[for-linus][PATCH 3/4] ftrace: Fix preemption accounting for stacktrace filter command

[Steven Rostedt <rostedt@goodmis.org>]

From: pengdonglin <pengdonglin@xiaomi.com>

The preemption count of the stacktrace filter command to trace ksys_read
is consistently incorrect:

$ echo ksys_read:stacktrace > set_ftrace_filter

   <...>-453     [004] ...1.    38.308956: <stack trace>
=> ksys_read
=> do_syscall_64
=> entry_SYSCALL_64_after_hwframe

The root cause is that the trace framework disables preemption when
invoking the filter command callback in function_trace_probe_call:

   preempt_disable_notrace();
   probe_ops->func(ip, parent_ip, probe_opsbe->tr, probe_ops, probe->data);
   preempt_enable_notrace();

Use tracing_gen_ctx_dec() to account for the preempt_disable_notrace(),
which will output the correct preemption count:

$ echo ksys_read:stacktrace > set_ftrace_filter

   <...>-410     [006] .....    31.420396: <stack trace>
=> ksys_read
=> do_syscall_64
=> entry_SYSCALL_64_after_hwframe

Cc: stable@vger.kernel.org
Fixes: 36590c50b2d07 ("tracing: Merge irqflags + preempt counter.")
Link: https://lore.kernel.org/20250512094246.1167956-2-dolinux.peng@gmail.com
Signed-off-by: pengdonglin <dolinux.peng@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/trace_functions.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 98ccf3f00c51..4e37a0f6aaa3 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -633,11 +633,7 @@ ftrace_traceoff(unsigned long ip, unsigned long parent_ip,
 
 static __always_inline void trace_stack(struct trace_array *tr)
 {
-	unsigned int trace_ctx;
-
-	trace_ctx = tracing_gen_ctx();
-
-	__trace_stack(tr, trace_ctx, FTRACE_STACK_SKIP);
+	__trace_stack(tr, tracing_gen_ctx_dec(), FTRACE_STACK_SKIP);
 }
 
 static void
-- 
2.47.2