linux-kernel - Re: question about save_xstate_sig()

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Message-ID: <54C6CD64.10208@redhat.com>
Date:	Mon, 26 Jan 2015 18:27:32 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>, "H. Peter Anvin" <hpa@...or.com>
CC:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

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On 01/24/2015 03:20 PM, Oleg Nesterov wrote:
> Let me abuse this thread to ask more questions.
> 
> Peter, could you help?
> 
> On 01/23, Rik van Riel wrote:
>> 
>> Not only is this broken with my new code, but it looks like it
>> may be broken with the current code, too...
> 
> As I already mentioned, at least math_error()->save_init_fpu()
> looks buggy. And unlazy_fpu() doesn't look right too.
> 
> Note that save_init_fpu() is calles after conditional_sti(), so
> unless I missed something the task can be preempted and we can
> actually hit WARN_ON_ONCE(!__thread_has_fpu()) if !use_eager_fpu()
> && .fpu_counter == 0.
> 
> Worse, the unconditional __save_init_fpu() is obviously wrong in
> this case.
> 
> I already have a patch which (like the patch from Rik) turns it
> into
> 
> static inline void save_init_fpu(struct task_struct *tsk) { 
> preempt_disable(); if (__thread_has_fpu(tsk)) { if
> (use_eager_fpu()) { __save_fpu(tsk); } else { 
> __save_init_fpu(tsk); __thread_fpu_end(tsk); } } preempt_enable(); 
> }

> Now the questions:
> 
> - This doesn't hurt, but does it really need __thread_fpu_end?
> 
> Perhaps this is because we do not check the error code returned by
> __save_init_fpu? although I am not sure I understand the comment 
> above fpu_save_init correctly...

Looking at the code some more, I do not see any call site of
save_init_fpu() that actually needs or wants __thread_fpu_end(),
with or without eager fpu mode.

It looks like we can get rid of that.

> - What about do_bounds() ? Should not it use save_init_fpu()
> rather than fpu_save_init() ?

I suppose do_bounds() probably should save the fpu context while
not preemptible, but that may also mean moving conditional_sti()
until after save_init_fpu() or __save_init_fpu() has been called.

> - Why unlazy_fpu() always does __save_init_fpu() even if
> use_eager_fpu?
> 
> and note that in this case __thread_fpu_end() is wrong if
> use_eager_fpu, but fortunately the only possible caller of
> unlazy_fpu() is coredump. fpu_copy() checks use_eager_fpu().
> 
> - Is unlazy_fpu()->__save_init_fpu() safe wrt __kernel_fpu_begin()
> from irq?
> 
> I mean, is it safe if __save_init_fpu() path is interrupted by
> another __save_init_fpu() + restore_fpu_checking() from
> __kernel_fpu_begin/end?

I got lost in the core dump code trying to figure out whether this is
safe or broken. I'll need some more time to look through that code...

- -- 
All rights reversed
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