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Message-ID: <f7af687f-2376-fede-fa22-f776811c48f1@huawei.com>
Date: Fri, 11 Aug 2023 20:37:07 +0800
From: Zheng Yejian <zhengyejian1@...wei.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
CC: <rostedt@...dmis.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 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:
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
... ...
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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