linux-kernel - Re: trace_pipe tentative fix

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Date:	Mon, 29 Sep 2008 19:49:46 +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

2008/9/28 Pekka Paalanen <pq@....fi>:
> On Sun, 28 Sep 2008 20:12:59 +0300
> Pekka Paalanen <pq@....fi> wrote:
>
>> 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?
>
> Whoops, sret was left in a bad state. Here's a new one.
>
> ---
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 6ada059..16b8a22 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2605,7 +2605,6 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>        sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
>        if (sret != -EBUSY)
>                return sret;
> -       sret = 0;
>
>        trace_seq_reset(&iter->seq);
>
> @@ -2616,6 +2615,8 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>                        goto out;
>        }
>
> +waitagain:
> +       sret = 0;
>        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);
>

Tested! And, no problem as far as I can see :)
So I'm going to send the new patchset.

Thanks.
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