linux-kernel - Re: trace_pipe tentative fix

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Message-ID: <20080928215815.3a9aa355@daedalus.pq.iki.fi>
Date:	Sun, 28 Sep 2008 21:58:15 +0300
From:	Pekka Paalanen <pq@....fi>
To:	"Frédéric Weisbecker" <fweisbec@...il.com>
Cc:	mingo@...e.hu, rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: trace_pipe tentative fix

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);
--
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