linux-kernel - Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

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Date:	Sat, 1 Feb 2014 17:06:53 -0800
From:	Suresh Siddha <sbsiddha@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nate Eldredge <nate@...tsmathematics.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	stable <stable@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Maarten Baert <maarten-baert@...mail.com>,
	Jan Kara <jack@...e.cz>, George Spelvin <linux@...izon.com>,
	Pekka Riikonen <priikone@....fi>
Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag

On Sat, Feb 1, 2014 at 11:27 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> That said, regardless of the allocation issue, I do think that it's
> stupid for kernel_fpu_{begin,end} to save the math state if
> "used_math" was not set.  So I do think__kernel_fpu_end() as-s is
> buggy and stupid.

For eager_fpu case, assumption was every task should always have
'used_math' set. But i think there is a race,  where we drop the fpu
explicitly by doing drop_fpu() and meanwhile if we get an interrupt
etc that ends up using fpu?

so I will Ack for option "b", as option "a" breaks the features which
don't take into account cr0.TS.

Meanwhile I have the patch removing the delayed dynamic allocation for
non-eager fpu. will post it after some testing.

thanks,
suresh
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