blocking, it registers a function and argument which are invoked
after all ongoing RCU read-side critical sections have completed.
This callback variant is particularly useful in situations where
- it is illegal to block.
+ it is illegal to block or where update-side performance is
+ critically important.
+
+ However, the call_rcu() API should not be used lightly, as use
+ of the synchronize_rcu() API generally results in simpler code.
+ In addition, the synchronize_rcu() API has the nice property
+ of automatically limiting update rate should grace periods
+ be delayed. This property results in system resilience in face
+ of denial-of-service attacks. Code using call_rcu() should limit
+ update rate in order to gain this same sort of resilience. See
+ checklist.txt for some approaches to limiting the update rate.
rcu_assign_pointer()
and release a global reader-writer lock. The synchronize_rcu()
primitive write-acquires this same lock, then immediately releases
it. This means that once synchronize_rcu() exits, all RCU read-side
-critical sections that were in progress before synchonize_rcu() was
+critical sections that were in progress before synchronize_rcu() was
called are guaranteed to have completed -- there is no way that
synchronize_rcu() would have been able to write-acquire the lock
otherwise.
+ spin_lock(&listmutex);
list_for_each_entry(p, head, lp) {
if (p->key == key) {
- list_del(&p->list);
+ - list_del(&p->list);
- write_unlock(&listmutex);
+ + list_del_rcu(&p->list);
+ spin_unlock(&listmutex);
+ synchronize_rcu();
kfree(p);
5 write_lock(&listmutex); 5 spin_lock(&listmutex);
6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
7 if (p->key == key) { 7 if (p->key == key) {
- 8 list_del(&p->list); 8 list_del(&p->list);
+ 8 list_del(&p->list); 8 list_del_rcu(&p->list);
9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
10 synchronize_rcu();
10 kfree(p); 11 kfree(p);
Either way, the differences are quite small. Read-side locking moves
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
-from a reader-writer lock to a simple spinlock, and a synchronize_rcu()
+a reader-writer lock to a simple spinlock, and a synchronize_rcu()
precedes the kfree().
However, there is one potential catch: the read-side and update-side
rcu_read_unlock
rcu_read_lock_bh
rcu_read_unlock_bh
+ srcu_read_lock
+ srcu_read_unlock
RCU pointer/list traversal:
RCU grace period:
- synchronize_kernel (deprecated)
synchronize_net
synchronize_sched
synchronize_rcu
+ synchronize_srcu
call_rcu
call_rcu_bh