https://github.com/torvalds/linux
Revision 85746e429f8e5dc8c5c0beadc0f099cb1feab93e authored by Linus Torvalds on 07 July 2011, 20:16:21 UTC, committed by Linus Torvalds on 07 July 2011, 20:16:21 UTC
* git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-2.6: (31 commits) sctp: fix missing send up SCTP_SENDER_DRY_EVENT when subscribe it net: refine {udp|tcp|sctp}_mem limits vmxnet3: round down # of queues to power of two net: sh_eth: fix the parameter for the ETHER of SH7757 net: sh_eth: fix cannot work half-duplex mode net: vlan: enable soft features regardless of underlying device vmxnet3: fix starving rx ring whenoc_skb kb fails bridge: Always flood broadcast packets greth: greth_set_mac_add would corrupt the MAC address. net: bind() fix error return on wrong address family natsemi: silence dma-debug warnings net: 8139too: Initial necessary vlan_features to support vlan Fix call trace when interrupts are disabled while sleeping function kzalloc is called qlge:Version change to v1.00.00.29 qlge: Fix printk priority so chip fatal errors are always reported. qlge:Fix crash caused by mailbox execution on wedged chip. xfrm4: Don't call icmp_send on local error ipv4: Don't use ufo handling on later transformed packets xfrm: Remove family arg from xfrm_bundle_ok ipv6: Don't put artificial limit on routing table size. ...
Tip revision: 85746e429f8e5dc8c5c0beadc0f099cb1feab93e authored by Linus Torvalds on 07 July 2011, 20:16:21 UTC
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-2.6
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-2.6
Tip revision: 85746e4
rcu.txt
RCU Concepts
The basic idea behind RCU (read-copy update) is to split destructive
operations into two parts, one that prevents anyone from seeing the data
item being destroyed, and one that actually carries out the destruction.
A "grace period" must elapse between the two parts, and this grace period
must be long enough that any readers accessing the item being deleted have
since dropped their references. For example, an RCU-protected deletion
from a linked list would first remove the item from the list, wait for
a grace period to elapse, then free the element. See the listRCU.txt
file for more information on using RCU with linked lists.
Frequently Asked Questions
o Why would anyone want to use RCU?
The advantage of RCU's two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory
barriers. The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages
in read-mostly situations. The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code.
o How can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done?
Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these
three states, we know that that CPU has exited any previous RCU
read-side critical sections. So, if we remove an item from a
linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can
safely free up that item.
Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters. These counters allow limited types of blocking
within RCU read-side critical sections. SRCU also uses
CPU-local counters, and permits general blocking within
RCU read-side critical sections. These two variants of
RCU detect grace periods by sampling these counters.
o If I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?
See the UP.txt file in this directory.
o How can I see where RCU is currently used in the Linux kernel?
Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
"synchronize_net", "synchronize_srcu", and the other RCU
primitives. Or grab one of the cscope databases from:
http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
o What guidelines should I follow when writing code that uses RCU?
See the checklist.txt file in this directory.
o Why the name "RCU"?
"RCU" stands for "read-copy update". The file listRCU.txt has
more information on where this name came from, search for
"read-copy update" to find it.
o I hear that RCU is patented? What is with that?
Yes, it is. There are several known patents related to RCU,
search for the string "Patent" in RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
There are now also LGPL implementations of user-level RCU
available (http://lttng.org/?q=node/18).
o I hear that RCU needs work in order to support realtime kernels?
This work is largely completed. Realtime-friendly RCU can be
enabled via the CONFIG_TREE_PREEMPT_RCU kernel configuration
parameter. However, work is in progress for enabling priority
boosting of preempted RCU read-side critical sections. This is
needed if you have CPU-bound realtime threads.
o Where can I find more information on RCU?
See the RTFP.txt file in this directory.
Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
o What are all these files in this directory?
See 00-INDEX for the list.
Computing file changes ...