tree a45808e0aa8e5f50ca934a571660f24d6b2d8e1d
parent 74bf4076f2ed79b5510440b72a561823a8852ec0
author Steven Rostedt <srostedt@redhat.com> 1264450667 -0500
committer Steven Rostedt <rostedt@goodmis.org> 1264540170 -0500

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>