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.
Add a non-NULL check on the return value of trace_find_next_entry_inc
prior to invoking trace_printk_seq, ensuring the `iter.seq` is properly
populated before subsequent operations.
Fixes: d769041f8653 ("ring_buffer: implement new locking")
Signed-off-by: Tengda Wu <[email protected]>
---
kernel/trace/trace.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4283ed4e8f59..efd82e5f1e25 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10632,10 +10632,10 @@ 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)
--
2.34.1
