Re: [PATCH v2] tracing: Introduce pipe_cpumask to avoid race on trace_pipes

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 18 Aug 2023 11:53:22 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 18 Aug 2023 23:23:01 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > It uses trace_pipe_raw. I guess if splice(from trace_pipe_raw to virtio-serial)
> > returns -1 and errno == EAGAIN, the trace data will be lost?
> 
> It shouldn't. If it does, then there's likely a bug. The code will block
> and if an interrupt comes in it will return immediately without reading
> from the buffer.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/trace.c#n8262
> 
> I don't see where it would return -EINTR and consume data, but I may be
> missing something.

Hmm, I suspect the case if the spilice_to_pipe() returns -EAGAIN.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/trace.c#n8491

It seems not handling such case.

Anyway, I also think something wrong in virtio-serial (or misusing?), since
it can not read anything from the host sometimes. I just setup the virtio-trace
with below patch (ignore EAGAIN).


From 92242480285448360c9390a743ea7b3751bb3e61 Mon Sep 17 00:00:00 2001
From: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Date: Thu, 17 Aug 2023 14:08:40 +0900
Subject: [PATCH 1/3] tools/virtio-trace: Ignore EAGAIN error on splice()

splice() can return EAGAIN error instead of returning 0 size read.
In that case, wait a while and try to call splice() again.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 tools/virtio/virtio-trace/trace-agent-rw.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/virtio/virtio-trace/trace-agent-rw.c b/tools/virtio/virtio-trace/trace-agent-rw.c
index ddfe7875eb16..e8a4c4f0c499 100644
--- a/tools/virtio/virtio-trace/trace-agent-rw.c
+++ b/tools/virtio/virtio-trace/trace-agent-rw.c
@@ -8,6 +8,7 @@
  */
 
 #define _GNU_SOURCE
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -127,10 +128,10 @@ static void *rw_thread_main(void *thread_info)
 		rlen = splice(ts->in_fd, NULL, ts->read_pipe, NULL,
 				ts->pipe_size, SPLICE_F_MOVE | SPLICE_F_MORE);
 
-		if (rlen < 0) {
-			pr_err("Splice_read in rw-thread(%d)\n", ts->cpu_num);
+		if (rlen < 0 && errno != EAGAIN) {
+			pr_err("Splice_read error (%d) in rw-thread(%d)\n", errno, ts->cpu_num);
 			goto error;
-		} else if (rlen == 0) {
+		} else if (rlen == 0 || errno == EAGAIN) {
 			/*
 			 * If trace data do not exist or are unreadable not
 			 * for exceeding the page size, splice_read returns
-- 
2.34.1


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>