[PATCH v3] RISC-V: Don't check text_mutex during stop_machine
Conor Dooley
conor.dooley at microchip.com
Fri Feb 24 03:07:42 PST 2023
On Thu, Feb 16, 2023 at 07:31:26PM +0800, Changbin Du wrote:
> On Wed, Feb 15, 2023 at 04:43:17PM +0000, Conor Dooley wrote:
> > From: Palmer Dabbelt <palmerdabbelt at google.com>
> >
> > We're currently using stop_machine() to update ftrace, which means that
> > the thread that takes text_mutex during ftrace_prepare() may not be the
> > same as the thread that eventually patches the code. This isn't
> > actually a race because the lock is still held (preventing any other
> > concurrent accesses) and there is only one thread running during
> > stop_machine(), but it does trigger a lockdep failure.
> >
> > This patch just elides the lockdep check during stop_machine.
> >
> > Fixes: c15ac4fd60d5 ("riscv/ftrace: Add dynamic function tracer support")
> > Suggested-by: Steven Rostedt <rostedt at goodmis.org>
> > Reported-by: Changbin Du <changbin.du at gmail.com>
> > Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
> > Signed-off-by: Palmer Dabbelt <palmer at rivosinc.com>
> > Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
> > ---
> > Resending this version as I am quite averse to deleting the assertion!
> >
> > Changes since v2 [<20220322022331.32136-1-palmer at rivosinc.com>]:
> > * rebase on riscv/for-next as it as been a year.
> > * incorporate Changbin's suggestion that init_nop should take the lock
> > rather than call prepare() & post_process().
> >
> > Changes since v1 [<20210506071041.417854-1-palmer at dabbelt.com>]:
> > * Use ftrace_arch_ocde_modify_{prepare,post_process}() to set the flag.
> > I remember having a reason I wanted the function when I wrote the v1,
> > but it's been almost a year and I forget what that was -- maybe I was
> > just crazy, the patch was sent at midnight.
> > * Fix DYNAMIC_FTRACE=n builds.
> > ---
> > arch/riscv/include/asm/ftrace.h | 7 +++++++
> > arch/riscv/kernel/ftrace.c | 15 +++++++++++++--
> > arch/riscv/kernel/patch.c | 10 +++++++++-
> > 3 files changed, 29 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> > index 04dad3380041..3ac7609f4ee9 100644
> > --- a/arch/riscv/include/asm/ftrace.h
> > +++ b/arch/riscv/include/asm/ftrace.h
> > @@ -81,8 +81,15 @@ do { \
> > struct dyn_ftrace;
> > int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
> > #define ftrace_init_nop ftrace_init_nop
> > +extern int riscv_ftrace_in_stop_machine;
> > #endif
> >
> > +#else /* CONFIG_DYNAMIC_FTRACE */
> > +
> > +#ifndef __ASSEMBLY__
> > +#define riscv_ftrace_in_stop_machine 0
> > #endif
> >
> > +#endif /* CONFIG_DYNAMIC_FTRACE */
> > +
> > #endif /* _ASM_RISCV_FTRACE_H */
> > diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> > index 2086f6585773..661bfa72f359 100644
> > --- a/arch/riscv/kernel/ftrace.c
> > +++ b/arch/riscv/kernel/ftrace.c
> > @@ -11,14 +11,25 @@
> > #include <asm/cacheflush.h>
> > #include <asm/patch.h>
> >
> > +int riscv_ftrace_in_stop_machine;
> > +
> > #ifdef CONFIG_DYNAMIC_FTRACE
> > void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
> > {
> > mutex_lock(&text_mutex);
> > +
> > + /*
> > + * The code sequences we use for ftrace can't be patched while the
> > + * kernel is running, so we need to use stop_machine() to modify them
> > + * for now. This doesn't play nice with text_mutex, we use this flag
> > + * to elide the check.
> > + */
> > + riscv_ftrace_in_stop_machine = true;
> > }
> >
> > void ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
> > {
> > + riscv_ftrace_in_stop_machine = false;
> > mutex_unlock(&text_mutex);
> > }
> >
> > @@ -134,9 +145,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> > {
> > int out;
> >
> > - ftrace_arch_code_modify_prepare();
> > + mutex_lock(&text_mutex);
> > out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> > - ftrace_arch_code_modify_post_process();
> > + mutex_unlock(&text_mutex);
> >
> > return out;
> > }
> > diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> > index 765004b60513..56b70271518d 100644
> > --- a/arch/riscv/kernel/patch.c
> > +++ b/arch/riscv/kernel/patch.c
> > @@ -11,6 +11,7 @@
> > #include <asm/kprobes.h>
> > #include <asm/cacheflush.h>
> > #include <asm/fixmap.h>
> > +#include <asm/ftrace.h>
> > #include <asm/patch.h>
> >
> > struct patch_insn {
> > @@ -59,8 +60,15 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
> > * Before reaching here, it was expected to lock the text_mutex
> > * already, so we don't need to give another lock here and could
> > * ensure that it was safe between each cores.
> > + *
> > + * We're currently using stop_machine() for ftrace, and while that
> > + * ensures text_mutex is held before installing the mappings it does
> > + * not ensure text_mutex is held by the calling thread. That's safe
> > + * but triggers a lockdep failure, so just elide it for that specific
> > + * case.
> > */
> > - lockdep_assert_held(&text_mutex);
> > + if (!riscv_ftrace_in_stop_machine)
> > + lockdep_assert_held(&text_mutex);
> >
> > if (across_pages)
> > patch_map(addr + len, FIX_TEXT_POKE1);
> This misses this function.
>
> int patch_text(void *addr, u32 insn)
So, with a corresponding rename to the symbol, does the following look
okay to you?
diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index f21592d20306..433b454e693f 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -27,9 +27,15 @@ static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
p->ainsn.api.restore = (unsigned long)p->addr + offset;
+ /*
+ * kprobes takes text_mutex, but patch_text() calls stop_machine and
+ * lockdep gets confused by the context in which the lock is taken.
+ */
+ riscv_patch_in_stop_machine = true;
patch_text(p->ainsn.api.insn, p->opcode);
patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
__BUG_INSN_32);
+ riscv_patch_in_stop_machine = false;
}
static void __kprobes arch_prepare_simulate(struct kprobe *p)
@@ -96,16 +102,28 @@ void *alloc_insn_page(void)
/* install breakpoint in text */
void __kprobes arch_arm_kprobe(struct kprobe *p)
{
+ /*
+ * kprobes takes text_mutex, but patch_text() calls stop_machine and
+ * lockdep gets confused by the context in which the lock is taken.
+ */
+ riscv_patch_in_stop_machine = true;
if ((p->opcode & __INSN_LENGTH_MASK) == __INSN_LENGTH_32)
patch_text(p->addr, __BUG_INSN_32);
else
patch_text(p->addr, __BUG_INSN_16);
+ riscv_patch_in_stop_machine = false;
}
/* remove breakpoint from text */
void __kprobes arch_disarm_kprobe(struct kprobe *p)
{
+ /*
+ * kprobes takes text_mutex, but patch_text() calls stop_machine and
+ * lockdep gets confused by the context in which the lock is taken.
+ */
+ riscv_patch_in_stop_machine = true;
patch_text(p->addr, p->opcode);
+ riscv_patch_in_stop_machine = false;
}
void __kprobes arch_remove_kprobe(struct kprobe *p)
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