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         Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
  40         same effect, but require that the readers manipulate CPU-local
  41         counters.  These counters allow limited types of blocking
  42         within RCU read-side critical sections.  SRCU also uses
  43         CPU-local counters, and permits general blocking within
  44         RCU read-side critical sections.  These two variants of
  45         RCU detect grace periods by sampling these counters.
  46
  47 o       If I am running on a uniprocessor kernel, which can only do one
  48         thing at a time, why should I wait for a grace period?
  49
  50         See the UP.txt file in this directory.
  51
  52 o       How can I see where RCU is currently used in the Linux kernel?
  53
  54         Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
  55         "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
  56         "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
  57         "synchronize_net", "synchronize_srcu", and the other RCU
  58         primitives.  Or grab one of the cscope databases from:
  59
  60         http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
  61
  62 o       What guidelines should I follow when writing code that uses RCU?
  63
  64         See the checklist.txt file in this directory.
  65
  66 o       Why the name "RCU"?
  67
  68         "RCU" stands for "read-copy update".  The file listRCU.txt has
  69         more information on where this name came from, search for
  70         "read-copy update" to find it.
  71
  72 o       I hear that RCU is patented?  What is with that?
  73
  74         Yes, it is.  There are several known patents related to RCU,
  75         search for the string "Patent" in RTFP.txt to find them.
  76         Of these, one was allowed to lapse by the assignee, and the
  77         others have been contributed to the Linux kernel under GPL.
  78
  79 o       I hear that RCU needs work in order to support realtime kernels?
  80
  81         This work is largely completed.  Realtime-friendly RCU can be
  82         enabled via the CONFIG_PREEMPT_RCU kernel configuration parameter.
  83         However, work is in progress for enabling priority boosting of
  84         preempted RCU read-side critical sections.This is needed if you
  85         have CPU-bound realtime threads.
  86
  87 o       Where can I find more information on RCU?
  88
  89         See the RTFP.txt file in this directory.
  90         Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
  91
  92 o       What are all these files in this directory?
  93
  94
  95         NMI-RCU.txt
  96
  97                 Describes how to use RCU to implement dynamic
  98                 NMI handlers, which can be revectored on the fly,
  99                 without rebooting.
 100
 101         RTFP.txt
 102
 103                 List of RCU-related publications and web sites.
 104
 105         UP.txt
 106
 107                 Discussion of RCU usage in UP kernels.
 108
 109         arrayRCU.txt
 110
 111                 Describes how to use RCU to protect arrays, with
 112                 resizeable arrays whose elements reference other
 113                 data structures being of the most interest.
 114
 115         checklist.txt
 116
 117                 Lists things to check for when inspecting code that
 118                 uses RCU.
 119
 120         listRCU.txt
 121
 122                 Describes how to use RCU to protect linked lists.
 123                 This is the simplest and most common use of RCU
 124                 in the Linux kernel.
 125
 126         rcu.txt
 127
 128                 You are reading it!
 129
 130         rcuref.txt
 131
 132                 Describes how to combine use of reference counts
 133                 with RCU.
 134
 135         whatisRCU.txt
 136
 137                 Overview of how the RCU implementation works.  Along
 138                 the way, presents a conceptual view of RCU.