linux-kernel - Re: math_state_restore and kernel_fpu

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Message-ID: <20140128185315.9014.qmail@science.horizon.com>
Date:	28 Jan 2014 13:53:15 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, nate@...tsmathematics.com
Cc:	adilger@...ger.ca, arjan@...ux.intel.com, jack@...e.cz,
	linux-kernel@...r.kernel.org, maarten-baert@...mail.com,
	mingo@...e.hu, suresh.b.siddha@...el.com, tglx@...utronix.de,
	viro@...iv.linux.org.uk
Subject: Re: math_state_restore and kernel_fpu_end disable interrupts?

> I'm trying it now.  But it takes a while for me to reproduce, and even
> longer to be sure the problem has gone away.  So anything you hear from
> me within a week will be bad news.

Well, it's been a week, and: good news!

I'd still wish for some review by someone who really understands this
code; in particular it seems dangerous to just enable interrupts for
a window without re-checking the condition afterward.

What if an interrupt hander wants to use the FPU and triggers the
allocate itself?  Shouldn't it be:
* Enable interrupts
* Allocate
* Disable interrupts
* Check that tsk->thread.xstate is still NULL
* (If it has been filled in, free and return.)
* Fill in tsk->thread.xstate

I don't feel I really understand the irq_fpu_usable() logic in
arch/x86/i387.c.

But this patch clearly doesn't make these issues any *worse*, so
these concerns are no reason to block it.

Would you like add an appropriate commit message and send in the patch?

Something like:

Subject: arch/x86/kernel/traps.c: make math_state_restore preserve IRQ status.

Commit aa283f4927 (in 2.6.26!) to add lazy FPU save are allocation did
an local_irq_enable()/local_irq_disable() around the allocate.  However,
that assumes that it is only called with interrupts disabled.

math_state_restore() can also be called from kernel_fpu_end() with
interrupts enabled.  Very occasionally, this triggers an FPU state
allocation.  Disabling interrupts unconditionally is Bad.

Not-yet-Signed-off-by: Nate Eldredge <nate@...tsmathematics.com>
Tested-by: George Spelvin <linux@...izon.com>
Cc: <stable@...r.kernel.org>
Fixes: aa283f4927
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