Re: [PATCH v2] tracing: Introduce pipe_cpumask to avoid race on trace_pipes
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Date: 2023-08-20 13:18:47
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On Sat, 19 Aug 2023 10:42:57 +0900 Masami Hiramatsu (Google) [off-list ref] wrote:
On Fri, 18 Aug 2023 11:53:22 -0400 Steven Rostedt [off-list ref] wrote:quoted
On Fri, 18 Aug 2023 23:23:01 +0900 Masami Hiramatsu (Google) [off-list ref] wrote:quoted
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).
Hmm, I couldn't reproduce it. (maybe a host security update change something?) Anyway, I confirmed that the ring buffer pages will not be consumed unless splice_to_pipe() succeeded. Thank you,
quoted hunk ↗ jump to hunk
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>
-- Masami Hiramatsu (Google) [off-list ref]