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Date: Fri, 6 Mar 2009 16:01:16 -0500 From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> To: Masami Hiramatsu <mhiramat@...hat.com> Cc: Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...ux-foundation.org>, Nick Piggin <npiggin@...e.de>, Steven Rostedt <rostedt@...dmis.org>, Andi Kleen <andi@...stfloor.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Frederic Weisbecker <fweisbec@...il.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Arjan van de Ven <arjan@...radead.org>, Rusty Russell <rusty@...tcorp.com.au>, "H. Peter Anvin" <hpa@...or.com> Subject: Re: [PATCH -tip 5/4] Expands irq-off region in text_poke() * Masami Hiramatsu (mhiramat@...hat.com) wrote: > Ingo Molnar wrote: > > * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote: > > > >> * Ingo Molnar (mingo@...e.hu) wrote: > >>> * Masami Hiramatsu <mhiramat@...hat.com> 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@...hat.com> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> > Cc: Ingo Molnar <mingo@...e.hu> > --- > 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]); I think irq off should cover the BUG_ON too. This safety check assumes we are the only ones modifying "addr". Mathieu > return addr; > -- > Masami Hiramatsu > > Software Engineer > Hitachi Computer Products (America) Inc. > Software Solutions Division > > e-mail: mhiramat@...hat.com > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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