linux-kernel - Re: [PATCH] tracing: Fix race when concurrently splice_read trace

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Date:   Wed, 16 Aug 2023 15:23:08 -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 Sat, 12 Aug 2023 10:22:43 +0800
Zheng Yejian <zhengyejian1@...wei.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