linux-kernel - Re: [PATCH 3/4] x86: Rewrite switch

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Date:	Mon, 23 May 2016 07:05:34 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH 3/4] x86: Rewrite switch_to() code

On Mon, May 23, 2016 at 07:49:37AM -0400, Brian Gerst wrote:
> On Mon, May 23, 2016 at 7:40 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Sun, May 22, 2016 at 09:47:22PM -0700, Andy Lutomirski wrote:
> >> On Sun, May 22, 2016 at 7:34 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >> > On Sun, May 22, 2016 at 10:59:38AM -0700, Andy Lutomirski wrote:
> >> >> cc: Josh Poimboeuf: do you care about the exact stack layout of the
> >> >> bottom of the stack of an inactive task?
> >> >
> >> > So there's one minor issue with this patch, relating to unwinding the
> >> > stack of a newly forked task.  For detecting reliable stacks, the
> >> > unwinder needs to unwind all the way to the syscall pt_regs to make sure
> >> > the stack is sane.  But for newly forked tasks, that won't be possible
> >> > here because the unwinding will stop at the fork_frame instead.
> >> >
> >> > So from an unwinder standpoint it might be nice for copy_thread_tls() to
> >> > place a frame pointer on the stack next to the ret_from_fork return
> >> > address, so that it would resemble an actual stack frame.  The frame
> >> > pointer could probably just be hard-coded to zero.  And then the first
> >> > bp in fork_frame would need to be a pointer to it instead of zero.  That
> >> > would make it nicely resemble the stack of any other task.
> >> >
> >> > Alternatively I could teach the unwinder that if the unwinding starts at
> >> > the fork_frame offset from the end of the stack page, and the saved rbp
> >> > is zero, it can assume that it's a newly forked task.  But that seems a
> >> > little more brittle to me, as it requires the unwinder to understand
> >> > more of the internal workings of the fork code.
> >> >
> >> > But overall I think this patch is a really nice cleanup, and other than
> >> > the above minor issue it should be fine with my reliable unwinder, since
> >> > rbp is still at the top of the stack.
> >>
> >> Is this a regression or is there some reason that it works right
> >> without the patch?
> >
> > Without the patch, it uses TIF_FORK to determine the stack is empty.
> 
> Where is this code?  I don't see it in the mainline kernel.

Yeah, it hasn't been merged.  Here's the last version:

  https://lkml.kernel.org/r/4d34d452bf8f85c7d6d5f93db1d3eeb4cba335c7.1461875890.git.jpoimboe@redhat.com

-- 
Josh