ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

When calling ftrace_dump_one() concurrently with reading trace_pipe,
a WARN_ON_ONCE() in trace_printk_seq() can be triggered due to a race
condition.

The issue occurs because:

CPU0 (ftrace_dump)                              CPU1 (reader)
echo z > /proc/sysrq-trigger

!trace_empty(&iter)
trace_iterator_reset(&iter) <- len = size = 0
                                                cat /sys/kernel/tracing/trace_pipe
trace_find_next_entry_inc(&iter)
  __find_next_entry
    ring_buffer_empty_cpu <- all empty
  return NULL

trace_printk_seq(&iter.seq)
  WARN_ON_ONCE(s->seq.len >= s->seq.size)

In the context between trace_empty() and trace_find_next_entry_inc()
during ftrace_dump, the ring buffer data was consumed by other readers.
This caused trace_find_next_entry_inc to return NULL, failing to populate
`iter.seq`. At this point, due to the prior trace_iterator_reset, both
`iter.seq.len` and `iter.seq.size` were set to 0. Since they are equal,
the WARN_ON_ONCE condition is triggered.

Move the trace_printk_seq() into the if block that checks to make sure the
return value of trace_find_next_entry_inc() is non-NULL in
ftrace_dump_one(), ensuring the 'iter.seq' is properly populated before
subsequent operations.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Ingo Molnar <mingo@elte.hu>
Link: https://lore.kernel.org/20250822033343.3000289-1-wutengda@huaweicloud.com
Fixes: d769041f8653 ("ring_buffer: implement new locking")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8d8935ed416d..1b7db732c0b1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10638,10 +10638,10 @@ static void ftrace_dump_one(struct trace_array *tr, enum ftrace_dump_mode dump_m
 			ret = print_trace_line(&iter);
 			if (ret != TRACE_TYPE_NO_CONSUME)
 				trace_consume(&iter);
+
+			trace_printk_seq(&iter.seq);
 		}
 		touch_nmi_watchdog();
-
-		trace_printk_seq(&iter.seq);
 	}
 
 	if (!cnt)