linux-kernel - Re: [PATCH -next 1/8] rcu: Fix rcu_read

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Message-ID: <20260102122807.7025fc87@gandalf.local.home>
Date: Fri, 2 Jan 2026 12:28:07 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: "Paul E . McKenney" <paulmck@...nel.org>, Boqun Feng
 <boqun.feng@...il.com>, rcu@...r.kernel.org, Frederic Weisbecker
 <frederic@...nel.org>, Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Josh
 Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan
 <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>, Shuah Khan
 <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, Yao Kai <yaokai34@...wei.com>, Tengda Wu
 <wutengda2@...wei.com>
Subject: Re: [PATCH -next 1/8] rcu: Fix rcu_read_unlock() deadloop due to
 softirq

On Thu,  1 Jan 2026 11:34:10 -0500
Joel Fernandes <joelagnelf@...dia.com> wrote:

>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  unwind_next_frame+0x203/0x9b0
>  __unwind_start+0x15d/0x1c0
>  arch_stack_walk+0x62/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180
>  trace_buffer_unlock_commit_regs+0x6d/0x220
>  trace_event_buffer_commit+0x5c/0x260
>  trace_event_raw_event_softirq+0x47/0x80
>  raise_softirq_irqoff+0x6e/0xa0
>  rcu_read_unlock_special+0xb1/0x160
>  unwind_next_frame+0x203/0x9b0
>  __unwind_start+0x15d/0x1c0
>  arch_stack_walk+0x62/0xf0
>  stack_trace_save+0x48/0x70
>  __ftrace_trace_stack.constprop.0+0x144/0x180

Stacktrace should have recursion protection too.

Can you try this patch to see if it would have fixed the problem too?

-- Steve

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index ae04054a1be3..e6ca052b2a85 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -34,6 +34,13 @@ enum {
 	TRACE_INTERNAL_SIRQ_BIT,
 	TRACE_INTERNAL_TRANSITION_BIT,
 
+	/* Internal event use recursion bits */
+	TRACE_INTERNAL_EVENT_BIT,
+	TRACE_INTERNAL_EVENT_NMI_BIT,
+	TRACE_INTERNAL_EVENT_IRQ_BIT,
+	TRACE_INTERNAL_EVENT_SIRQ_BIT,
+	TRACE_INTERNAL_EVENT_TRANSITION_BIT,
+
 	TRACE_BRANCH_BIT,
 /*
  * Abuse of the trace_recursion.
@@ -58,6 +65,8 @@ enum {
 
 #define TRACE_LIST_START	TRACE_INTERNAL_BIT
 
+#define TRACE_EVENT_START	TRACE_INTERNAL_EVENT_BIT
+
 #define TRACE_CONTEXT_MASK	((1 << (TRACE_LIST_START + TRACE_CONTEXT_BITS)) - 1)
 
 /*
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2d387d56dcd4..e145d1c7f604 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3013,6 +3013,11 @@ static void __ftrace_trace_stack(struct trace_array *tr,
 	struct ftrace_stack *fstack;
 	struct stack_entry *entry;
 	int stackidx;
+	int bit;
+
+	bit = trace_test_and_set_recursion(_THIS_IP_, _RET_IP_, TRACE_EVENT_START);
+	if (bit < 0)
+		return;
 
 	/*
 	 * Add one, for this function and the call to save_stack_trace()
@@ -3081,6 +3086,7 @@ static void __ftrace_trace_stack(struct trace_array *tr,
 	/* Again, don't let gcc optimize things here */
 	barrier();
 	__this_cpu_dec(ftrace_stack_reserve);
+	trace_clear_recursion(bit);
 }
 
 static inline void ftrace_trace_stack(struct trace_array *tr,