diff options
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/events/internal.h | 4 | ||||
| -rw-r--r-- | kernel/events/ring_buffer.c | 64 |
2 files changed, 52 insertions, 16 deletions
diff --git a/kernel/events/internal.h b/kernel/events/internal.h index 79c47076700a..3aef4191798c 100644 --- a/kernel/events/internal.h +++ b/kernel/events/internal.h @@ -24,7 +24,7 @@ struct ring_buffer { atomic_t poll; /* POLL_ for wakeups */ local_t head; /* write position */ - local_t nest; /* nested writers */ + unsigned int nest; /* nested writers */ local_t events; /* event limit */ local_t wakeup; /* wakeup stamp */ local_t lost; /* nr records lost */ @@ -41,7 +41,7 @@ struct ring_buffer { /* AUX area */ long aux_head; - local_t aux_nest; + unsigned int aux_nest; long aux_wakeup; /* last aux_watermark boundary crossed by aux_head */ unsigned long aux_pgoff; int aux_nr_pages; diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 674b35383491..ffb59a4ef4ff 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -38,7 +38,12 @@ static void perf_output_get_handle(struct perf_output_handle *handle) struct ring_buffer *rb = handle->rb; preempt_disable(); - local_inc(&rb->nest); + + /* + * Avoid an explicit LOAD/STORE such that architectures with memops + * can use them. + */ + (*(volatile unsigned int *)&rb->nest)++; handle->wakeup = local_read(&rb->wakeup); } @@ -46,17 +51,35 @@ static void perf_output_put_handle(struct perf_output_handle *handle) { struct ring_buffer *rb = handle->rb; unsigned long head; + unsigned int nest; + + /* + * If this isn't the outermost nesting, we don't have to update + * @rb->user_page->data_head. + */ + nest = READ_ONCE(rb->nest); + if (nest > 1) { + WRITE_ONCE(rb->nest, nest - 1); + goto out; + } again: + /* + * In order to avoid publishing a head value that goes backwards, + * we must ensure the load of @rb->head happens after we've + * incremented @rb->nest. + * + * Otherwise we can observe a @rb->head value before one published + * by an IRQ/NMI happening between the load and the increment. + */ + barrier(); head = local_read(&rb->head); /* - * IRQ/NMI can happen here, which means we can miss a head update. + * IRQ/NMI can happen here and advance @rb->head, causing our + * load above to be stale. */ - if (!local_dec_and_test(&rb->nest)) - goto out; - /* * Since the mmap() consumer (userspace) can run on a different CPU: * @@ -84,14 +107,23 @@ again: * See perf_output_begin(). */ smp_wmb(); /* B, matches C */ - rb->user_page->data_head = head; + WRITE_ONCE(rb->user_page->data_head, head); /* - * Now check if we missed an update -- rely on previous implied - * compiler barriers to force a re-read. + * We must publish the head before decrementing the nest count, + * otherwise an IRQ/NMI can publish a more recent head value and our + * write will (temporarily) publish a stale value. */ + barrier(); + WRITE_ONCE(rb->nest, 0); + + /* + * Ensure we decrement @rb->nest before we validate the @rb->head. + * Otherwise we cannot be sure we caught the 'last' nested update. + */ + barrier(); if (unlikely(head != local_read(&rb->head))) { - local_inc(&rb->nest); + WRITE_ONCE(rb->nest, 1); goto again; } @@ -330,6 +362,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle, struct perf_event *output_event = event; unsigned long aux_head, aux_tail; struct ring_buffer *rb; + unsigned int nest; if (output_event->parent) output_event = output_event->parent; @@ -360,13 +393,16 @@ void *perf_aux_output_begin(struct perf_output_handle *handle, if (!refcount_inc_not_zero(&rb->aux_refcount)) goto err; + nest = READ_ONCE(rb->aux_nest); /* * Nesting is not supported for AUX area, make sure nested * writers are caught early */ - if (WARN_ON_ONCE(local_xchg(&rb->aux_nest, 1))) + if (WARN_ON_ONCE(nest)) goto err_put; + WRITE_ONCE(rb->aux_nest, nest + 1); + aux_head = rb->aux_head; handle->rb = rb; @@ -394,7 +430,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle, if (!handle->size) { /* A, matches D */ event->pending_disable = smp_processor_id(); perf_output_wakeup(handle); - local_set(&rb->aux_nest, 0); + WRITE_ONCE(rb->aux_nest, 0); goto err_put; } } @@ -471,7 +507,7 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size) perf_event_aux_event(handle->event, aux_head, size, handle->aux_flags); - rb->user_page->aux_head = rb->aux_head; + WRITE_ONCE(rb->user_page->aux_head, rb->aux_head); if (rb_need_aux_wakeup(rb)) wakeup = true; @@ -483,7 +519,7 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size) handle->event = NULL; - local_set(&rb->aux_nest, 0); + WRITE_ONCE(rb->aux_nest, 0); /* can't be last */ rb_free_aux(rb); ring_buffer_put(rb); @@ -503,7 +539,7 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size) rb->aux_head += size; - rb->user_page->aux_head = rb->aux_head; + WRITE_ONCE(rb->user_page->aux_head, rb->aux_head); if (rb_need_aux_wakeup(rb)) { perf_output_wakeup(handle); handle->wakeup = rb->aux_wakeup + rb->aux_watermark; |