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SubjectRe: math_state_restore and kernel_fpu_end disable interrupts?
[Here's my original message, since George's reply didn't quote or 
reference it: https://lkml.org/lkml/2014/1/18/3. Summary:
math_state_restore() always leaves interrupts disabled, and I think this
is a bug.]

On Sun, 19 Jan 2014, George Spelvin wrote:

> THANK YOU!
>
> I've been having a problem with ext4 metadata checksums, which use SSE
> for large blocks, and traced it to kernel_fpu_end() disabling interrupts,
> but had paused to debug this (I assumed well-tested) piece of kernel
> code before pushing it harder.

Interesting. I guess it's not surprising this has other effects.

Here's the commit that added the code in question (about 6 years ago):
https://github.com/torvalds/linux/commit/aa283f49276e7d840a40fb01eee6de97eaa7e012

It's credited to Suresh Siddha, whom I've cc'ed (along with others who
signed off). Suresh, if you're still around, could you comment on why
math_state_restore always leaves interrupts disabled, regardless of their
state on entry? Is there a deep reason or is it a bug?

Assuming it's a bug, here's the obvious patch:

--- linux-source-3.11.0/arch/x86/kernel/traps.c 2013-09-02 14:46:10.000000000 -0600
+++ linux-source-3.11.0-nate/arch/x86/kernel/traps.c 2014-01-19 11:25:32.977221476 -0700
@@ -624,6 +624,9 @@
struct task_struct *tsk = current;

if (!tsk_used_math(tsk)) {
+ unsigned long flags;
+
+ local_save_flags(flags);
local_irq_enable();
/*
* does a slab alloc which can sleep
@@ -635,7 +638,7 @@
do_group_exit(SIGKILL);
return;
}
- local_irq_disable();
+ local_irq_restore(flags);
}

__thread_fpu_begin(tsk);
I tested it briefly: the kernel still boots fine, and it fixes the problem
I was seeing (BUG() when core dumping on ecryptfs). George, does it help
your problem?

Thanks everyone!

> (Search October-December LKML archives for "3.11.4: kernel BUG at
> fs/buffer.c:1268".")

--
Nate Eldredge
nate@thatsmathematics.com



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