Documentation/RCU/UP.txt

   1 RCU on Uniprocessor Systems
   2
   3
   4 A common misconception is that, on UP systems, the call_rcu() primitive
   5 may immediately invoke its function.  The basis of this misconception
   6 is that since there is only one CPU, it should not be necessary to
   7 wait for anything else to get done, since there are no other CPUs for
   8 anything else to be happening on.  Although this approach will -sort- -of-
   9 work a surprising amount of the time, it is a very bad idea in general.
  10 This document presents three examples that demonstrate exactly how bad
  11 an idea this is.
  12
  13
  14 Example 1: softirq Suicide
  15
  16 Suppose that an RCU-based algorithm scans a linked list containing
  17 elements A, B, and C in process context, and can delete elements from
  18 this same list in softirq context.  Suppose that the process-context scan
  19 is referencing element B when it is interrupted by softirq processing,
  20 which deletes element B, and then invokes call_rcu() to free element B
  21 after a grace period.
  22
  23 Now, if call_rcu() were to directly invoke its arguments, then upon return
  24 from softirq, the list scan would find itself referencing a newly freed
  25 element B.  This situation can greatly decrease the life expectancy of
  26 your kernel.
  27
  28 This same problem can occur if call_rcu() is invoked from a hardware
  29 interrupt handler.
  30
  31
  32 Example 2: Function-Call Fatality
  33
  34 Of course, one could avert the suicide described in the preceding example
  35 by having call_rcu() directly invoke its arguments only if it was called
  36 from process context.  However, this can fail in a similar manner.
  37
  38 Suppose that an RCU-based algorithm again scans a linked list containing
  39 elements A, B, and C in process contexts, but that it invokes a function
  40 on each element as it is scanned.  Suppose further that this function
  41 deletes element B from the list, then passes it to call_rcu() for deferred
  42 freeing.  This may be a bit unconventional, but it is perfectly legal
  43 RCU usage, since call_rcu() must wait for a grace period to elapse.
  44 Therefore, in this case, allowing call_rcu() to immediately invoke
  45 its arguments would cause it to fail to make the fundamental guarantee
  46 underlying RCU, namely that call_rcu() defers invoking its arguments until
  47 all RCU read-side critical sections currently executing have completed.
  48
  49 Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in
  50         this case?
  51
  52
  53 Example 3: Death by Deadlock
  54
  55 Suppose that call_rcu() is invoked while holding a lock, and that the
  56 callback function must acquire this same lock.  In this case, if
  57 call_rcu() were to directly invoke the callback, the result would
  58 be self-deadlock.
  59
  60 In some cases, it would possible to restructure to code so that
  61 the call_rcu() is delayed until after the lock is released.  However,
  62 there are cases where this can be quite ugly:
  63
  64 1.      If a number of items need to be passed to call_rcu() within
  65         the same critical section, then the code would need to create
  66         a list of them, then traverse the list once the lock was
  67         released.
  68
  69 2.      In some cases, the lock will be held across some kernel API,
  70         so that delaying the call_rcu() until the lock is released
  71         requires that the data item be passed up via a common API.
  72         It is far better to guarantee that callbacks are invoked
  73         with no locks held than to have to modify such APIs to allow
  74         arbitrary data items to be passed back up through them.
  75
  76 If call_rcu() directly invokes the callback, painful locking restrictions
  77 or API changes would be required.
  78
  79 Quick Quiz #2: What locking restriction must RCU callbacks respect?
  80
  81
  82 Summary
  83
  84 Permitting call_rcu() to immediately invoke its arguments breaks RCU,
  85 even on a UP system.  So do not do it!  Even on a UP system, the RCU
  86 infrastructure -must- respect grace periods, and -must- invoke callbacks
  87 from a known environment in which no locks are held.
  88
  89 It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return
  90 immediately on an UP system.  It is also safe for synchronize_rcu()
  91 to return immediately on UP systems, except when running preemptable
  92 RCU.
  93
  94 Quick Quiz #3: Why can't synchronize_rcu() return immediately on
  95         UP systems running preemptable RCU?
  96
  97
  98 Answer to Quick Quiz #1:
  99         Why is it -not- legal to invoke synchronize_rcu() in this case?
 100
 101         Because the calling function is scanning an RCU-protected linked
 102         list, and is therefore within an RCU read-side critical section.
 103         Therefore, the called function has been invoked within an RCU
 104         read-side critical section, and is not permitted to block.
 105
 106 Answer to Quick Quiz #2:
 107         What locking restriction must RCU callbacks respect?
 108
 109         Any lock that is acquired within an RCU callback must be
 110         acquired elsewhere using an _irq variant of the spinlock
 111         primitive.  For example, if "mylock" is acquired by an
 112         RCU callback, then a process-context acquisition of this
 113         lock must use something like spin_lock_irqsave() to
 114         acquire the lock.
 115
 116         If the process-context code were to simply use spin_lock(),
 117         then, since RCU callbacks can be invoked from softirq context,
 118         the callback might be called from a softirq that interrupted
 119         the process-context critical section.  This would result in
 120         self-deadlock.
 121
 122         This restriction might seem gratuitous, since very few RCU
 123         callbacks acquire locks directly.  However, a great many RCU
 124         callbacks do acquire locks -indirectly-, for example, via
 125         the kfree() primitive.
 126
 127 Answer to Quick Quiz #3:
 128         Why can't synchronize_rcu() return immediately on UP systems
 129         running preemptable RCU?
 130
 131         Because some other task might have been preempted in the middle
 132         of an RCU read-side critical section.  If synchronize_rcu()
 133         simply immediately returned, it would prematurely signal the
 134         end of the grace period, which would come as a nasty shock to
 135         that other thread when it started running again.