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SubjectRe: trace_pipe tentative fix
On Sun, 28 Sep 2008 20:12:59 +0300
Pekka Paalanen <pq@iki.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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