From: Al Viro <viro@zeniv.linux.org.uk>
Date: Sat, 19 Apr 2014 16:30:58 +0000 (-0400)
Subject: fix races between __d_instantiate() and checks of dentry flags
X-Git-Tag: omap-for-v3.16/fixes-against-rc1~209^2~10
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=22213318af7ae265bc6cd8aef2febbc2d69a2440;p=pandora-kernel.git

fix races between __d_instantiate() and checks of dentry flags

in non-lazy walk we need to be careful about dentry switching from
negative to positive - both ->d_flags and ->d_inode are updated,
and in some places we might see only one store.  The cases where
dentry has been obtained by dcache lookup with ->i_mutex held on
parent are safe - ->d_lock and ->i_mutex provide all the barriers
we need.  However, there are several places where we run into
trouble:
	* do_last() fetches ->d_inode, then checks ->d_flags and
assumes that inode won't be NULL unless d_is_negative() is true.
Race with e.g. creat() - we might have fetched the old value of
->d_inode (still NULL) and new value of ->d_flags (already not
DCACHE_MISS_TYPE).  Lin Ming has observed and reported the resulting
oops.
	* a bunch of places checks ->d_inode for being non-NULL,
then checks ->d_flags for "is it a symlink".  Race with symlink(2)
in case if our CPU sees ->d_inode update first - we see non-NULL
there, but ->d_flags still contains DCACHE_MISS_TYPE instead of
DCACHE_SYMLINK_TYPE.  Result: false negative on "should we follow
link here?", with subsequent unpleasantness.

Cc: stable@vger.kernel.org # 3.13 and 3.14 need that one
Reported-and-tested-by: Lin Ming <minggr@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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