linux-kernel - Re: [PATCH 1/2] coredump: flush the fpu exit state for proper multi-threaded core dump

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Message-ID: <1336421341.19423.4.camel@sbsiddha-desk.sc.intel.com>
Date:	Mon, 07 May 2012 13:09:01 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	hpa@...or.com, mingo@...e.hu, oleg@...hat.com,
	suresh@...stanetworks.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] coredump: flush the fpu exit state for proper
 multi-threaded core dump

On Mon, 2012-05-07 at 12:15 -0700, Linus Torvalds wrote:
> On Mon, May 7, 2012 at 12:07 PM, Suresh Siddha
> <suresh.b.siddha@...el.com> wrote:
> >
> > Fix this by explicitly flushing the fpu state in do_exit() by calling
> > prepare_to_copy()
> 
> Ugh, I hate this one.
> 
> We're making the exit path more expensive for almost no gain. The FPU
> state is dead in 99.9% of all cases.
> 
> Why isn't this a core-dump-only case?
> 

Today we do this unlazy_fpu() (which is what prepare_to_copy does)
already as part of the first schedule() in the exit path. I am just
making it explicit by calling prepare_to_copy() before exit_mm() which
synchronizes all the threads before the first thread goes with the core
dump.

I don't think I am adding any more cost to the existing path, unless I
am missing something.

thanks,
suresh

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