ftrace: Fix preemption accounting for stacktrace trigger command

When using the stacktrace trigger command to trace syscalls, the
preemption count was consistently reported as 1 when the system call
event itself had 0 (".").

For example:

root@ubuntu22-vm:/sys/kernel/tracing/events/syscalls/sys_enter_read
$ echo stacktrace > trigger
$ echo 1 > enable

    sshd-416     [002] .....   232.864910: sys_read(fd: a, buf: 556b1f3221d0, count: 8000)
    sshd-416     [002] ...1.   232.864913: <stack trace>
 => ftrace_syscall_enter
 => syscall_trace_enter
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe

The root cause is that the trace framework disables preemption in __DO_TRACE before
invoking the trigger callback.

Use the tracing_gen_ctx_dec() that will accommodate for the increase of
the preemption count in __DO_TRACE when calling the callback. The result
is the accurate reporting of:

    sshd-410     [004] .....   210.117660: sys_read(fd: 4, buf: 559b725ba130, count: 40000)
    sshd-410     [004] .....   210.117662: <stack trace>
 => ftrace_syscall_enter
 => syscall_trace_enter
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe

Cc: stable@vger.kernel.org
Fixes: ce33c845b030c ("tracing: Dump stacktrace trigger to the corresponding instance")
Link: https://lore.kernel.org/20250512094246.1167956-1-dolinux.peng@gmail.com
Signed-off-by: pengdonglin <dolinux.peng@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index b66b6d235d9130c49ed1c02ad280baa7e30de667..6e87ae2a1a66bf2cb1274ff5569de3d1f072abcc 100644 (file)
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1560,7 +1560,7 @@ stacktrace_trigger(struct event_trigger_data *data,
        struct trace_event_file *file = data->private_data;
 
        if (file)
-               __trace_stack(file->tr, tracing_gen_ctx(), STACK_SKIP);
+               __trace_stack(file->tr, tracing_gen_ctx_dec(), STACK_SKIP);
        else
                trace_dump_stack(STACK_SKIP);
 }