ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

[PATCH] ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

Posted by Steven Rostedt 1 month, 1 week ago

From: Steven Rostedt <rostedt@goodmis.org>

When function trace PID filtering is enabled, the function tracer will
attach a callback to the fork tracepoint as well as the exit tracepoint
that will add the forked child PID to the PID filtering list as well as
remove the PID that is exiting.

Commit a46023d5616e ("tracing: Guard __DECLARE_TRACE() use of
__DO_TRACE_CALL() with SRCU-fast") removed the disabling of preemption
when calling tracepoint callbacks.

The callbacks used for the PID filtering accounting depended on preemption
being disabled, and now the trigger a "suspicious RCU usage" warning message.

Make them explicitly disable preemption.

Fixes: a46023d5616e ("tracing: Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/ftrace.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 1ce17c8af409..5274e0cab8f3 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -8611,6 +8611,7 @@ ftrace_pid_follow_sched_process_fork(void *data,
 	struct trace_pid_list *pid_list;
 	struct trace_array *tr = data;
 
+	guard(preempt)();
 	pid_list = rcu_dereference_sched(tr->function_pids);
 	trace_filter_add_remove_task(pid_list, self, task);
 
@@ -8624,6 +8625,7 @@ ftrace_pid_follow_sched_process_exit(void *data, struct task_struct *task)
 	struct trace_pid_list *pid_list;
 	struct trace_array *tr = data;
 
+	guard(preempt)();
 	pid_list = rcu_dereference_sched(tr->function_pids);
 	trace_filter_add_remove_task(pid_list, NULL, task);
 
-- 
2.51.0

Re: [PATCH] ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

Posted by Masami Hiramatsu (Google) 1 month, 1 week ago

On Mon, 2 Mar 2026 21:35:46 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> When function trace PID filtering is enabled, the function tracer will
> attach a callback to the fork tracepoint as well as the exit tracepoint
> that will add the forked child PID to the PID filtering list as well as
> remove the PID that is exiting.
> 
> Commit a46023d5616e ("tracing: Guard __DECLARE_TRACE() use of
> __DO_TRACE_CALL() with SRCU-fast") removed the disabling of preemption
> when calling tracepoint callbacks.
> 
> The callbacks used for the PID filtering accounting depended on preemption
> being disabled, and now the trigger a "suspicious RCU usage" warning message.
> 
> Make them explicitly disable preemption.
> 

Hmm, we still have another "WARNING: suspicious RCU usage" 

[37] event tracing - restricts events based on pid notrace filtering[  155.874095] 
[  155.874869] =============================
[  155.876037] WARNING: suspicious RCU usage
[  155.877287] 7.0.0-rc1-00004-g8cd473a19bc7 #7 Not tainted
[  155.879263] -----------------------------
[  155.882839] kernel/trace/trace_events.c:1057 suspicious rcu_dereference_check() usage!
[  155.889281] 
[  155.889281] other info that might help us debug this:
[  155.889281] 
[  155.894519] 
[  155.894519] rcu_scheduler_active = 2, debug_locks = 1
[  155.898068] no locks held by ftracetest/4364.
[  155.900524] 
[  155.900524] stack backtrace:
[  155.902645] CPU: 1 UID: 0 PID: 4364 Comm: ftracetest Not tainted 7.0.0-rc1-00004-g8cd473a19bc7 #7 PREEMPT(lazy) 
[  155.902648] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
[  155.902651] Call Trace:
[  155.902655]  <TASK>
[  155.902659]  dump_stack_lvl+0x67/0x90
[  155.902665]  lockdep_rcu_suspicious+0x154/0x1a0
[  155.902672]  event_filter_pid_sched_process_fork+0x9a/0xd0
[  155.902678]  kernel_clone+0x367/0x3a0
[  155.902689]  __x64_sys_clone+0x116/0x140
[  155.902696]  do_syscall_64+0x158/0x460
[  155.902700]  ? entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  155.902702]  ? trace_irq_disable+0x1d/0xc0
[  155.902709]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  155.902711] RIP: 0033:0x4697c3
[  155.902716] Code: 1f 84 00 00 00 00 00 64 48 8b 04 25 10 00 00 00 45 31 c0 31 d2 31 f6 bf 11 00 20 01 4c 8d 90 d0 02 00 00 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 35 89 c2 85 c0 75 2c 64 48 8b 04 25 10 00 00
[  155.902718] RSP: 002b:00007ffc41150428 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
[  155.902721] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004697c3
[  155.902722] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
[  155.902724] RBP: 0000000000000000 R08: 0000000000000000 R09: 000000003fccf990
[  155.902725] R10: 000000003fccd690 R11: 0000000000000246 R12: 0000000000000001
[  155.902726] R13: 000000003fce8103 R14: 0000000000000001 R15: 0000000000000000
[  155.902733]  </TASK>
[  155.902747] 

It seems we need 2 more guard()s.

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 9928da636c9d..3b3aaf4831e9 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1039,6 +1039,7 @@ event_filter_pid_sched_process_exit(void *data, struct task_struct *task)
 	struct trace_pid_list *pid_list;
 	struct trace_array *tr = data;
 
+	guard(preempt)();
 	pid_list = rcu_dereference_raw(tr->filtered_pids);
 	trace_filter_add_remove_task(pid_list, NULL, task);
 
@@ -1054,6 +1055,7 @@ event_filter_pid_sched_process_fork(void *data,
 	struct trace_pid_list *pid_list;
 	struct trace_array *tr = data;
 
+	guard(preempt)();
 	pid_list = rcu_dereference_sched(tr->filtered_pids);
 	trace_filter_add_remove_task(pid_list, self, task);
 

Thanks,

> Fixes: a46023d5616e ("tracing: Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast")
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/ftrace.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 1ce17c8af409..5274e0cab8f3 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -8611,6 +8611,7 @@ ftrace_pid_follow_sched_process_fork(void *data,
>  	struct trace_pid_list *pid_list;
>  	struct trace_array *tr = data;
>  
> +	guard(preempt)();
>  	pid_list = rcu_dereference_sched(tr->function_pids);
>  	trace_filter_add_remove_task(pid_list, self, task);
>  
> @@ -8624,6 +8625,7 @@ ftrace_pid_follow_sched_process_exit(void *data, struct task_struct *task)
>  	struct trace_pid_list *pid_list;
>  	struct trace_array *tr = data;
>  
> +	guard(preempt)();
>  	pid_list = rcu_dereference_sched(tr->function_pids);
>  	trace_filter_add_remove_task(pid_list, NULL, task);
>  
> -- 
> 2.51.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH] ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

Posted by Steven Rostedt 1 month ago

On Tue, 3 Mar 2026 13:17:06 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 9928da636c9d..3b3aaf4831e9 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -1039,6 +1039,7 @@ event_filter_pid_sched_process_exit(void *data, struct task_struct *task)
>  	struct trace_pid_list *pid_list;
>  	struct trace_array *tr = data;
>  
> +	guard(preempt)();
>  	pid_list = rcu_dereference_raw(tr->filtered_pids);
>  	trace_filter_add_remove_task(pid_list, NULL, task);
>  
> @@ -1054,6 +1055,7 @@ event_filter_pid_sched_process_fork(void *data,
>  	struct trace_pid_list *pid_list;
>  	struct trace_array *tr = data;
>  
> +	guard(preempt)();
>  	pid_list = rcu_dereference_sched(tr->filtered_pids);
>  	trace_filter_add_remove_task(pid_list, self, task);
>  

Yep. I can make them two different patches. Do you want me to take this as
your patch? I'll need a SoB from you then.

-- Steve

Re: [PATCH] ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

Posted by Masami Hiramatsu (Google) 1 month ago

On Tue, 3 Mar 2026 09:39:49 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 3 Mar 2026 13:17:06 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> > index 9928da636c9d..3b3aaf4831e9 100644
> > --- a/kernel/trace/trace_events.c
> > +++ b/kernel/trace/trace_events.c
> > @@ -1039,6 +1039,7 @@ event_filter_pid_sched_process_exit(void *data, struct task_struct *task)
> >  	struct trace_pid_list *pid_list;
> >  	struct trace_array *tr = data;
> >  
> > +	guard(preempt)();
> >  	pid_list = rcu_dereference_raw(tr->filtered_pids);
> >  	trace_filter_add_remove_task(pid_list, NULL, task);
> >  
> > @@ -1054,6 +1055,7 @@ event_filter_pid_sched_process_fork(void *data,
> >  	struct trace_pid_list *pid_list;
> >  	struct trace_array *tr = data;
> >  
> > +	guard(preempt)();
> >  	pid_list = rcu_dereference_sched(tr->filtered_pids);
> >  	trace_filter_add_remove_task(pid_list, self, task);
> >  
> 
> Yep. I can make them two different patches. Do you want me to take this as
> your patch? I'll need a SoB from you then.

I think we can combine those since it fixes the same commit.
(just for bisectability)

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH] ftrace: Disable preemption in the tracepoint callbacks handling filtered pids

Posted by Steven Rostedt 1 month ago

On Wed, 4 Mar 2026 10:48:33 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> I think we can combine those since it fixes the same commit.
> (just for bisectability)

They can still be separate. One is for function tracing and the other
is for events. They both may have the same fixes tag, but it shouldn't
affect bisectablility, as they require different things to be enabled
to trigger.

> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

-- Steve