97 lines
4.0 KiB
Plaintext
97 lines
4.0 KiB
Plaintext
RCU Concepts
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The basic idea behind RCU (read-copy update) is to split destructive
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operations into two parts, one that prevents anyone from seeing the data
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item being destroyed, and one that actually carries out the destruction.
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A "grace period" must elapse between the two parts, and this grace period
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must be long enough that any readers accessing the item being deleted have
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since dropped their references. For example, an RCU-protected deletion
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from a linked list would first remove the item from the list, wait for
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a grace period to elapse, then free the element. See the listRCU.txt
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file for more information on using RCU with linked lists.
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Frequently Asked Questions
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o Why would anyone want to use RCU?
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The advantage of RCU's two-part approach is that RCU readers need
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not acquire any locks, perform any atomic instructions, write to
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shared memory, or (on CPUs other than Alpha) execute any memory
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barriers. The fact that these operations are quite expensive
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on modern CPUs is what gives RCU its performance advantages
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in read-mostly situations. The fact that RCU readers need not
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acquire locks can also greatly simplify deadlock-avoidance code.
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o How can the updater tell when a grace period has completed
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if the RCU readers give no indication when they are done?
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Just as with spinlocks, RCU readers are not permitted to
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block, switch to user-mode execution, or enter the idle loop.
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Therefore, as soon as a CPU is seen passing through any of these
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three states, we know that that CPU has exited any previous RCU
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read-side critical sections. So, if we remove an item from a
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linked list, and then wait until all CPUs have switched context,
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executed in user mode, or executed in the idle loop, we can
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safely free up that item.
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Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the
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same effect, but require that the readers manipulate CPU-local
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counters. These counters allow limited types of blocking within
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RCU read-side critical sections. SRCU also uses CPU-local
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counters, and permits general blocking within RCU read-side
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critical sections. These variants of RCU detect grace periods
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by sampling these counters.
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o If I am running on a uniprocessor kernel, which can only do one
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thing at a time, why should I wait for a grace period?
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See the UP.txt file in this directory.
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o How can I see where RCU is currently used in the Linux kernel?
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Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
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"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
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"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
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"synchronize_net", "synchronize_srcu", and the other RCU
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primitives. Or grab one of the cscope databases from:
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http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
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o What guidelines should I follow when writing code that uses RCU?
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See the checklist.txt file in this directory.
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o Why the name "RCU"?
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"RCU" stands for "read-copy update". The file listRCU.txt has
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more information on where this name came from, search for
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"read-copy update" to find it.
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o I hear that RCU is patented? What is with that?
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Yes, it is. There are several known patents related to RCU,
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search for the string "Patent" in RTFP.txt to find them.
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Of these, one was allowed to lapse by the assignee, and the
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others have been contributed to the Linux kernel under GPL.
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There are now also LGPL implementations of user-level RCU
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available (http://lttng.org/?q=node/18).
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o I hear that RCU needs work in order to support realtime kernels?
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This work is largely completed. Realtime-friendly RCU can be
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enabled via the CONFIG_TREE_PREEMPT_RCU kernel configuration
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parameter. However, work is in progress for enabling priority
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boosting of preempted RCU read-side critical sections. This is
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needed if you have CPU-bound realtime threads.
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o Where can I find more information on RCU?
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See the RTFP.txt file in this directory.
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Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
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o What are all these files in this directory?
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See 00-INDEX for the list.
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