Documentation/RCU/checklist.txt

   1 Review Checklist for RCU Patches
   2
   3
   4 This document contains a checklist for producing and reviewing patches
   5 that make use of RCU.  Violating any of the rules listed below will
   6 result in the same sorts of problems that leaving out a locking primitive
   7 would cause.  This list is based on experiences reviewing such patches
   8 over a rather long period of time, but improvements are always welcome!
   9
  10 0.      Is RCU being applied to a read-mostly situation?  If the data
  11         structure is updated more than about 10% of the time, then
  12         you should strongly consider some other approach, unless
  13         detailed performance measurements show that RCU is nonetheless
  14         the right tool for the job.
  15
  16         The other exception would be where performance is not an issue,
  17         and RCU provides a simpler implementation.  An example of this
  18         situation is the dynamic NMI code in the Linux 2.6 kernel,
  19         at least on architectures where NMIs are rare.
  20
  21 1.      Does the update code have proper mutual exclusion?
  22
  23         RCU does allow -readers- to run (almost) naked, but -writers- must
  24         still use some sort of mutual exclusion, such as:
  25
  26         a.      locking,
  27         b.      atomic operations, or
  28         c.      restricting updates to a single task.
  29
  30         If you choose #b, be prepared to describe how you have handled
  31         memory barriers on weakly ordered machines (pretty much all of
  32         them -- even x86 allows reads to be reordered), and be prepared
  33         to explain why this added complexity is worthwhile.  If you
  34         choose #c, be prepared to explain how this single task does not
  35         become a major bottleneck on big multiprocessor machines.
  36
  37 2.      Do the RCU read-side critical sections make proper use of
  38         rcu_read_lock() and friends?  These primitives are needed
  39         to suppress preemption (or bottom halves, in the case of
  40         rcu_read_lock_bh()) in the read-side critical sections,
  41         and are also an excellent aid to readability.
  42
  43 3.      Does the update code tolerate concurrent accesses?
  44
  45         The whole point of RCU is to permit readers to run without
  46         any locks or atomic operations.  This means that readers will
  47         be running while updates are in progress.  There are a number
  48         of ways to handle this concurrency, depending on the situation:
  49
  50         a.      Make updates appear atomic to readers.  For example,
  51                 pointer updates to properly aligned fields will appear
  52                 atomic, as will individual atomic primitives.  Operations
  53                 performed under a lock and sequences of multiple atomic
  54                 primitives will -not- appear to be atomic.
  55
  56                 This is almost always the best approach.
  57
  58         b.      Carefully order the updates and the reads so that
  59                 readers see valid data at all phases of the update.
  60                 This is often more difficult than it sounds, especially
  61                 given modern CPUs' tendency to reorder memory references.
  62                 One must usually liberally sprinkle memory barriers
  63                 (smp_wmb(), smp_rmb(), smp_mb()) through the code,
  64                 making it difficult to understand and to test.
  65
  66                 It is usually better to group the changing data into
  67                 a separate structure, so that the change may be made
  68                 to appear atomic by updating a pointer to reference
  69                 a new structure containing updated values.
  70
  71 4.      Weakly ordered CPUs pose special challenges.  Almost all CPUs
  72         are weakly ordered -- even i386 CPUs allow reads to be reordered.
  73         RCU code must take all of the following measures to prevent
  74         memory-corruption problems:
  75
  76         a.      Readers must maintain proper ordering of their memory
  77                 accesses.  The rcu_dereference() primitive ensures that
  78                 the CPU picks up the pointer before it picks up the data
  79                 that the pointer points to.  This really is necessary
  80                 on Alpha CPUs.  If you don't believe me, see:
  81
  82                         http://www.openvms.compaq.com/wizard/wiz_2637.html
  83
  84                 The rcu_dereference() primitive is also an excellent
  85                 documentation aid, letting the person reading the code
  86                 know exactly which pointers are protected by RCU.
  87
  88                 The rcu_dereference() primitive is used by the various
  89                 "_rcu()" list-traversal primitives, such as the
  90                 list_for_each_entry_rcu().
  91
  92         b.      If the list macros are being used, the list_del_rcu(),
  93                 list_add_tail_rcu(), and list_del_rcu() primitives must
  94                 be used in order to prevent weakly ordered machines from
  95                 misordering structure initialization and pointer planting.
  96                 Similarly, if the hlist macros are being used, the
  97                 hlist_del_rcu() and hlist_add_head_rcu() primitives
  98                 are required.
  99
 100         c.      Updates must ensure that initialization of a given
 101                 structure happens before pointers to that structure are
 102                 publicized.  Use the rcu_assign_pointer() primitive
 103                 when publicizing a pointer to a structure that can
 104                 be traversed by an RCU read-side critical section.
 105
 106                 [The rcu_assign_pointer() primitive is in process.]
 107
 108 5.      If call_rcu(), or a related primitive such as call_rcu_bh(),
 109         is used, the callback function must be written to be called
 110         from softirq context.  In particular, it cannot block.
 111
 112 6.      Since synchronize_kernel() blocks, it cannot be called from
 113         any sort of irq context.
 114
 115 7.      If the updater uses call_rcu(), then the corresponding readers
 116         must use rcu_read_lock() and rcu_read_unlock().  If the updater
 117         uses call_rcu_bh(), then the corresponding readers must use
 118         rcu_read_lock_bh() and rcu_read_unlock_bh().  Mixing things up
 119         will result in confusion and broken kernels.
 120
 121         One exception to this rule: rcu_read_lock() and rcu_read_unlock()
 122         may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
 123         in cases where local bottom halves are already known to be
 124         disabled, for example, in irq or softirq context.  Commenting
 125         such cases is a must, of course!  And the jury is still out on
 126         whether the increased speed is worth it.
 127
 128 8.      Although synchronize_kernel() is a bit slower than is call_rcu(),
 129         it usually results in simpler code.  So, unless update performance
 130         is important or the updaters cannot block, synchronize_kernel()
 131         should be used in preference to call_rcu().
 132
 133 9.      All RCU list-traversal primitives, which include
 134         list_for_each_rcu(), list_for_each_entry_rcu(),
 135         list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
 136         must be within an RCU read-side critical section.  RCU
 137         read-side critical sections are delimited by rcu_read_lock()
 138         and rcu_read_unlock(), or by similar primitives such as
 139         rcu_read_lock_bh() and rcu_read_unlock_bh().
 140
 141         Use of the _rcu() list-traversal primitives outside of an
 142         RCU read-side critical section causes no harm other than
 143         a slight performance degradation on Alpha CPUs and some
 144         confusion on the part of people trying to read the code.
 145
 146         Another way of thinking of this is "If you are holding the
 147         lock that prevents the data structure from changing, why do
 148         you also need RCU-based protection?"  That said, there may
 149         well be situations where use of the _rcu() list-traversal
 150         primitives while the update-side lock is held results in
 151         simpler and more maintainable code.  The jury is still out
 152         on this question.
 153
 154 10.     Conversely, if you are in an RCU read-side critical section,
 155         you -must- use the "_rcu()" variants of the list macros.
 156         Failing to do so will break Alpha and confuse people reading
 157         your code.