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Cc: rcu@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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.. _rcu_doc:
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RCU Concepts
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============
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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 listRCU.rst for more
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information on using RCU with linked lists.
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Frequently Asked Questions
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--------------------------
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- 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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- 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_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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- 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 UP.rst for more information.
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- 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", "srcu_read_lock",
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"srcu_read_unlock", "synchronize_rcu", "synchronize_net",
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"synchronize_srcu", and the other RCU primitives. Or grab one
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of the cscope databases from:
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(http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).
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- What guidelines should I follow when writing code that uses RCU?
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See checklist.rst.
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- Why the name "RCU"?
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"RCU" stands for "read-copy update".
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listRCU.rst has more information on where this name came from, search
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for "read-copy update" to find it.
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- 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 Documentation/RCU/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 (https://liburcu.org/).
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- I hear that RCU needs work in order to support realtime kernels?
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Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
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kernel configuration parameter.
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- Where can I find more information on RCU?
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See the Documentation/RCU/RTFP.txt file.
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Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).
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