Commit 32dc0042 authored May 23, 2025 by Masami Hiramatsu (Google) Committed by Steven Rostedt (Google) May 29, 2025

tracing: Reset last-boot buffers when reading out all cpu buffers

Reset the last-boot ring buffers when read() reads out all cpu
buffers through trace_pipe/trace_pipe_raw. This prevents ftrace to
unwind ring buffer read pointer next boot.

Note that this resets only when all per-cpu buffers are empty, and
read via read(2) syscall. For example, if you read only one of the
per-cpu trace_pipe, it does not reset it. Also, reading buffer by
splice(2) syscall does not reset because some data in the reader
(the last) page.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/174792929202.496143.8184644221859580999.stgit@mhiramat.tok.corp.google.com


Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

parent c2a08311

kernel/trace/trace.c

+22 −0

Original line number	Diff line number	Diff line
		@@ -6610,6 +6610,22 @@ static int tracing_wait_pipe(struct file *filp)
		return 1;
		}

		static bool update_last_data_if_empty(struct trace_array *tr)
		{
		if (!(tr->flags & TRACE_ARRAY_FL_LAST_BOOT))
		return false;

		if (!ring_buffer_empty(tr->array_buffer.buffer))
		return false;

		/*
		* If the buffer contains the last boot data and all per-cpu
		* buffers are empty, reset it from the kernel side.
		*/
		update_last_data(tr);
		return true;
		}

		/*
		* Consumer reader.
		*/
		@@ -6641,6 +6657,9 @@ tracing_read_pipe(struct file filp, char __user ubuf,
		}

		waitagain:
		if (update_last_data_if_empty(iter->tr))
		return 0;

		sret = tracing_wait_pipe(filp);
		if (sret <= 0)
		return sret;
		@@ -8167,6 +8186,9 @@ tracing_buffers_read(struct file filp, char __user ubuf,

		if (ret < 0) {
		if (trace_empty(iter) && !iter->closed) {
		if (update_last_data_if_empty(iter->tr))
		return 0;

		if ((filp->f_flags & O_NONBLOCK))
		return -EAGAIN;