Commit 6865aff6 authored Dec 28, 2023 by Steven Rostedt (Google) Committed by sanglipeng May 10, 2024

tracing: Fix blocked reader of snapshot buffer

stable inclusion
from stable-v5.10.206
commit 8bf79dec73fe9ec97996a292cfacf89e12f1e74d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9O5W8

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8bf79dec73fe9ec97996a292cfacf89e12f1e74d

--------------------------------

commit 39a7dc23a1ed0fe81141792a09449d124c5953bd upstream.

If an application blocks on the snapshot or snapshot_raw files, expecting
to be woken up when a snapshot occurs, it will not happen. Or it may
happen with an unexpected result.

That result is that the application will be reading the main buffer
instead of the snapshot buffer. That is because when the snapshot occurs,
the main and snapshot buffers are swapped. But the reader has a descriptor
still pointing to the buffer that it originally connected to.

This is fine for the main buffer readers, as they may be blocked waiting
for a watermark to be hit, and when a snapshot occurs, the data that the
main readers want is now on the snapshot buffer.

But for waiters of the snapshot buffer, they are waiting for an event to
occur that will trigger the snapshot and they can then consume it quickly
to save the snapshot before the next snapshot occurs. But to do this, they
need to read the new snapshot buffer, not the old one that is now
receiving new data.

Also, it does not make sense to have a watermark "buffer_percent" on the
snapshot buffer, as the snapshot buffer is static and does not receive new
data except all at once.

Link: https://lore.kernel.org/linux-trace-kernel/20231228095149.77f5b45d@gandalf.local.home



Cc: stable@vger.kernel.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Fixes: debdd57f ("tracing: Make a snapshot feature available from userspace")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>

parent a054c3d9

kernel/trace/ring_buffer.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -866,7 +866,8 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
		/* make sure the waiters see the new index */
		smp_wmb();

		rb_wake_up_waiters(&rbwork->work);
		/* This can be called in any context */
		irq_work_queue(&rbwork->work);
		}

		/**

kernel/trace/trace.c

+17 −3

Original line number	Diff line number	Diff line
		@@ -1886,17 +1886,31 @@ update_max_tr_single(struct trace_array tr, struct task_struct tsk, int cpu)

		__update_max_tr(tr, tsk, cpu);
		arch_spin_unlock(&tr->max_lock);

		/* Any waiters on the old snapshot buffer need to wake up */
		ring_buffer_wake_waiters(tr->array_buffer.buffer, RING_BUFFER_ALL_CPUS);
		}
		#endif /* CONFIG_TRACER_MAX_TRACE */

		static int wait_on_pipe(struct trace_iterator *iter, int full)
		{
		int ret;

		/* Iterators are static, they should be filled or empty */
		if (trace_buffer_iter(iter, iter->cpu_file))
		return 0;

		return ring_buffer_wait(iter->array_buffer->buffer, iter->cpu_file,
		full);
		ret = ring_buffer_wait(iter->array_buffer->buffer, iter->cpu_file, full);

		#ifdef CONFIG_TRACER_MAX_TRACE
		/*
		* Make sure this is still the snapshot buffer, as if a snapshot were
		* to happen, this would now be the main buffer.
		*/
		if (iter->snapshot)
		iter->array_buffer = &iter->tr->max_buffer;
		#endif
		return ret;
		}

		#ifdef CONFIG_FTRACE_STARTUP_TEST
		@@ -7961,7 +7975,7 @@ tracing_buffers_splice_read(struct file file, loff_t ppos,
		if ((file->f_flags & O_NONBLOCK) \|\| (flags & SPLICE_F_NONBLOCK))
		goto out;

		ret = wait_on_pipe(iter, iter->tr->buffer_percent);
		ret = wait_on_pipe(iter, iter->snapshot ? 0 : iter->tr->buffer_percent);
		if (ret)
		goto out;