[PATCH v8 11/21] ring-buffer: Use irqsave for the reader lock in ring_buffer_poll_remote

[Vincent Donnefort <vdonnefort@google.com>]

Calling rb_wakeups with the reader lock held but interrupts enabled can
lead to a deadlock: the irq_work might run on the same CPU, but will
block when acquiring that very same reader spinlock.

First, rb_wakeups does not even need to be called under the reader lock.
Move the function outside of the reader lock scope.

Second, the reader lock must be acquired with IRQs disabled anyway. Use
the irqsave variant of the spinlock.

Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260602171146.2238998-1-vdonnefort%40google.com # patch 8
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 kernel/trace/ring_buffer.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 1fd28a72e6ee..4dab77883bd5 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6902,13 +6902,17 @@ int ring_buffer_poll_remote(struct trace_buffer *buffer, int cpu)
 	struct ring_buffer_per_cpu *cpu_buffer;
 
 	if (cpu != RING_BUFFER_ALL_CPUS) {
+		bool wakeup;
+
 		if (!cpumask_test_cpu(cpu, buffer->cpumask))
 			return -EINVAL;
 
 		cpu_buffer = buffer->buffers[cpu];
 
-		guard(raw_spinlock)(&cpu_buffer->reader_lock);
-		if (rb_read_remote_meta_page(cpu_buffer))
+		scoped_guard(raw_spinlock_irqsave, &cpu_buffer->reader_lock)
+			wakeup = rb_read_remote_meta_page(cpu_buffer);
+
+		if (wakeup)
 			rb_wakeups(buffer, cpu_buffer);
 
 		return 0;
@@ -6923,7 +6927,7 @@ int ring_buffer_poll_remote(struct trace_buffer *buffer, int cpu)
 	for_each_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
-		guard(raw_spinlock)(&cpu_buffer->reader_lock);
+		guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
 		rb_read_remote_meta_page(cpu_buffer);
 	}
 
-- 
2.56.0.455.g6c5a503b38-goog