linux-kernel - Re: [PATCH] tracing/ftrace: add missing wake-up on some callsites

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Date:	Mon, 23 Feb 2009 10:37:52 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] tracing/ftrace: add missing wake-up on some callsites



On Sun, 22 Feb 2009, Frederic Weisbecker wrote:

> Impact: fix unwaken pipe
> 
> Now that we use a common wakeup infrastructure, we must append a wakeup
> on few callsites which lack it or tasks reading trace_pipe will not be
> awaken when events come on few tracers.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/trace/trace.c             |    2 ++
>  kernel/trace/trace_branch.c      |    2 ++
>  kernel/trace/trace_hw_branches.c |    2 ++
>  3 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index e1f3b99..7f450b6 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3055,6 +3055,8 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
>   out:
>  	preempt_enable_notrace();
>  
> +	trace_wake_up();
> +
>  	return len;
>  }
>  EXPORT_SYMBOL_GPL(trace_vprintk);
> diff --git a/kernel/trace/trace_branch.c b/kernel/trace/trace_branch.c
> index c2e68d4..8c8f8c0 100644
> --- a/kernel/trace/trace_branch.c
> +++ b/kernel/trace/trace_branch.c
> @@ -78,6 +78,8 @@ probe_likely_condition(struct ftrace_branch_data *f, int val, int expect)
>   out:
>  	atomic_dec(&tr->data[cpu]->disabled);
>  	local_irq_restore(flags);
> +
> +	trace_wake_up();
>  }
>  
>  static inline
> diff --git a/kernel/trace/trace_hw_branches.c b/kernel/trace/trace_hw_branches.c
> index 3561aac..ddd87fd 100644
> --- a/kernel/trace/trace_hw_branches.c
> +++ b/kernel/trace/trace_hw_branches.c
> @@ -202,6 +202,8 @@ void trace_hw_branch(u64 from, u64 to)
>   out:
>  	atomic_dec(&tr->data[cpu]->disabled);
>  	local_irq_restore(irq1);
> +
> +	trace_wake_up();
>  }
>  
>  static void trace_bts_at(const struct bts_trace *trace, void *at)

Ah, we don't wake up purposely on these three places. ftrace_printk is 
meant to be called anywhere (including the scheduler). And the branch 
tracers are also allowed to be called anywhere (they usually are).

Calling "wake_up" from any of these can easily cause a dead lock with the 
run queue lock, because all three can be called from with in the 
scheduler.

Sorry, but I have to NACK this change.

-- Steve
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