linux-kernel - Re: [PATCH 2/5] x86, fpu: don't drop_fpu() in __restore_xstate_sig() if use_eager

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Message-ID: <20140827170233.GA6092@redhat.com>
Date:	Wed, 27 Aug 2014 19:02:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Bean Anderson <bean@...lsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] x86, fpu: don't drop_fpu() in
	__restore_xstate_sig() if use_eager_fpu()

I'm afraid you all already hate me, but let me continue to spam your
mailboxes.

On 08/25, Oleg Nesterov wrote:
>
> I'll try to play with copy_from_user_in_atomic(), if nothing else just
> to complete the discussion and see how the code can look in this case.

OK, to complete the discussion, the code looks simple unless I missed
something,

	if (ia32_fxstate) {
		/*
		 * For 32-bit frames with fxstate, copy the user state to the
		 * thread's fpu state, reconstruct fxstate from the fsave
		 * header. Sanitize the copied state etc.
		 */
		struct xsave_struct *xsave = &tsk->thread.fpu.state->xsave;
		struct user_i387_ia32_struct env;
		bool done;

		if (__copy_from_user(&env, buf, sizeof(env))
			return -1;

		for (done = false; !done; ) {
			if (fatal_signal_pending(current) ||
			    fault_in_pages_readable(buf_fx, state_size))
				return -1;

			preempt_disable();
			pagefault_disable(); /* not really needed */
			done = !__copy_from_user_inatomic(xsave, buf_fx, state_size);
			pagefault_enable();
			if (likely(done)) {
				sanitize_restored_xstate(tsk, &env, xstate_bv, fx_only);
				if (__thread_has_fpu(tsk))
					math_state_restore();
			} else {
				fpu_finit(&tsk->thread.fpu);
			}
			preempt_enable();
		}

		return 0;
	}

and in some sense it is even simpler because we do not care about
use_eager_fpu() or set/clear_used_math().

Does it look better than switch_fpu_xstate() hack?


However, this code can race with kernel_fpu_begin() if use_eager_fpu().
I _think_ that kernel_fpu_begin/end and irq_fpu_usable() need cleanups
too and in any case. Will try to do, but I am not sure.

And to spam you even more, I'll send you a couple of other, more simple
cleanups.

Oleg.

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