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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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      Last update: 2008-09-28 21:01    [W:0.022 / U:32.004 seconds]
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