Messages in this thread | | | Date | Wed, 16 Aug 2023 15:23:08 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] tracing: Fix race when concurrently splice_read trace_pipe |
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On Sat, 12 Aug 2023 10:22:43 +0800 Zheng Yejian <zhengyejian1@huawei.com> wrote:
> > >>>> 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? > > If 'iter->ent' was not consumed, trace_find_next_entry_inc() will find > it again, will it? > > -- Zheng Yejian > > > > >> > >> 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; > > } > > > > Thanks for pointing this out!
Just to get this moving again, I believe we should add a ref count to trace_pipe and the per_cpu trace_pipes, where if they are opened, nothing else can read it.
Opening trace_pipe locks all per_cpu ref counts, if any of them are open, then the trace_pipe open will fail (and releases any ref counts it had taken).
Opening a per_cpu trace_pipe will up the ref count for just that CPU buffer. This will allow multiple tasks to read different per_cpu trace_pipe files, but will prevent the main trace_pipe file from being opened.
Does that work for this?
-- Steve
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