| Message ID | 20220322022331.32136-1-palmer@rivosinc.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | RISC-V: Don't check text_mutex during stop_machine | expand |
On Mon, Mar 21, 2022 at 07:23:31PM -0700, Palmer Dabbelt wrote: > From: Palmer Dabbelt <palmerdabbelt@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@goodmis.org> > Reported-by: Changbin Du <changbin.du@gmail.com> > Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com> > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com> > > -- > > Changes since v1 [<20210506071041.417854-1-palmer@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 | 12 ++++++++++++ > arch/riscv/kernel/patch.c | 10 +++++++++- > 3 files changed, 28 insertions(+), 1 deletion(-) > > 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 4716f4cdc038..c5f77922d7ea 100644 > --- a/arch/riscv/kernel/ftrace.c > +++ b/arch/riscv/kernel/ftrace.c > @@ -11,15 +11,27 @@ > #include <asm/cacheflush.h> > #include <asm/patch.h> > > +int riscv_ftrace_in_stop_machine; > + > #ifdef CONFIG_DYNAMIC_FTRACE > int 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; > + > return 0; > } > > int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex) > { > + riscv_ftrace_in_stop_machine = false; > mutex_unlock(&text_mutex); > return 0; > } The function ftrace_init_nop() should be updated. That's not in stop-machine context. @@ -136,9 +124,9 @@ 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);
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 4716f4cdc038..c5f77922d7ea 100644 --- a/arch/riscv/kernel/ftrace.c +++ b/arch/riscv/kernel/ftrace.c @@ -11,15 +11,27 @@ #include <asm/cacheflush.h> #include <asm/patch.h> +int riscv_ftrace_in_stop_machine; + #ifdef CONFIG_DYNAMIC_FTRACE int 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; + return 0; } int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex) { + riscv_ftrace_in_stop_machine = false; mutex_unlock(&text_mutex); return 0; } diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c index 0b552873a577..7983dba477f0 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);