From: Peter Hurley <peter@hurleysoftware.com>
Date: Fri, 2 May 2014 14:56:12 +0000 (-0400)
Subject: tty: Fix lockless tty buffer race
X-Git-Tag: omap-for-v3.16/fixes-against-rc1~216^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2;p=pandora-kernel.git

tty: Fix lockless tty buffer race

Commit 6a20dbd6caa2358716136144bf524331d70b1e03,
"tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc"
correctly identifies an unsafe race condition between
__tty_buffer_request_room() and flush_to_ldisc(), where the consumer
flush_to_ldisc() prematurely advances the head before consuming the
last of the data committed. For example:

           CPU 0                     |            CPU 1
__tty_buffer_request_room            | flush_to_ldisc
  ...                                |   ...
                                     |   count = head->commit - head->read
  n = tty_buffer_alloc()             |
  b->commit = b->used                |
  b->next = n                        |
                                     |   if (!count)                /* T */
                                     |     if (head->next == NULL)  /* F */
                                     |     buf->head = head->next

In this case, buf->head has been advanced but head->commit may have
been updated with a new value.

Instead of reintroducing an unnecessary lock, fix the race locklessly.
Read the commit-next pair in the reverse order of writing, which guarantees
the commit value read is the latest value written if the head is
advancing.

Reported-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Cc: <stable@vger.kernel.org> # 3.12.x+
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

Reading git-diff-tree failed