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Date: Tue, 19 Aug 2008 17:08:15 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Scott Wood <scottwood@freescale.com>,
       Eran Liberty <liberty@extricom.com>, linux-kernel@vger.kernel.org,
       linuxppc-dev@ozlabs.org, Steven Rostedt <srostedt@redhat.com>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
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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=blob;f=arch/x86/kernel/immediate.c;h=7789e2c75bf03e645f15759d5dff0c1698493f92;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.

-- Steve