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

Steven Rostedt
2009-02-23T15:38:08Z

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@goodmis.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.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