RCU on Uniprocessor Systems¶
A common misconception is that, on UP systems, the call_rcu() primitive
may immediately invoke its function. The basis of this misconception
is that since there is only one CPU, it should not be necessary to
wait for anything else to get done, since there are no other CPUs for
anything else to be happening on. Although this approach will sort of
work a surprising amount of the time, it is a very bad idea in general.
This document presents three examples that demonstrate exactly how bad
an idea this is.
Example 1: softirq Suicide¶
Suppose that an RCU-based algorithm scans a linked list containing
elements A, B, and C in process context, and can delete elements from
this same list in softirq context. Suppose that the process-context scan
is referencing element B when it is interrupted by softirq processing,
which deletes element B, and then invokes call_rcu() to free element B
after a grace period.
Now, if call_rcu() were to directly invoke its arguments, then upon return
from softirq, the list scan would find itself referencing a newly freed
element B. This situation can greatly decrease the life expectancy of
your kernel.
This same problem can occur if call_rcu() is invoked from a hardware
interrupt handler.
Example 2: Function-Call Fatality¶
Of course, one could avert the suicide described in the preceding example
by having call_rcu() directly invoke its arguments only if it was called
from process context. However, this can fail in a similar manner.
Suppose that an RCU-based algorithm again scans a linked list containing
elements A, B, and C in process context, but that it invokes a function
on each element as it is scanned. Suppose further that this function
deletes element B from the list, then passes it to call_rcu() for deferred
freeing. This may be a bit unconventional, but it is perfectly legal
RCU usage, since call_rcu() must wait for a grace period to elapse.
Therefore, in this case, allowing call_rcu() to immediately invoke
its arguments would cause it to fail to make the fundamental guarantee
underlying RCU, namely that call_rcu() defers invoking its arguments until
all RCU read-side critical sections currently executing have completed.
- Quick Quiz #1:
Why is it not legal to invoke
synchronize_rcu()in this case?
Example 3: Death by Deadlock¶
Suppose that call_rcu() is invoked while holding a lock, and that the
callback function must acquire this same lock. In this case, if
call_rcu() were to directly invoke the callback, the result would
be self-deadlock even if this invocation occurred from a later
call_rcu() invocation a full grace period later.
In some cases, it would possible to restructure to code so that
the call_rcu() is delayed until after the lock is released. However,
there are cases where this can be quite ugly:
If a number of items need to be passed to
call_rcu()within the same critical section, then the code would need to create a list of them, then traverse the list once the lock was released.In some cases, the lock will be held across some kernel API, so that delaying the
call_rcu()until the lock is released requires that the data item be passed up via a common API. It is far better to guarantee that callbacks are invoked with no locks held than to have to modify such APIs to allow arbitrary data items to be passed back up through them.
If call_rcu() directly invokes the callback, painful locking restrictions
or API changes would be required.
- Quick Quiz #2:
What locking restriction must RCU callbacks respect?
It is important to note that userspace RCU implementations do
permit call_rcu() to directly invoke callbacks, but only if a full
grace period has elapsed since those callbacks were queued. This is
the case because some userspace environments are extremely constrained.
Nevertheless, people writing userspace RCU implementations are strongly
encouraged to avoid invoking callbacks from call_rcu(), thus obtaining
the deadlock-avoidance benefits called out above.
Summary¶
Permitting call_rcu() to immediately invoke its arguments breaks RCU,
even on a UP system. So do not do it! Even on a UP system, the RCU
infrastructure must respect grace periods, and must invoke callbacks
from a known environment in which no locks are held.
Note that it is safe for synchronize_rcu() to return immediately on
UP systems, including PREEMPT SMP builds running on UP systems.
- Quick Quiz #3:
Why can’t
synchronize_rcu()return immediately on UP systems running preemptible RCU?
- Answer to Quick Quiz #1:
Why is it not legal to invoke
synchronize_rcu()in this case?Because the calling function is scanning an RCU-protected linked list, and is therefore within an RCU read-side critical section. Therefore, the called function has been invoked within an RCU read-side critical section, and is not permitted to block.
- Answer to Quick Quiz #2:
What locking restriction must RCU callbacks respect?
Any lock that is acquired within an RCU callback must be acquired elsewhere using an _bh variant of the spinlock primitive. For example, if “mylock” is acquired by an RCU callback, then a process-context acquisition of this lock must use something like spin_lock_bh() to acquire the lock. Please note that it is also OK to use _irq variants of spinlocks, for example, spin_lock_irqsave().
If the process-context code were to simply use spin_lock(), then, since RCU callbacks can be invoked from softirq context, the callback might be called from a softirq that interrupted the process-context critical section. This would result in self-deadlock.
This restriction might seem gratuitous, since very few RCU callbacks acquire locks directly. However, a great many RCU callbacks do acquire locks indirectly, for example, via the
kfree()primitive.- Answer to Quick Quiz #3:
Why can’t
synchronize_rcu()return immediately on UP systems running preemptible RCU?Because some other task might have been preempted in the middle of an RCU read-side critical section. If
synchronize_rcu()simply immediately returned, it would prematurely signal the end of the grace period, which would come as a nasty shock to that other thread when it started running again.