Documentation/RCU/rcu.txt

   1 RCU Concepts
   2
   3
   4 The basic idea behind RCU (read-copy update) is to split destructive
   5 operations into two parts, one that prevents anyone from seeing the data
   6 item being destroyed, and one that actually carries out the destruction.
   7 A "grace period" must elapse between the two parts, and this grace period
   8 must be long enough that any readers accessing the item being deleted have
   9 since dropped their references.  For example, an RCU-protected deletion
  10 from a linked list would first remove the item from the list, wait for
  11 a grace period to elapse, then free the element.  See the listRCU.txt
  12 file for more information on using RCU with linked lists.
  13
  14
  15 Frequently Asked Questions
  16
  17 o       Why would anyone want to use RCU?
  18
  19         The advantage of RCU's two-part approach is that RCU readers need
  20         not acquire any locks, perform any atomic instructions, write to
  21         shared memory, or (on CPUs other than Alpha) execute any memory
  22         barriers.  The fact that these operations are quite expensive
  23         on modern CPUs is what gives RCU its performance advantages
  24         in read-mostly situations.  The fact that RCU readers need not
  25         acquire locks can also greatly simplify deadlock-avoidance code.
  26
  27 o       How can the updater tell when a grace period has completed
  28         if the RCU readers give no indication when they are done?
  29
  30         Just as with spinlocks, RCU readers are not permitted to
  31         block, switch to user-mode execution, or enter the idle loop.
  32         Therefore, as soon as a CPU is seen passing through any of these
  33         three states, we know that that CPU has exited any previous RCU
  34         read-side critical sections.  So, if we remove an item from a
  35         linked list, and then wait until all CPUs have switched context,
  36         executed in user mode, or executed in the idle loop, we can
  37         safely free up that item.
  38
  39 o       If I am running on a uniprocessor kernel, which can only do one
  40         thing at a time, why should I wait for a grace period?
  41
  42         See the UP.txt file in this directory.
  43
  44 o       How can I see where RCU is currently used in the Linux kernel?
  45
  46         Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
  47         "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
  48         "synchronize_rcu", and "synchronize_net".
  49
  50 o       What guidelines should I follow when writing code that uses RCU?
  51
  52         See the checklist.txt file in this directory.
  53
  54 o       Why the name "RCU"?
  55
  56         "RCU" stands for "read-copy update".  The file listRCU.txt has
  57         more information on where this name came from, search for
  58         "read-copy update" to find it.
  59
  60 o       I hear that RCU is patented?  What is with that?
  61
  62         Yes, it is.  There are several known patents related to RCU,
  63         search for the string "Patent" in RTFP.txt to find them.
  64         Of these, one was allowed to lapse by the assignee, and the
  65         others have been contributed to the Linux kernel under GPL.
  66
  67 o       I hear that RCU needs work in order to support realtime kernels?
  68
  69         Yes, work in progress.
  70
  71 o       Where can I find more information on RCU?
  72
  73         See the RTFP.txt file in this directory.
  74         Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
  75
  76 o       What are all these files in this directory?
  77
  78
  79         NMI-RCU.txt
  80
  81                 Describes how to use RCU to implement dynamic
  82                 NMI handlers, which can be revectored on the fly,
  83                 without rebooting.
  84
  85         RTFP.txt
  86
  87                 List of RCU-related publications and web sites.
  88
  89         UP.txt
  90
  91                 Discussion of RCU usage in UP kernels.
  92
  93         arrayRCU.txt
  94
  95                 Describes how to use RCU to protect arrays, with
  96                 resizeable arrays whose elements reference other
  97                 data structures being of the most interest.
  98
  99         checklist.txt
 100
 101                 Lists things to check for when inspecting code that
 102                 uses RCU.
 103
 104         listRCU.txt
 105
 106                 Describes how to use RCU to protect linked lists.
 107                 This is the simplest and most common use of RCU
 108                 in the Linux kernel.
 109
 110         rcu.txt
 111
 112                 You are reading it!
 113
 114         rcuref.txt
 115
 116                 Describes how to combine use of reference counts
 117                 with RCU.
 118
 119         whatisRCU.txt
 120
 121                 Overview of how the RCU implementation works.  Along
 122                 the way, presents a conceptual view of RCU.