linux-kernel - Re: [PATCH] tracing: wake up tasks reading trace_pipe on write to trace

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Message-ID: <1281127136.3352.13.camel@gandalf.stny.rr.com>
Date:	Fri, 06 Aug 2010 16:38:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] tracing: wake up tasks reading trace_pipe on write to
 trace_marker

On Wed, 2010-07-28 at 00:44 +0200, Marcin Slusarz wrote:
> Currently we rely on other code periodically waking up trace reader.
> If there aren't any other data than markers, reader will never be woken up.
> Fix it.
> 
> Signed-off-by: Marcin Slusarz <marcin.slusarz@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> ---
>  kernel/trace/trace.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 086d363..02e04c8 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1520,6 +1520,7 @@ int trace_array_vprintk(struct trace_array *tr,
>  	if (!filter_check_discard(call, entry, buffer, event)) {
>  		ring_buffer_unlock_commit(buffer, event);
>  		ftrace_trace_stack(buffer, irq_flags, 6, pc);
> +		trace_wake_up();
>  	}
>  

This can't work. trace_printk() and friends must be able to be used
anywhere. This can cause race conditions with the rq locks in the
scheduler.

But you do bring up a good idea. That is, perhaps we should have a way
to attach to known safe tracepoints that we can hook to to check if a
wake up should happen or not.

-- Steve


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