ftrace: Fix preemption accounting for stacktrace filter command

JIRA: https://issues.redhat.com/browse/RHEL-115358

commit 11aff32
Author: pengdonglin <[email protected]>
Date:   Mon May 12 17:42:46 2025 +0800

    ftrace: Fix preemption accounting for stacktrace filter command

    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: [email protected]
    Fixes: 36590c5 ("tracing: Merge irqflags + preempt counter.")
    Link: https://lore.kernel.org/[email protected]
    Signed-off-by: pengdonglin <[email protected]>
    Signed-off-by: Steven Rostedt (Google) <[email protected]>

Signed-off-by: Jerome Marchand <[email protected]>

Author: jeromemarchand

kernel/trace/trace_functions.c

@@ -574,11 +574,7 @@ 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