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Message-ID: <20090307014219.GA27154@Krystal> Date: Fri, 6 Mar 2009 20:42:20 -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: > > > Mathieu Desnoyers wrote: > > * 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". > > I think others don't change without text_mutex, don't it? > They shouldn't, but given we decided to grow the irq off region to contain all the code that needs to be executed atomically, it should also contain the BUG_ON, because it is expected to be as atomic as the rest of the code. Mathieu > Thank you, > > -- > 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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