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Date: Mon, 05 Jan 2009 10:09:36 -0500
From: Steven Rostedt <srostedt@...hat.com>
To: Indan Zupancic <indan@....nu>
Cc: Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix race in ring_buffer_consume(): Replace
ring_buffer_consume and ring_buffer_peek with ring_buffer_get_event
On Mon, 2008-12-29 at 18:34 +1100, Indan Zupancic wrote:
> Original mail was mangled, patch resent via git.
>
Indan,
I like the patch except for one thing. Could you make the
ring_buffer_get_event into a static function called rb_get_event. And
then you could have:
struct ring_buffer_event *
ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
{
rb_get_event(buffer, cpu, ts, 0);
}
struct ring_buffer_event *
ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
{
rb_get_event(buffer, cpu, ts, 1);
}
I just hate public functions that have flags that can get confusing.
Looking at code where I see:
ring_buffer_get_event(buffer, cpu, ts, 0);
I find it hard to know if the ",0" is correct or should be a ",1"
instead.
This would also make the patch only need to touch ring_buffer.c and not
all the places that use it.
Thanks,
-- Steve
> Signed-off-by: Indan Zupancic <indan@....nu>
> ---
> include/linux/ring_buffer.h | 4 +---
> kernel/trace/ring_buffer.c | 39 ++++++++-------------------------------
> kernel/trace/trace.c | 15 ++++++++-------
> kernel/trace/trace_selftest.c | 2 +-
> 4 files changed, 18 insertions(+), 42 deletions(-)
>
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index e097c2e..d9320bd 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -83,9 +83,7 @@ int ring_buffer_write(struct ring_buffer *buffer,
> unsigned long length, void *data);
>
> struct ring_buffer_event *
> -ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts);
> -struct ring_buffer_event *
> -ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts);
> +ring_buffer_get_event(struct ring_buffer *buffer, int cpu, u64 *ts, int consume);
>
> struct ring_buffer_iter *
> ring_buffer_read_start(struct ring_buffer *buffer, int cpu);
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 668bbb5..d4e5dbc 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1727,16 +1727,18 @@ static void rb_advance_iter(struct ring_buffer_iter *iter)
> }
>
> /**
> - * ring_buffer_peek - peek at the next event to be read
> + * ring_buffer_get_event - get the next event to be read
> * @buffer: The ring buffer to read
> * @cpu: The cpu to peak at
> * @ts: The timestamp counter of this event.
> + * @consume: Whether the event should be consumed.
> + * If true, sequential reads will keep returning a different event,
> + * and eventually empty the ring buffer if the producer is slower.
> *
> - * This will return the event that will be read next, but does
> - * not consume the data.
> + * This will return the event that will be read next.
> */
> struct ring_buffer_event *
> -ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
> +ring_buffer_get_event(struct ring_buffer *buffer, int cpu, u64 *ts, int consume)
> {
> struct ring_buffer_per_cpu *cpu_buffer;
> struct ring_buffer_event *event;
> @@ -1789,6 +1791,8 @@ ring_buffer_peek(struct ring_buffer *buffer, int cpu, u64 *ts)
> *ts = cpu_buffer->read_stamp + event->time_delta;
> ring_buffer_normalize_time_stamp(cpu_buffer->cpu, ts);
> }
> + if (consume)
> + rb_advance_reader(cpu_buffer);
> return event;
>
> default:
> @@ -1869,33 +1873,6 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
> }
>
> /**
> - * ring_buffer_consume - return an event and consume it
> - * @buffer: The ring buffer to get the next event from
> - *
> - * Returns the next event in the ring buffer, and that event is consumed.
> - * Meaning, that sequential reads will keep returning a different event,
> - * and eventually empty the ring buffer if the producer is slower.
> - */
> -struct ring_buffer_event *
> -ring_buffer_consume(struct ring_buffer *buffer, int cpu, u64 *ts)
> -{
> - struct ring_buffer_per_cpu *cpu_buffer;
> - struct ring_buffer_event *event;
> -
> - if (!cpu_isset(cpu, buffer->cpumask))
> - return NULL;
> -
> - event = ring_buffer_peek(buffer, cpu, ts);
> - if (!event)
> - return NULL;
> -
> - cpu_buffer = buffer->buffers[cpu];
> - rb_advance_reader(cpu_buffer);
> -
> - return event;
> -}
> -
> -/**
> * ring_buffer_read_start - start a non consuming read of the buffer
> * @buffer: The ring buffer to read from
> * @cpu: The cpu buffer to iterate over
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d86e325..f1329ed 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -938,7 +938,7 @@ peek_next_entry(struct trace_iterator *iter, int cpu, u64 *ts)
> if (buf_iter)
> event = ring_buffer_iter_peek(buf_iter, ts);
> else
> - event = ring_buffer_peek(iter->tr->buffer, cpu, ts);
> + event = ring_buffer_get_event(iter->tr->buffer, cpu, ts, 0);
>
> ftrace_enable_cpu();
>
> @@ -1002,7 +1002,7 @@ static void trace_consume(struct trace_iterator *iter)
> {
> /* Don't allow ftrace to trace into the ring buffers */
> ftrace_disable_cpu();
> - ring_buffer_consume(iter->tr->buffer, iter->cpu, &iter->ts);
> + ring_buffer_get_event(iter->tr->buffer, iter->cpu, &iter->ts, 1);
> ftrace_enable_cpu();
> }
>
> @@ -1310,8 +1310,9 @@ void trace_seq_print_cont(struct trace_seq *s, struct trace_iterator *iter)
> struct trace_entry *ent;
> struct trace_field_cont *cont;
> bool ok = true;
> + int cpu = iter->cpu;
>
> - ent = peek_next_entry(iter, iter->cpu, NULL);
> + ent = peek_next_entry(iter, cpu, NULL);
> if (!ent || ent->type != TRACE_CONT) {
> trace_seq_putc(s, '\n');
> return;
> @@ -1324,14 +1325,14 @@ void trace_seq_print_cont(struct trace_seq *s, struct trace_iterator *iter)
>
> ftrace_disable_cpu();
>
> - if (iter->buffer_iter[iter->cpu])
> - ring_buffer_read(iter->buffer_iter[iter->cpu], NULL);
> + if (iter->buffer_iter[cpu])
> + ring_buffer_read(iter->buffer_iter[cpu], NULL);
> else
> - ring_buffer_consume(iter->tr->buffer, iter->cpu, NULL);
> + ring_buffer_get_event(iter->tr->buffer, cpu, NULL, 1);
>
> ftrace_enable_cpu();
>
> - ent = peek_next_entry(iter, iter->cpu, NULL);
> + ent = peek_next_entry(iter, cpu, NULL);
> } while (ent && ent->type == TRACE_CONT);
>
> if (!ok)
> diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
> index 90bc752..3894989 100644
> --- a/kernel/trace/trace_selftest.c
> +++ b/kernel/trace/trace_selftest.c
> @@ -23,7 +23,7 @@ static int trace_test_buffer_cpu(struct trace_array *tr, int cpu)
> struct ring_buffer_event *event;
> struct trace_entry *entry;
>
> - while ((event = ring_buffer_consume(tr->buffer, cpu, NULL))) {
> + while ((event = ring_buffer_get_event(tr->buffer, cpu, NULL, 1))) {
> entry = ring_buffer_event_data(event);
>
> if (!trace_valid_entry(entry)) {
--
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