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Message-Id: <20230811204257.99df8ba60d591f5bace38615@kernel.org>
Date: Fri, 11 Aug 2023 20:42:57 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Zheng Yejian <zhengyejian1@...wei.com>
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 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?
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
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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