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Message-ID: <20230811152413.76d5b72e@gandalf.local.home>
Date: Fri, 11 Aug 2023 15:24:13 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Zheng Yejian <zhengyejian1@...wei.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
<laijs@...fujitsu.com>, <linux-kernel@...r.kernel.org>,
<linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix race when concurrently splice_read
trace_pipe
On Fri, 11 Aug 2023 20:37:07 +0800
Zheng Yejian <zhengyejian1@...wei.com> wrote:
> On 2023/8/11 19:42, Masami Hiramatsu (Google) wrote:
> > On Thu, 10 Aug 2023 20:39:05 +0800
> > Zheng Yejian <zhengyejian1@...wei.com> wrote:
> >
> >> When concurrently splice_read file trace_pipe and per_cpu/cpu*/trace_pipe,
> >> there are more data being read out than expected.
> >>
> >> The root cause is that in tracing_splice_read_pipe(), an entry is found
> >> outside locks, it may be read by multiple readers or consumed by other
> >> reader as starting printing it.
> >>
> >> To fix it, change to find entry after holding locks.
> >>
> >> Fixes: 7e53bd42d14c ("tracing: Consolidate protection of reader access to the ring buffer")
> >> Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
> >> ---
> >> kernel/trace/trace.c | 10 ++++++----
> >> 1 file changed, 6 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> >> index b8870078ef58..f169d33b948f 100644
> >> --- a/kernel/trace/trace.c
> >> +++ b/kernel/trace/trace.c
> >> @@ -7054,14 +7054,16 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
> >> if (ret <= 0)
> >> goto out_err;
> >>
> >> - if (!iter->ent && !trace_find_next_entry_inc(iter)) {
> >> + trace_event_read_lock();
> >> + trace_access_lock(iter->cpu_file);
> >> +
> >> + if (!trace_find_next_entry_inc(iter)) {
> >
> > It seems you skips '!iter->ent' check. Is there any reason for this change?
>
> IIUC, 'iter->ent' may be the entry that was found but not consumed
> in last call tracing_splice_read_pipe(), and in this call, 'iter->ent'
> may have being consumed, so we may should find a new 'iter->ent' before
> printing it in tracing_fill_pipe_page(), see following reduced codes:
And if it wasn't consumed? We just lost it?
>
> tracing_splice_read_pipe() {
> if (!iter->ent && !trace_find_next_entry_inc(iter)) { // 1. find
> entry here
> ... ...
> }
> tracing_fill_pipe_page() {
> for (;;) {
> ... ...
> ret = print_trace_line(iter); // 2. print entry
> ... ...
You missed:
count = trace_seq_used(&iter->seq) - save_len;
if (rem < count) {
rem = 0;
iter->seq.seq.len = save_len;
Where the above just threw away what was printed in the above
"print_trace_line()", and it never went to console.
break;
}
-- Steve
> if (!trace_find_next_entry_inc()) { // 3. find next entry
> ... ...
> break;
> }
> }
>
> --
>
> Thanks,
> Zheng Yejian
>
> >
> > Thank you,
> >
> >> + trace_access_unlock(iter->cpu_file);
> >> + trace_event_read_unlock();
> >> ret = -EFAULT;
> >> goto out_err;
> >> }
> >>
> >> - trace_event_read_lock();
> >> - trace_access_lock(iter->cpu_file);
> >> -
> >> /* Fill as many pages as possible. */
> >> for (i = 0, rem = len; i < spd.nr_pages_max && rem; i++) {
> >> spd.pages[i] = alloc_page(GFP_KERNEL);
> >> --
> >> 2.25.1
> >>
> >
> >
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