1RCU on Uniprocessor Systems
   4A common misconception is that, on UP systems, the call_rcu() primitive
   5may immediately invoke its function.  The basis of this misconception
   6is that since there is only one CPU, it should not be necessary to
   7wait for anything else to get done, since there are no other CPUs for
   8anything else to be happening on.  Although this approach will -sort- -of-
   9work a surprising amount of the time, it is a very bad idea in general.
  10This document presents three examples that demonstrate exactly how bad
  11an idea this is.
  14Example 1: softirq Suicide
  16Suppose that an RCU-based algorithm scans a linked list containing
  17elements A, B, and C in process context, and can delete elements from
  18this same list in softirq context.  Suppose that the process-context scan
  19is referencing element B when it is interrupted by softirq processing,
  20which deletes element B, and then invokes call_rcu() to free element B
  21after a grace period.
  23Now, if call_rcu() were to directly invoke its arguments, then upon return
  24from softirq, the list scan would find itself referencing a newly freed
  25element B.  This situation can greatly decrease the life expectancy of
  26your kernel.
  28This same problem can occur if call_rcu() is invoked from a hardware
  29interrupt handler.
  32Example 2: Function-Call Fatality
  34Of course, one could avert the suicide described in the preceding example
  35by having call_rcu() directly invoke its arguments only if it was called
  36from process context.  However, this can fail in a similar manner.
  38Suppose that an RCU-based algorithm again scans a linked list containing
  39elements A, B, and C in process contexts, but that it invokes a function
  40on each element as it is scanned.  Suppose further that this function
  41deletes element B from the list, then passes it to call_rcu() for deferred
  42freeing.  This may be a bit unconventional, but it is perfectly legal
  43RCU usage, since call_rcu() must wait for a grace period to elapse.
  44Therefore, in this case, allowing call_rcu() to immediately invoke
  45its arguments would cause it to fail to make the fundamental guarantee
  46underlying RCU, namely that call_rcu() defers invoking its arguments until
  47all RCU read-side critical sections currently executing have completed.
  49Quick Quiz #1: why is it -not- legal to invoke synchronize_rcu() in
  50        this case?
  53Example 3: Death by Deadlock
  55Suppose that call_rcu() is invoked while holding a lock, and that the
  56callback function must acquire this same lock.  In this case, if
  57call_rcu() were to directly invoke the callback, the result would
  58be self-deadlock.
  60In some cases, it would possible to restructure to code so that
  61the call_rcu() is delayed until after the lock is released.  However,
  62there are cases where this can be quite ugly:
  641.      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.
  692.      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.
  76If call_rcu() directly invokes the callback, painful locking restrictions
  77or API changes would be required.
  79Quick Quiz #2: What locking restriction must RCU callbacks respect?
  84Permitting call_rcu() to immediately invoke its arguments breaks RCU,
  85even on a UP system.  So do not do it!  Even on a UP system, the RCU
  86infrastructure -must- respect grace periods, and -must- invoke callbacks
  87from a known environment in which no locks are held.
  89It -is- safe for synchronize_sched() and synchronize_rcu_bh() to return
  90immediately on an UP system.  It is also safe for synchronize_rcu()
  91to return immediately on UP systems, except when running preemptable
  94Quick Quiz #3: Why can't synchronize_rcu() return immediately on
  95        UP systems running preemptable RCU?
  98Answer to Quick Quiz #1:
  99        Why is it -not- legal to invoke synchronize_rcu() in this case?
 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.
 106Answer to Quick Quiz #2:
 107        What locking restriction must RCU callbacks respect?
 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.
 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.
 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.
 127Answer to Quick Quiz #3:
 128        Why can't synchronize_rcu() return immediately on UP systems
 129        running preemptable RCU?
 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.
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