linux-kernel - [PATCH] CRED: Holding a spinlock does not imply the holding of RCU read lock

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Message-ID: <20100426105849.26489.72831.stgit@warthog.procyon.org.uk>
Date:	Mon, 26 Apr 2010 11:58:49 +0100
From:	David Howells <dhowells@...hat.com>
To:	linux-security-module@...r.kernel.org
Cc:	dhowells@...hat.com, serue@...ibm.com, paulmck@...ibm.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH] CRED: Holding a spinlock does not imply the holding of RCU
	read lock

From: Serge E. Hallyn <serue@...ibm.com>

Change the credentials documentation to make it clear that the RCU read lock
must be explicitly held when accessing credentials pointers in some other task
than current.  Holding a spinlock does not implicitly hold the RCU read lock.

Signed-off-by: Serge Hallyn <serue@...ibm.com>
Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 Documentation/credentials.txt |   14 +++++---------
 1 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt
index df03169..a2db352 100644
--- a/Documentation/credentials.txt
+++ b/Documentation/credentials.txt
@@ -408,9 +408,6 @@ This should be used inside the RCU read lock, as in the following example:
 		...
 	}
 
-A function need not get RCU read lock to use __task_cred() if it is holding a
-spinlock at the time as this implicitly holds the RCU read lock.
-
 Should it be necessary to hold another task's credentials for a long period of
 time, and possibly to sleep whilst doing so, then the caller should get a
 reference on them using:
@@ -426,17 +423,16 @@ credentials, hiding the RCU magic from the caller:
 	uid_t task_uid(task)		Task's real UID
 	uid_t task_euid(task)		Task's effective UID
 
-If the caller is holding a spinlock or the RCU read lock at the time anyway,
-then:
+If the caller is holding the RCU read lock at the time anyway, then:
 
 	__task_cred(task)->uid
 	__task_cred(task)->euid
 
 should be used instead.  Similarly, if multiple aspects of a task's credentials
-need to be accessed, RCU read lock or a spinlock should be used, __task_cred()
-called, the result stored in a temporary pointer and then the credential
-aspects called from that before dropping the lock.  This prevents the
-potentially expensive RCU magic from being invoked multiple times.
+need to be accessed, RCU read lock should be used, __task_cred() called, the
+result stored in a temporary pointer and then the credential aspects called
+from that before dropping the lock.  This prevents the potentially expensive
+RCU magic from being invoked multiple times.
 
 Should some other single aspect of another task's credentials need to be
 accessed, then this can be used:

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