Re: [PATCH 14/26] trace: Convert various locks to raw_spinlock

Frederic Weisbecker
2010-01-12T03:39:37Z

On Mon, Jan 11, 2010 at 10:26:44PM +0100, John Kacur wrote:
> Convert locks that cannot sleep in preempt-rt to raw_spinlocks.
>
> See also: 87654a70523a8c5baadcbbc07d80cbae8f912837
>
> Signed-off-by: John Kacur <jkacur@redhat.com>
> ---
> kernel/trace/ring_buffer.c | 52 +++++++++++++++++++++---------------------
> kernel/trace/trace.c | 10 ++++----
> kernel/trace/trace_irqsoff.c | 6 ++--
> 3 files changed, 34 insertions(+), 34 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 2326b04..ffaddc5 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -422,7 +422,7 @@ int ring_buffer_print_page_header(struct trace_seq *s)
> struct ring_buffer_per_cpu {
> int cpu;
> struct ring_buffer *buffer;
> - spinlock_t reader_lock; /* serialize readers */
> + raw_spinlock_t reader_lock; /* serialize readers */

Why this one? This is a reader lock, not taken in any tracing fast path
places. Why should it never sleep in rt, it's taken by a reader of the ring
buffer, which doesn't seem to me involved in any critical work.

I may be wrong though, better wait for Steve to correct me
if needed.

In any case, the changelog needs more details about the individual
purpose of this patch.

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 0df1b0f..0c6bbcb 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -258,7 +258,7 @@ unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
> TRACE_ITER_GRAPH_TIME;
>
> static int trace_stop_count;
> -static DEFINE_SPINLOCK(tracing_start_lock);
> +static DEFINE_RAW_SPINLOCK(tracing_start_lock);

Same here. I don't understand why this should never sleep in -rt.
This is not a critical lock. It is taken in rare and not critical
places, mostly in reader places when we open/release a trace
file and also in tracing selftests.

> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
> index 2974bc7..60ba58e 100644
> --- a/kernel/trace/trace_irqsoff.c
> +++ b/kernel/trace/trace_irqsoff.c
> @@ -23,7 +23,7 @@ static int tracer_enabled __read_mostly;
>
> static DEFINE_PER_CPU(int, tracing_cpu);
>
> -static DEFINE_SPINLOCK(max_trace_lock);
> +static DEFINE_RAW_SPINLOCK(max_trace_lock);

But there yeah, it does seem necessary as it is involved
in irqsoff tracing fastpath.

This needs a comment though.

Thanks.