them -- even x86 allows reads to be reordered), and be prepared
to explain why this added complexity is worthwhile. If you
choose #c, be prepared to explain how this single task does not
- become a major bottleneck on big multiprocessor machines.
+ become a major bottleneck on big multiprocessor machines (for
+ example, if the task is updating information relating to itself
+ that other tasks can read, there by definition can be no
+ bottleneck).
2. Do the RCU read-side critical sections make proper use of
rcu_read_lock() and friends? These primitives are needed
rcu_read_lock_bh()) in the read-side critical sections,
and are also an excellent aid to readability.
+ As a rough rule of thumb, any dereference of an RCU-protected
+ pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
+ or by the appropriate update-side lock.
+
3. Does the update code tolerate concurrent accesses?
The whole point of RCU is to permit readers to run without
The rcu_dereference() primitive is used by the various
"_rcu()" list-traversal primitives, such as the
- list_for_each_entry_rcu().
-
- b. If the list macros are being used, the list_del_rcu(),
- list_add_tail_rcu(), and list_del_rcu() primitives must
- be used in order to prevent weakly ordered machines from
- misordering structure initialization and pointer planting.
+ list_for_each_entry_rcu(). Note that it is perfectly
+ legal (if redundant) for update-side code to use
+ rcu_dereference() and the "_rcu()" list-traversal
+ primitives. This is particularly useful in code
+ that is common to readers and updaters.
+
+ b. If the list macros are being used, the list_add_tail_rcu()
+ and list_add_rcu() primitives must be used in order
+ to prevent weakly ordered machines from misordering
+ structure initialization and pointer planting.
Similarly, if the hlist macros are being used, the
- hlist_del_rcu() and hlist_add_head_rcu() primitives
- are required.
+ hlist_add_head_rcu() primitive is required.
+
+ c. If the list macros are being used, the list_del_rcu()
+ primitive must be used to keep list_del()'s pointer
+ poisoning from inflicting toxic effects on concurrent
+ readers. Similarly, if the hlist macros are being used,
+ the hlist_del_rcu() primitive is required.
+
+ The list_replace_rcu() primitive may be used to
+ replace an old structure with a new one in an
+ RCU-protected list.
- c. Updates must ensure that initialization of a given
+ d. Updates must ensure that initialization of a given
structure happens before pointers to that structure are
publicized. Use the rcu_assign_pointer() primitive
when publicizing a pointer to a structure that can
be traversed by an RCU read-side critical section.
- [The rcu_assign_pointer() primitive is in process.]
-
5. If call_rcu(), or a related primitive such as call_rcu_bh(),
is used, the callback function must be written to be called
from softirq context. In particular, it cannot block.
-6. Since synchronize_kernel() blocks, it cannot be called from
+6. Since synchronize_rcu() can block, it cannot be called from
any sort of irq context.
7. If the updater uses call_rcu(), then the corresponding readers
such cases is a must, of course! And the jury is still out on
whether the increased speed is worth it.
-8. Although synchronize_kernel() is a bit slower than is call_rcu(),
+8. Although synchronize_rcu() is a bit slower than is call_rcu(),
it usually results in simpler code. So, unless update performance
- is important or the updaters cannot block, synchronize_kernel()
+ is important or the updaters cannot block, synchronize_rcu()
should be used in preference to call_rcu().
9. All RCU list-traversal primitives, which include
Use of the _rcu() list-traversal primitives outside of an
RCU read-side critical section causes no harm other than
- a slight performance degradation on Alpha CPUs and some
- confusion on the part of people trying to read the code.
-
- Another way of thinking of this is "If you are holding the
- lock that prevents the data structure from changing, why do
- you also need RCU-based protection?" That said, there may
- well be situations where use of the _rcu() list-traversal
- primitives while the update-side lock is held results in
- simpler and more maintainable code. The jury is still out
- on this question.
+ a slight performance degradation on Alpha CPUs. It can
+ also be quite helpful in reducing code bloat when common
+ code is shared between readers and updaters.
10. Conversely, if you are in an RCU read-side critical section,
you -must- use the "_rcu()" variants of the list macros.
Failing to do so will break Alpha and confuse people reading
your code.
+
+11. Note that synchronize_rcu() -only- guarantees to wait until
+ all currently executing rcu_read_lock()-protected RCU read-side
+ critical sections complete. It does -not- necessarily guarantee
+ that all currently running interrupts, NMIs, preempt_disable()
+ code, or idle loops will complete. Therefore, if you do not have
+ rcu_read_lock()-protected read-side critical sections, do -not-
+ use synchronize_rcu().
+
+ If you want to wait for some of these other things, you might
+ instead need to use synchronize_irq() or synchronize_sched().
projects / powerpc.git / blobdiff