| Message ID | 20230207182135.2671106-5-revest@chromium.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Add ftrace direct call for arm64 | expand |
On Tue, 7 Feb 2023 19:21:29 +0100 Florent Revest <revest@chromium.org> wrote: > @@ -5445,6 +5445,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > /* Enable the tmp_ops to have the same functions as the direct ops */ > ftrace_ops_init(&tmp_ops); > tmp_ops.func_hash = ops->func_hash; > + tmp_ops.direct_call = addr; > > err = register_ftrace_function_nolock(&tmp_ops); > if (err) > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > entry->direct = addr; > } > } > + WRITE_ONCE(ops->direct_call, addr); I'm curious about the use of WRITE_ONCE(). It should not go outside the mutex barrier. -- Steve > > mutex_unlock(&ftrace_lock); >
On Thu, Mar 16, 2023 at 12:43 AM Steven Rostedt <rostedt@goodmis.org> wrote: > > On Tue, 7 Feb 2023 19:21:29 +0100 > Florent Revest <revest@chromium.org> wrote: > > > @@ -5445,6 +5445,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > /* Enable the tmp_ops to have the same functions as the direct ops */ > > ftrace_ops_init(&tmp_ops); > > tmp_ops.func_hash = ops->func_hash; > > + tmp_ops.direct_call = addr; > > > > err = register_ftrace_function_nolock(&tmp_ops); > > if (err) > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > entry->direct = addr; > > } > > } > > + WRITE_ONCE(ops->direct_call, addr); > > I'm curious about the use of WRITE_ONCE(). It should not go outside the > mutex barrier. This WRITE_ONCE was originally suggested by Mark here: https://lore.kernel.org/all/Y9vW99htjOphDXqY@FVFF77S0Q05N.cambridge.arm.com/#t My understanding is that it's not so much about avoiding re-ordering but rather about avoiding store tearing since a ftrace_caller trampoline could concurrently read ops->direct_call. Does that make sense ? > -- Steve > > > > > mutex_unlock(&ftrace_lock); > >
On Thu, 16 Mar 2023 16:40:48 +0100 Florent Revest <revest@chromium.org> wrote: > > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > > entry->direct = addr; > > > } > > > } > > > + WRITE_ONCE(ops->direct_call, addr); > > > > I'm curious about the use of WRITE_ONCE(). It should not go outside the > > mutex barrier. > > This WRITE_ONCE was originally suggested by Mark here: > https://lore.kernel.org/all/Y9vW99htjOphDXqY@FVFF77S0Q05N.cambridge.arm.com/#t > > My understanding is that it's not so much about avoiding re-ordering > but rather about avoiding store tearing since a ftrace_caller > trampoline could concurrently read ops->direct_call. Does that make > sense ? Yes, but a comment needs to be added: /* Prevent store tearing on some archs */ WRITE_ONCE(ops->direct_call, addr); Or something to that affect. Otherwise I can see it confusing others in the future. And probably me too, as I'll forget why it was a WRITE_ONCE() by next month. ;-) -- Steve
On Thu, Mar 16, 2023 at 4:45 PM Steven Rostedt <rostedt@goodmis.org> wrote: > > On Thu, 16 Mar 2023 16:40:48 +0100 > Florent Revest <revest@chromium.org> wrote: > > > > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) > > > > entry->direct = addr; > > > > } > > > > } > > > > + WRITE_ONCE(ops->direct_call, addr); > > > > > > I'm curious about the use of WRITE_ONCE(). It should not go outside the > > > mutex barrier. > > > > This WRITE_ONCE was originally suggested by Mark here: > > https://lore.kernel.org/all/Y9vW99htjOphDXqY@FVFF77S0Q05N.cambridge.arm.com/#t > > > > My understanding is that it's not so much about avoiding re-ordering > > but rather about avoiding store tearing since a ftrace_caller > > trampoline could concurrently read ops->direct_call. Does that make > > sense ? > > Yes, but a comment needs to be added: > > /* Prevent store tearing on some archs */ > WRITE_ONCE(ops->direct_call, addr); > > Or something to that affect. Otherwise I can see it confusing others in the > future. And probably me too, as I'll forget why it was a WRITE_ONCE() by > next month. ;-) Definitely :) I was myself confused after a few weeks of adding it so I'll add a clarifying comment. Thanks!
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h index a7dbd307c3a4..84f717f8959e 100644 --- a/include/linux/ftrace.h +++ b/include/linux/ftrace.h @@ -321,6 +321,9 @@ struct ftrace_ops { unsigned long trampoline_size; struct list_head list; ftrace_ops_func_t ops_func; +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS + unsigned long direct_call; +#endif #endif }; diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index cb77a0a208c7..dfa5f34ec320 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -2577,9 +2577,8 @@ ftrace_add_rec_direct(unsigned long ip, unsigned long addr, static void call_direct_funcs(unsigned long ip, unsigned long pip, struct ftrace_ops *ops, struct ftrace_regs *fregs) { - unsigned long addr; + unsigned long addr = ops->direct_call; - addr = ftrace_find_rec_direct(ip); if (!addr) return; @@ -5375,6 +5374,7 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) ops->func = call_direct_funcs; ops->flags = MULTI_FLAGS; ops->trampoline = FTRACE_REGS_ADDR; + ops->direct_call = addr; err = register_ftrace_function_nolock(ops); @@ -5445,6 +5445,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) /* Enable the tmp_ops to have the same functions as the direct ops */ ftrace_ops_init(&tmp_ops); tmp_ops.func_hash = ops->func_hash; + tmp_ops.direct_call = addr; err = register_ftrace_function_nolock(&tmp_ops); if (err) @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) entry->direct = addr; } } + WRITE_ONCE(ops->direct_call, addr); mutex_unlock(&ftrace_lock);
All direct calls are now registered using the register_ftrace_direct API so each ops can jump to only one direct-called trampoline. By storing the direct called trampoline address directly in the ops we can save one hashmap lookup in the direct call ops and implement arm64 direct calls on top of call ops. Signed-off-by: Florent Revest <revest@chromium.org> --- include/linux/ftrace.h | 3 +++ kernel/trace/ftrace.c | 6 ++++-- 2 files changed, 7 insertions(+), 2 deletions(-)