621934ee7e
Updated patch adding a variant of RCU that permits sleeping in read-side critical sections. SRCU is as follows: o Each use of SRCU creates its own srcu_struct, and each srcu_struct has its own set of grace periods. This is critical, as it prevents one subsystem with a blocking reader from holding up SRCU grace periods for other subsystems. o The SRCU primitives (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu()) all take a pointer to a srcu_struct. o The SRCU primitives must be called from process context. o srcu_read_lock() returns an int that must be passed to the matching srcu_read_unlock(). Realtime RCU avoids the need for this by storing the state in the task struct, but SRCU needs to allow a given code path to pass through multiple SRCU domains -- storing state in the task struct would therefore require either arbitrary space in the task struct or arbitrary limits on SRCU nesting. So I kicked the state-storage problem up to the caller. Of course, it is not permitted to call synchronize_srcu() while in an SRCU read-side critical section. o There is no call_srcu(). It would not be hard to implement one, but it seems like too easy a way to OOM the system. (Hey, we have enough trouble with call_rcu(), which does -not- permit readers to sleep!!!) So, if you want it, please tell me why... [josht@us.ibm.com: sparse notation] Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Josh Triplett <josh@freedesktop.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
123 lines
3.9 KiB
Text
123 lines
3.9 KiB
Text
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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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", and "synchronize_srcu".
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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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o I hear that RCU needs work in order to support realtime kernels?
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Yes, work in progress.
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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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NMI-RCU.txt
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Describes how to use RCU to implement dynamic
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NMI handlers, which can be revectored on the fly,
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without rebooting.
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RTFP.txt
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List of RCU-related publications and web sites.
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UP.txt
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Discussion of RCU usage in UP kernels.
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arrayRCU.txt
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Describes how to use RCU to protect arrays, with
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resizeable arrays whose elements reference other
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data structures being of the most interest.
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checklist.txt
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Lists things to check for when inspecting code that
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uses RCU.
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listRCU.txt
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Describes how to use RCU to protect linked lists.
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This is the simplest and most common use of RCU
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in the Linux kernel.
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rcu.txt
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You are reading it!
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rcuref.txt
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Describes how to combine use of reference counts
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with RCU.
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whatisRCU.txt
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Overview of how the RCU implementation works. Along
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the way, presents a conceptual view of RCU.
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