From: Josh Triplett <josh@joshtriplett.org>
Subject: Re: What does this sparse warning mean in posix_acl.h?
Date: Sat, 17 Aug 2013 09:17:18 -0700
Message-ID: <20130817161718.GA18897@leaf>
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To: Theodore Ts'o <tytso@thunk.org>
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On Sat, Aug 17, 2013 at 10:28:22AM -0400, Theodore Ts'o wrote:
> It apparently has something to do with rcu and "address spaces" but I'm
> not completely sure what sparse is complaining about --- and whether it
> is a false positive or a bug in the posix_acl.h.
> 
> Can someone explain what's going on, and whether it's something to be
> concerned about?
[...]
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9: warning: incorrect type in assignment (different address spaces)
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9:    expected struct posix_acl *<noident>
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9:    got struct posix_acl [noderef] <asn:4>*<noident>
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9: warning: incorrect type in assignment (different address spaces)
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9:    expected struct posix_acl *<noident>
> /usr/projects/linux/ext4/include/linux/posix_acl.h:137:9:    got struct posix_acl [noderef] <asn:4>*<noident>

Address space 4 is __rcu.  This warning means you accessed an RCU
pointer directly, without using the appropriate RCU primitive
(rcu_assign_pointer, rcu_dereference, etc).

- Josh Triplett