[PATCH] Notifier chain update: API changes

The kernel's implementation of notifier chains is unsafe.  There is no
protection against entries being added to or removed from a chain while the
chain is in use.  The issues were discussed in this thread:

    http://marc.theaimsgroup.com/?l=linux-kernel&m=113018709002036&w=2

We noticed that notifier chains in the kernel fall into two basic usage
classes:

	"Blocking" chains are always called from a process context
	and the callout routines are allowed to sleep;

	"Atomic" chains can be called from an atomic context and
	the callout routines are not allowed to sleep.

We decided to codify this distinction and make it part of the API.  Therefore
this set of patches introduces three new, parallel APIs: one for blocking
notifiers, one for atomic notifiers, and one for "raw" notifiers (which is
really just the old API under a new name).  New kinds of data structures are
used for the heads of the chains, and new routines are defined for
registration, unregistration, and calling a chain.  The three APIs are
explained in include/linux/notifier.h and their implementation is in
kernel/sys.c.

With atomic and blocking chains, the implementation guarantees that the chain
links will not be corrupted and that chain callers will not get messed up by
entries being added or removed.  For raw chains the implementation provides no
guarantees at all; users of this API must provide their own protections.  (The
idea was that situations may come up where the assumptions of the atomic and
blocking APIs are not appropriate, so it should be possible for users to
handle these things in their own way.)

There are some limitations, which should not be too hard to live with.  For
atomic/blocking chains, registration and unregistration must always be done in
a process context since the chain is protected by a mutex/rwsem.  Also, a
callout routine for a non-raw chain must not try to register or unregister
entries on its own chain.  (This did happen in a couple of places and the code
had to be changed to avoid it.)

Since atomic chains may be called from within an NMI handler, they cannot use
spinlocks for synchronization.  Instead we use RCU.  The overhead falls almost
entirely in the unregister routine, which is okay since unregistration is much
less frequent that calling a chain.

Here is the list of chains that we adjusted and their classifications.  None
of them use the raw API, so for the moment it is only a placeholder.

  ATOMIC CHAINS
  -------------
arch/i386/kernel/traps.c:		i386die_chain
arch/ia64/kernel/traps.c:		ia64die_chain
arch/powerpc/kernel/traps.c:		powerpc_die_chain
arch/sparc64/kernel/traps.c:		sparc64die_chain
arch/x86_64/kernel/traps.c:		die_chain
drivers/char/ipmi/ipmi_si_intf.c:	xaction_notifier_list
kernel/panic.c:				panic_notifier_list
kernel/profile.c:			task_free_notifier
net/bluetooth/hci_core.c:		hci_notifier
net/ipv4/netfilter/ip_conntrack_core.c:	ip_conntrack_chain
net/ipv4/netfilter/ip_conntrack_core.c:	ip_conntrack_expect_chain
net/ipv6/addrconf.c:			inet6addr_chain
net/netfilter/nf_conntrack_core.c:	nf_conntrack_chain
net/netfilter/nf_conntrack_core.c:	nf_conntrack_expect_chain
net/netlink/af_netlink.c:		netlink_chain

  BLOCKING CHAINS
  ---------------
arch/powerpc/platforms/pseries/reconfig.c:	pSeries_reconfig_chain
arch/s390/kernel/process.c:		idle_chain
arch/x86_64/kernel/process.c		idle_notifier
drivers/base/memory.c:			memory_chain
drivers/cpufreq/cpufreq.c		cpufreq_policy_notifier_list
drivers/cpufreq/cpufreq.c		cpufreq_transition_notifier_list
drivers/macintosh/adb.c:		adb_client_list
drivers/macintosh/via-pmu.c		sleep_notifier_list
drivers/macintosh/via-pmu68k.c		sleep_notifier_list
drivers/macintosh/windfarm_core.c	wf_client_list
drivers/usb/core/notify.c		usb_notifier_list
drivers/video/fbmem.c			fb_notifier_list
kernel/cpu.c				cpu_chain
kernel/module.c				module_notify_list
kernel/profile.c			munmap_notifier
kernel/profile.c			task_exit_notifier
kernel/sys.c				reboot_notifier_list
net/core/dev.c				netdev_chain
net/decnet/dn_dev.c:			dnaddr_chain
net/ipv4/devinet.c:			inetaddr_chain

It's possible that some of these classifications are wrong.  If they are,
please let us know or submit a patch to fix them.  Note that any chain that
gets called very frequently should be atomic, because the rwsem read-locking
used for blocking chains is very likely to incur cache misses on SMP systems.
(However, if the chain's callout routines may sleep then the chain cannot be
atomic.)

The patch set was written by Alan Stern and Chandra Seetharaman, incorporating
material written by Keith Owens and suggestions from Paul McKenney and Andrew
Morton.

[jes@sgi.com: restructure the notifier chain initialization macros]
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Jes Sorensen <jes@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 89 insertions, 7 deletions

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 5937dd60..51dbab9 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -10,25 +10,107 @@
 #ifndef _LINUX_NOTIFIER_H
 #define _LINUX_NOTIFIER_H
 #include <linux/errno.h>
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
 
-struct notifier_block
-{
-	int (*notifier_call)(struct notifier_block *self, unsigned long, void *);
+/*
+ * Notifier chains are of three types:
+ *
+ *	Atomic notifier chains: Chain callbacks run in interrupt/atomic
+ *		context. Callouts are not allowed to block.
+ *	Blocking notifier chains: Chain callbacks run in process context.
+ *		Callouts are allowed to block.
+ *	Raw notifier chains: There are no restrictions on callbacks,
+ *		registration, or unregistration.  All locking and protection
+ *		must be provided by the caller.
+ *
+ * atomic_notifier_chain_register() may be called from an atomic context,
+ * but blocking_notifier_chain_register() must be called from a process
+ * context.  Ditto for the corresponding _unregister() routines.
+ *
+ * atomic_notifier_chain_unregister() and blocking_notifier_chain_unregister()
+ * _must not_ be called from within the call chain.
+ */
+
+struct notifier_block {
+	int (*notifier_call)(struct notifier_block *, unsigned long, void *);
 	struct notifier_block *next;
 	int priority;
 };
 
+struct atomic_notifier_head {
+	spinlock_t lock;
+	struct notifier_block *head;
+};
+
+struct blocking_notifier_head {
+	struct rw_semaphore rwsem;
+	struct notifier_block *head;
+};
+
+struct raw_notifier_head {
+	struct notifier_block *head;
+};
+
+#define ATOMIC_INIT_NOTIFIER_HEAD(name) do {	\
+		spin_lock_init(&(name)->lock);	\
+		(name)->head = NULL;		\
+	} while (0)
+#define BLOCKING_INIT_NOTIFIER_HEAD(name) do {	\
+		init_rwsem(&(name)->rwsem);	\
+		(name)->head = NULL;		\
+	} while (0)
+#define RAW_INIT_NOTIFIER_HEAD(name) do {	\
+		(name)->head = NULL;		\
+	} while (0)
+
+#define ATOMIC_NOTIFIER_INIT(name) {				\
+		.lock = SPIN_LOCK_UNLOCKED,			\
+		.head = NULL }
+#define BLOCKING_NOTIFIER_INIT(name) {				\
+		.rwsem = __RWSEM_INITIALIZER((name).rwsem),	\
+		.head = NULL }
+#define RAW_NOTIFIER_INIT(name)	{				\
+		.head = NULL }
+
+#define ATOMIC_NOTIFIER_HEAD(name)				\
+	struct atomic_notifier_head name =			\
+		ATOMIC_NOTIFIER_INIT(name)
+#define BLOCKING_NOTIFIER_HEAD(name)				\
+	struct blocking_notifier_head name =			\
+		BLOCKING_NOTIFIER_INIT(name)
+#define RAW_NOTIFIER_HEAD(name)					\
+	struct raw_notifier_head name =				\
+		RAW_NOTIFIER_INIT(name)
 
 #ifdef __KERNEL__
 
-extern int notifier_chain_register(struct notifier_block **list, struct notifier_block *n);
-extern int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n);
-extern int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v);
+extern int atomic_notifier_chain_register(struct atomic_notifier_head *,
+		struct notifier_block *);
+extern int blocking_notifier_chain_register(struct blocking_notifier_head *,
+		struct notifier_block *);
+extern int raw_notifier_chain_register(struct raw_notifier_head *,
+		struct notifier_block *);
+
+extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *,
+		struct notifier_block *);
+extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *,
+		struct notifier_block *);
+extern int raw_notifier_chain_unregister(struct raw_notifier_head *,
+		struct notifier_block *);
+
+extern int atomic_notifier_call_chain(struct atomic_notifier_head *,
+		unsigned long val, void *v);
+extern int blocking_notifier_call_chain(struct blocking_notifier_head *,
+		unsigned long val, void *v);
+extern int raw_notifier_call_chain(struct raw_notifier_head *,
+		unsigned long val, void *v);
 
 #define NOTIFY_DONE		0x0000		/* Don't care */
 #define NOTIFY_OK		0x0001		/* Suits me */
 #define NOTIFY_STOP_MASK	0x8000		/* Don't call further */
-#define NOTIFY_BAD		(NOTIFY_STOP_MASK|0x0002)	/* Bad/Veto action	*/
+#define NOTIFY_BAD		(NOTIFY_STOP_MASK|0x0002)
+						/* Bad/Veto action */
 /*
  * Clean way to return from the notifier and stop further calls.
  */