[PATCH -tip 5/4] Expands irq-off region in text_poke()

Ingo Molnar wrote:
* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:

* Ingo Molnar (mingo@xxxxxxx) wrote:
* Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

@@ -523,14 +526,17 @@ void *__kprobes text_poke(void *addr, co
pages[1] = virt_to_page(addr + PAGE_SIZE);
}
BUG_ON(!pages[0]);
- if (!pages[1])
- nr_pages = 1;
- vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
- BUG_ON(!vaddr);
- local_irq_disable();
+ local_irq_save(flags);
+ set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
+ if (pages[1])
+ set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
+ vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
- local_irq_enable();
- vunmap(vaddr);
+ clear_fixmap(FIX_TEXT_POKE0);
+ if (pages[1])
+ clear_fixmap(FIX_TEXT_POKE1);
+ local_flush_tlb();
+ local_irq_restore(flags);
sync_core();
I'm not sure at all about this widening of the irq-atomic
section and the idea of allowing non-locked access on single-CPU
situations - we dont really want to micro-optimize any of this
on such a level, holding the text lock is a robust rule all code
should be listening to. (Creating locking assymetry always
inserts a certain amount of fragility - adding to an already
fragile concept here.)

And note that there's no reason why text_poke could not be used
in stop_machine_run() - the stop_machine_run() handler must not
take the text_lock of course - but outside code calling
stop_machine_run() can do it and can hence serialize properly.

Note that even if we did this then your v2 patch is not fully
correct: you need to move the sync_core() at the end of the
sequence inside the critical section too. (right now this is
mostly harmless because the INVLPG inside the clear_fixmap()
happens to be serializing so it has an implicit sync_core()
property - but nevertheless we better do this straight away to
not cause problems later down the line.)

Ingo
Agreed. The alternatives_smp_lock/alternatives_smp_unlock
specific case does not bring us much if it has no perceivable
performance impact. It's better to keep a standard interface
and clear requirements.

Note that i dont object to another aspect of this same change:
the fact that it makes the whole sequence more atomic and more
defensive [which is never bad of fragile interfaces].

I only got worried about the "lets use this without the text
lock" ideas.

So if Masami-san sends a delta patch with a different changelog
and with the sync_core() bit moved inside the critical section,
i'll apply that too.

OK, here is the delta patch.

Expand irq-atomic region to cover fixmap using code and sync_core.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
---
arch/x86/kernel/alternative.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-2.6-tip/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6-tip.orig/arch/x86/kernel/alternative.c
+++ linux-2.6-tip/arch/x86/kernel/alternative.c
@@ -526,13 +526,12 @@ void *__kprobes text_poke(void *addr, co
pages[1] = virt_to_page(addr + PAGE_SIZE);
}
BUG_ON(!pages[0]);
+ local_irq_save(flags);
set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
if (pages[1])
set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
- local_irq_save(flags);
memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
- local_irq_restore(flags);
clear_fixmap(FIX_TEXT_POKE0);
if (pages[1])
clear_fixmap(FIX_TEXT_POKE1);
@@ -540,6 +539,7 @@ void *__kprobes text_poke(void *addr, co
sync_core();
/* Could also do a CLFLUSH here to speed up CPU recovery; but
that causes hangs on some VIA CPUs. */
+ local_irq_restore(flags);
for (i = 0; i < len; i++)
BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
return addr;
--
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Relevant Pages

  • Re: [PATCH -tip 5/4] Expands irq-off region in text_poke()
    ... defensive [which is never bad of fragile interfaces]. ... So if Masami-san sends a delta patch with a different changelog ... Software Solutions Division ...
    (Linux-Kernel)
  • Re: [PATCH -tip 5/4] Expands irq-off region in text_poke()
    ... Masami Hiramatsu wrote: ... Note that i dont object to another aspect of this same change: ... defensive [which is never bad of fragile interfaces]. ... So if Masami-san sends a delta patch with a different changelog ...
    (Linux-Kernel)
  • Re: Review Fragile
    ... english, thank goodness :-). ... I could send you a copy of half light and fragile if you want. ... services and if you dont use them they close your account, ...
    (rec.arts.horror.movies)
  • Re: Review Fragile
    ... english, thank goodness :-). ... I could send you a copy of half light and fragile if you want. ... services and if you dont use them they close your account, ...
    (rec.arts.horror.movies)
  • Re: [PATCH -tip 4/4] Atomic text_poke() with fixmap take2
    ... sequence inside the critical section too. ... It's better to keep a standard interface ... Note that i dont object to another aspect of this same change: ... defensive [which is never bad of fragile interfaces]. ...
    (Linux-Kernel)