Messages in this thread | | | Date | Sat, 3 Oct 2009 08:39:00 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 1/4] jump label - make init_kernel_text() global |
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* Ingo Molnar (mingo@elte.hu) wrote: > > * Jason Baron <jbaron@redhat.com> wrote: > > > On Thu, Oct 01, 2009 at 01:20:03PM +0200, Ingo Molnar wrote: > > > * Jason Baron <jbaron@redhat.com> wrote: > > > > > > > allow usage of init_kernel_text - we need this in jump labeling to > > > > avoid attemtpting to patch code that has been freed as in the __init > > > > sections > > > > > > s/attemtpting/attempting > > > > > > > Signed-off-by: Jason Baron <jbaron@redhat.com> > > > > --- > > > > include/linux/kernel.h | 1 + > > > > kernel/extable.c | 2 +- > > > > 2 files changed, 2 insertions(+), 1 deletions(-) > > > > > > > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > > > > index f61039e..9d3419f 100644 > > > > --- a/include/linux/kernel.h > > > > +++ b/include/linux/kernel.h > > > > @@ -295,6 +295,7 @@ extern int get_option(char **str, int *pint); > > > > extern char *get_options(const char *str, int nints, int *ints); > > > > extern unsigned long long memparse(const char *ptr, char **retptr); > > > > > > > > +extern int init_kernel_text(unsigned long addr); > > > > extern int core_kernel_text(unsigned long addr); > > > > extern int __kernel_text_address(unsigned long addr); > > > > extern int kernel_text_address(unsigned long addr); > > > > diff --git a/kernel/extable.c b/kernel/extable.c > > > > index 7f8f263..f6893ad 100644 > > > > --- a/kernel/extable.c > > > > +++ b/kernel/extable.c > > > > @@ -52,7 +52,7 @@ const struct exception_table_entry *search_exception_tables(unsigned long addr) > > > > return e; > > > > } > > > > > > > > -static inline int init_kernel_text(unsigned long addr) > > > > +int init_kernel_text(unsigned long addr) > > > > { > > > > if (addr >= (unsigned long)_sinittext && > > > > addr <= (unsigned long)_einittext) > > > > > > i'm confused. Later on jump_label_update() does: > > > > > > + if (!(system_state == SYSTEM_RUNNING && > > > + (init_kernel_text(iter->code)))) > > > + jump_label_transform(iter, type); > > > > > > which is: > > > > > > + if (system_state != SYSTEM_RUNNING || > > > + !init_kernel_text(iter->code))) > > > + jump_label_transform(iter, type); > > > > > > What is the logic behind that? System going into SYSTEM_RUNNING does not > > > coincide with free_initmem() precisely. > > > > > > > The specific case I hit was in modifying code in arch_kdebugfs_init() > > which is '__init' after the system was up and running. The tracepoint is > > in 'kmalloc()' which is marked as __always_inline. > > > > > > > Also, do we ever want to patch init-text tracepoints? I think we want to > > > stay away from them as much as possible. > > > > I was trying to make sure that tracepoints in init-text were honored. > > > > > > > > It appears to me that what we want here is a straight: > > > > > > if (kernel_text(iter->code)) > > > jump_label_transform(iter, type); > > > > > > Also, maybe a WARN_ONCE(!kernel_text()) - we should never even attempt > > > to transform non-patchable code. If yes then we want to know about that > > > in a noisy way and not skip it silently. > > > > > > > hmmm....indeed, kernel_text_address() does do what I want here (I must > > have mis-read its definition). Although, I'm not sure there isn't a > > race here betweeen freeing the init sections and possibly updating > > them. For modules, there is no race since the module init free code > > takes the module_mutex, and I do as well in this code... > > > > I've now also tested this code on 32-bit x86 system, and it seems to > > perform nicely. I'm seeing a 15 cycle improvement per tracepoint. > > > > I've based the text section updating on text_poke_fixup(), which has > > recently come into question about safety of cross modifying code. I > > could rebase my patches back to use stop_machine()? I guess I'm > > looking for some advice on how to proceed here. > > I think this very limited form of code patching that you are using here > (patching a JMP) _should_ be safe - so we can avoid stop_machine(). >
I might be missing a bit of context here, I just want to make sure we are on the same page: patching a jmp instruction is safe on UP, safe with stop_machine(), is very likely safe with the breakpoint-ipi approach (but we need the confirmation from Intel, which hpa is trying to get), but is definitely _not_ safe if neither of these methods are used on a SMP system. If a non-aligned multi-word jump is modified while another CPU is fetching the instruction, bad things could happen.
BTW, patching kernel and module init sections can be done without sop_machine(), because only one CPU is ever accessing the init code.
But again, I might be missing some context. If so, sorry for the noise.
Thanks,
Mathieu
> Ingo
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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