linux-kernel - Re: question about save_xstate_sig()

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Date:	Sat, 24 Jan 2015 21:20:21 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...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?

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();
	}

and I think this fix needs the separate patch/changelog.

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...

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

- 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?

Thanks,

Oleg.

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