dd81eca83c
Update RCU documentation based on discussions and review of RCU-based tree patches. Add an introductory whatisRCU.txt file. Signed-off-by: <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
119 lines
5 KiB
Text
119 lines
5 KiB
Text
RCU on Uniprocessor Systems
|
|
|
|
|
|
A common misconception is that, on UP systems, the call_rcu() primitive
|
|
may immediately invoke its function, and that the synchronize_rcu()
|
|
primitive may return immediately. 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 contexts, 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.
|
|
|
|
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:
|
|
|
|
1. 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.
|
|
|
|
2. 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?
|
|
|
|
|
|
Summary
|
|
|
|
Permitting call_rcu() to immediately invoke its arguments or permitting
|
|
synchronize_rcu() to immediately return 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.
|
|
|
|
|
|
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 _irq 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_irqsave() to
|
|
acquire the lock.
|
|
|
|
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.
|