[for-linus][PATCH 07/12] ring-buffer: Use current_context for safe per-CPU buffer swap

From: Tengda Wu <[email protected]>

The ring_buffer_swap_cpu() function currently checks the per-CPU
committing counter to determine if a buffer is actively being written to
before performing the swap. However, there exists a race window where
this check can be bypassed:

    ring_buffer_lock_reserve
        cpu_buffer = buffer->buffers[cpu];       // cpu_buffer_a
        rb_reserve_next_event
            rb_start_commit // inc committing
            if (unlikely(READ_ONCE(cpu_buffer->buffer) != buffer)) {...}
            __rb_reserve_next
                rb_move_tail
                    rb_end_commit(cpu_buffer);   // dec committing => 0
                    /* interrupt hits here, successfully swaps! */
                    local_inc(&cpu_buffer->committing);

    ring_buffer_unlock_commit
        cpu_buffer = buffer->buffers[cpu];      // cpu_buffer_b
        rb_commit
            rb_end_commit
            RB_WARN_ON(cpu_buffer, !local_read(&cpu_buffer->committing))
                                                // triggers warning

The committing counter can temporarily drop to 0 during a single write
operation (within rb_move_tail), creating a window where swap can
succeed even though the write is still in progress. This leads to
inconsistent buffer state and triggers the RB_WARN_ON in rb_commit().

Replace the committing counter check with current_context checks, which
are set at the entry of ring_buffer_lock_reserve() and remain valid
throughout the entire write operation, providing a reliable indicator of
buffer busy state during swap.

Cc: [email protected]
Fixes: 4239c38fe0b3 ("ring-buffer: Process commits whenever moving to a new page.")
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Tengda Wu <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
---
 kernel/trace/ring_buffer.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8e2485bb3aa8..58dc8995a88d 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6852,7 +6852,7 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
 {
 	struct ring_buffer_per_cpu *cpu_buffer_a;
 	struct ring_buffer_per_cpu *cpu_buffer_b;
-	int ret = -EINVAL;
+	int ret = -EBUSY;
 
 	if (!cpumask_test_cpu(cpu, buffer_a->cpumask) ||
 	    !cpumask_test_cpu(cpu, buffer_b->cpumask))
@@ -6893,10 +6893,10 @@ int ring_buffer_swap_cpu(struct trace_buffer *buffer_a,
 	atomic_inc(&cpu_buffer_a->record_disabled);
 	atomic_inc(&cpu_buffer_b->record_disabled);
 
-	ret = -EBUSY;
-	if (local_read(&cpu_buffer_a->committing))
+	/* Do not swap if either buffer is in the process of writing */
+	if (cpu_buffer_a->current_context)
 		goto out_dec;
-	if (local_read(&cpu_buffer_b->committing))
+	if (cpu_buffer_b->current_context)
 		goto out_dec;
 
 	/*
-- 
2.53.0