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.
Fixes: 4239c38fe0b3 ("ring-buffer: Process commits whenever moving to a new
page.")
Cc: [email protected]
Signed-off-by: Tengda Wu <[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 e863eb0d10f7..28d6a3812da0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6850,7 +6850,7 @@ 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))
@@ -6891,10 +6891,10 @@ 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;
/*
--
2.34.1
