linux-kernel - Re: trace_pipe tentative fix (Re: [PATCH -tip 2/3] Tracing/ftrace: Adapt mmiotrace to the new type of print

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Date:	Mon, 29 Sep 2008 11:20:59 +0200
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Pekka Paalanen" <pq@....fi>
Cc:	mingo@...e.hu, rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: trace_pipe tentative fix (Re: [PATCH -tip 2/3] Tracing/ftrace: Adapt mmiotrace to the new type of print_line)

2008/9/28 Pekka Paalanen <pq@....fi>:
> On Sat, 27 Sep 2008 14:17:47 +0200
> "Frédéric Weisbecker" <fweisbec@...il.com> wrote:
>
>> 2008/9/26 Pekka Paalanen <pq@....fi>:
>> > Yes, I do disagree about printing stuff that doesn't belong there.
>> > First, I doubt printing bogus text is a right way to fix the
>> > early pipe EOF problem. Second, if you really have to do that,
>> > do it so that it obeys the mmiotrace log format specification.
>> > (I'd recommend a MARK entry.) The spec is in
>> > Documentation/tracers/mmiotrace.txt.
>>
>> No problem, I have one other option: ignoring and sleep again before
>> receiving another entries.
>> I can do another patch this week end to implement that.
>> The third patch only concerns the boot tracer. It relays to other
>> output functions.
>>
>> Just tell me if you agree with the ignoring...
>
> If I understand you suggestion, it looks like the right thing to do.
> Here is a tentative fix, which has not even been compile-tested.
>
> Is it so that the problem is triggered by consuming a trace entry
> which does not produce any output? If that entry is all there is
> in the ring at a time of a read call, then the last call to
> trace_seq_to_user() returns -EBUSY, because there is nothing to
> copy to user. What I failed to understand when I wrote that
> piece of code, is that returning 0 means EOF. The only cases
> when we do want to return an EOF are near the
>        while (trace_empty(iter)) {
> loop.
>
> Frederic, could you test the fix, and if it works, send it to Ingo?
>
>
> Thanks.
>
> ---
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 6ada059..d85659a 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2616,6 +2616,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>                        goto out;
>        }
>
> +waitagain:
>        while (trace_empty(iter)) {
>
>                if ((filp->f_flags & O_NONBLOCK)) {
> @@ -2749,8 +2750,13 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>        sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
>        if (iter->seq.readpos >= iter->seq.len)
>                trace_seq_reset(&iter->seq);
> +
> +       /*
> +        * If there was nothing to send to user, inspite of consuming trace
> +        * entries, go back to wait for more entries.
> +        */
>        if (sret == -EBUSY)
> -               sret = 0;
> +               goto waitagain;
>
>  out:
>        mutex_unlock(&trace_types_lock);
>

Ok I will test it this evening.
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