Commit 492a74f4 authored Jan 25, 2010 by Steven Rostedt Committed by Steven Rostedt Jan 26, 2010

ring-buffer: Check if ring buffer iterator has stale data



Usually reads of the ring buffer is performed by a single task.
There are two types of reads from the ring buffer.

One is a consuming read which will consume the entry that was read
and the next read will be the entry that follows.

The other is an iterator that will let the user read the contents of
the ring buffer without modifying it. When an iterator is allocated,
writes to the ring buffer are disabled to protect the iterator.

The problem exists when consuming reads happen while an iterator is
allocated. Specifically, the kind of read that swaps out an entire
page (used by splice) and replaces it with a new read. If the iterator
is on the page that is swapped out, then the next read may read
from this swapped out page and return garbage.

This patch adds a check when reading the iterator to make sure that
the iterator contents are still valid. If a consuming read has taken
place, the iterator is reset.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

parent 74bf4076

kernel/trace/ring_buffer.c

+13 −0

Original line number	Diff line number	Diff line
		@@ -464,6 +464,8 @@ struct ring_buffer_iter {
		struct ring_buffer_per_cpu *cpu_buffer;
		unsigned long head;
		struct buffer_page *head_page;
		struct buffer_page *cache_reader_page;
		unsigned long cache_read;
		u64 read_stamp;
		};

		@@ -2716,6 +2718,8 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
		iter->read_stamp = cpu_buffer->read_stamp;
		else
		iter->read_stamp = iter->head_page->page->time_stamp;
		iter->cache_reader_page = cpu_buffer->reader_page;
		iter->cache_read = cpu_buffer->read;
		}

		/**
		@@ -3066,6 +3070,15 @@ rb_iter_peek(struct ring_buffer_iter iter, u64 ts)
		cpu_buffer = iter->cpu_buffer;
		buffer = cpu_buffer->buffer;

		/*
		* Check if someone performed a consuming read to
		* the buffer. A consuming read invalidates the iterator
		* and we need to reset the iterator in this case.
		*/
		if (unlikely(iter->cache_read != cpu_buffer->read \|\|
		iter->cache_reader_page != cpu_buffer->reader_page))
		rb_iter_reset(iter);

		again:
		/*
		* We repeat when a timestamp is encountered.

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