Re: [PATCH] trace/ring_buffer: Fix locking order in ring_buffer_map()
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2025-10-08 14:50:12
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On Sun, 5 Oct 2025 19:46:36 +0530 ssrane_b23@ee.vjti.ac.in wrote:
From: Shaurya Rane <redacted> The kernel's lockdep validator detected a circular locking dependency in ring_buffer_map(). The function was acquiring the per-CPU 'cpu_buffer->mapping_lock' before the global 'buffer->mutex'.
You should either have a link to the email reporting the lockdep splat, or post it in the change log. I'd like to know exactly what the race was.
quoted hunk ↗ jump to hunk
This violates the established locking hierarchy where 'buffer->mutex' should be acquired first, leading to a potential deadlock. Fix this by reordering the mutex acquisition to lock 'buffer->mutex' before 'cpu_buffer->mapping_lock', satisfying the lockdep requirements and preventing the deadlock. Reported-by: syzbot+c530b4d95ec5cd4f33a7@syzkaller.appspotmail.com Signed-off-by: Shaurya Rane <redacted> --- kernel/trace/ring_buffer.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 43460949ad3f..82c3d5d2dcf6 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c@@ -7222,9 +7222,10 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu, if (!cpumask_test_cpu(cpu, buffer->cpumask)) return -EINVAL; - +
Added white space?
cpu_buffer = buffer->buffers[cpu]; - +
More added white space?
+ guard(mutex)(&buffer->mutex); guard(mutex)(&cpu_buffer->mapping_lock);
You state that you are reversing the order here, but all I see is you added taking the buffer->mutex lock. If there was a reverse order, then I'm assuming that later on in this function the buffer->mutex is taken again. That would cause a deadlock. What exactly are you reversing? -- Steve
if (cpu_buffer->user_mapped) {