diff options
| -rw-r--r-- | include/uapi/linux/perf_event.h | 12 | ||||
| -rw-r--r-- | kernel/events/ring_buffer.c | 31 |
2 files changed, 34 insertions, 9 deletions
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h index 009a655a5d3..2fc1602e23b 100644 --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -456,13 +456,15 @@ struct perf_event_mmap_page { /* * Control data for the mmap() data buffer. * - * User-space reading the @data_head value should issue an rmb(), on - * SMP capable platforms, after reading this value -- see - * perf_event_wakeup(). + * User-space reading the @data_head value should issue an smp_rmb(), + * after reading this value. * * When the mapping is PROT_WRITE the @data_tail value should be - * written by userspace to reflect the last read data. In this case - * the kernel will not over-write unread data. + * written by userspace to reflect the last read data, after issueing + * an smp_mb() to separate the data read from the ->data_tail store. + * In this case the kernel will not over-write unread data. + * + * See perf_output_put_handle() for the data ordering. */ __u64 data_head; /* head in the data section */ __u64 data_tail; /* user-space written tail */ diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index cd55144270b..9c2ddfbf452 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -87,10 +87,31 @@ again: goto out; /* - * Publish the known good head. Rely on the full barrier implied - * by atomic_dec_and_test() order the rb->head read and this - * write. + * Since the mmap() consumer (userspace) can run on a different CPU: + * + * kernel user + * + * READ ->data_tail READ ->data_head + * smp_mb() (A) smp_rmb() (C) + * WRITE $data READ $data + * smp_wmb() (B) smp_mb() (D) + * STORE ->data_head WRITE ->data_tail + * + * Where A pairs with D, and B pairs with C. + * + * I don't think A needs to be a full barrier because we won't in fact + * write data until we see the store from userspace. So we simply don't + * issue the data WRITE until we observe it. Be conservative for now. + * + * OTOH, D needs to be a full barrier since it separates the data READ + * from the tail WRITE. + * + * For B a WMB is sufficient since it separates two WRITEs, and for C + * an RMB is sufficient since it separates two READs. + * + * See perf_output_begin(). */ + smp_wmb(); rb->user_page->data_head = head; /* @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output_handle *handle, * Userspace could choose to issue a mb() before updating the * tail pointer. So that all reads will be completed before the * write is issued. + * + * See perf_output_put_handle(). */ tail = ACCESS_ONCE(rb->user_page->data_tail); - smp_rmb(); + smp_mb(); offset = head = local_read(&rb->head); head += size; if (unlikely(!perf_output_space(rb, tail, offset, head))) |