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

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


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