linux-kernel - Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

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Message-Id: <200808201940.51916.nickpiggin@yahoo.com.au>
Date:	Wed, 20 Aug 2008 19:40:51 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Scott Wood <scottwood@...escale.com>,
	Eran Liberty <liberty@...ricom.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Steven Rostedt <srostedt@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

On Wednesday 20 August 2008 07:08, Steven Rostedt wrote:
> On Tue, 19 Aug 2008, Mathieu Desnoyers wrote:
> > Ok, there are two cases where it's ok :
> >
> > 1 - in stop_machine, considering we are not touching code executed in
> > NMI handlers.
> > 2 - when using my replace_instruction_safe() which uses a temporary
> > breakpoint when doing the instruction replacement.
> >
> > In those cases you could use text_poke_early().
> >
> > See
> > http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=b
> >lob;f=arch/x86/kernel/immediate.c;h=7789e2c75bf03e645f15759d5dff0c1698493f
> >92;hb=HEAD
> >
> > For a use example. Basically it looks like :
> >
> >
> > 360                 pages[0] = virt_to_page((void *)bypass_eip);
> > 361                 vaddr = vmap(pages, 1, VM_MAP, PAGE_KERNEL);
> > 362                 BUG_ON(!vaddr);
> > 363                 text_poke_early(&vaddr[bypass_eip & ~PAGE_MASK],
> > 364                         (void *)addr, size);
> > 365                 /*
> > 366                  * Fill the rest with nops.
> > 367                  */
> > 368                 len = NR_NOPS - size;
> > 369                 add_nops((void *)
> > 370                         &vaddr[(bypass_eip & ~PAGE_MASK) + size],
> > 371                         len);
> > 372                 print_dbg_bytes("inserted nops",
> > 373                         &vaddr[(bypass_eip & ~PAGE_MASK) + size],
> > len); 374                 vunmap(vaddr);
>
> vunmap can not be called with interrupts disabled, and this is exactly
> what my code does.

It could be after the vmap rewrite (with a few other small tweaks).
But a) it would be less robust when called from interrupt context
and this code looks broken as it is WRT error handling; and b) it
still costs several thousand cycles to vmap+touch+vunmap...
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