• Alan Stern's avatar
    [PATCH] Notifier chain update: API changes · e041c683
    Alan Stern authored
    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:
    We noticed that notifier chains in the kernel fall into two basic usage
    	"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
    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.
    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
    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
    The patch set was written by Alan Stern and Chandra Seetharaman, incorporating
    material written by Keith Owens and suggestions from Paul McKenney and Andrew
    [jes@sgi.com: restructure the notifier chain initialization macros]
    Signed-off-by: default avatarAlan Stern <stern@rowland.harvard.edu>
    Signed-off-by: default avatarChandra Seetharaman <sekharan@us.ibm.com>
    Signed-off-by: default avatarJes Sorensen <jes@sgi.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>