or NULL, and then use __builtin_frame_address(0) in the caller.. 
But we have a _lot_ of "dump_stack()" users.

		Linus
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