+Please note that the "What is RCU?" LWN series is an excellent place
+to start learning about RCU:
+
+1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
+2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
+3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
+
+
What is RCU?
RCU is a synchronization mechanism that was added to the Linux kernel
Used by a reader to inform the reclaimer that the reader is
entering an RCU read-side critical section. It is illegal
to block while in an RCU read-side critical section, though
- kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side
- critical sections. Any RCU-protected data structure accessed
- during an RCU read-side critical section is guaranteed to remain
- unreclaimed for the full duration of that critical section.
+ kernels built with CONFIG_TREE_PREEMPT_RCU can preempt RCU
+ read-side critical sections. Any RCU-protected data structure
+ accessed during an RCU read-side critical section is guaranteed to
+ remain unreclaimed for the full duration of that critical section.
Reference counts may be used in conjunction with RCU to maintain
longer-term references to data structures.
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()
Defer Protect
a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
- call_rcu()
+ call_rcu() rcu_dereference()
b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
+ rcu_dereference_bh()
-c. synchronize_sched() preempt_disable() / preempt_enable()
+c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
+ preempt_disable() / preempt_enable()
local_irq_save() / local_irq_restore()
hardirq enter / hardirq exit
NMI enter / NMI exit
+ rcu_dereference_sched()
These three mechanisms are used as follows:
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.
{
int cpu;
- for_each_cpu(cpu)
+ for_each_possible_cpu(cpu)
run_on(cpu);
}
+ 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
APIs, since there does not appear to be a way to categorize them
in docbook. Here is the list, by category.
-Markers for RCU read-side critical sections:
-
- rcu_read_lock
- rcu_read_unlock
- rcu_read_lock_bh
- rcu_read_unlock_bh
-
-RCU pointer/list traversal:
+RCU list traversal:
- rcu_dereference
- list_for_each_rcu (to be deprecated in favor of
- list_for_each_entry_rcu)
list_for_each_entry_rcu
+ hlist_for_each_entry_rcu
+ hlist_nulls_for_each_entry_rcu
+
list_for_each_continue_rcu (to be deprecated in favor of new
list_for_each_entry_continue_rcu)
- hlist_for_each_entry_rcu
-RCU pointer update:
+RCU pointer/list update:
rcu_assign_pointer
list_add_rcu
list_del_rcu
list_replace_rcu
hlist_del_rcu
+ hlist_add_after_rcu
+ hlist_add_before_rcu
hlist_add_head_rcu
+ hlist_replace_rcu
+ list_splice_init_rcu()
+
+RCU: Critical sections Grace period Barrier
+
+ rcu_read_lock synchronize_net rcu_barrier
+ rcu_read_unlock synchronize_rcu
+ rcu_dereference synchronize_rcu_expedited
+ call_rcu
+
+
+bh: Critical sections Grace period Barrier
+
+ rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
+ rcu_read_unlock_bh synchronize_rcu_bh
+ rcu_dereference_bh synchronize_rcu_bh_expedited
+
+
+sched: Critical sections Grace period Barrier
+
+ rcu_read_lock_sched synchronize_sched rcu_barrier_sched
+ rcu_read_unlock_sched call_rcu_sched
+ [preempt_disable] synchronize_sched_expedited
+ [and friends]
+ rcu_dereference_sched
+
+
+SRCU: Critical sections Grace period Barrier
+
+ srcu_read_lock synchronize_srcu N/A
+ srcu_read_unlock synchronize_srcu_expedited
+ srcu_dereference
+
+SRCU: Initialization/cleanup
+ init_srcu_struct
+ cleanup_srcu_struct
-RCU grace period:
+All: lockdep-checked RCU-protected pointer access
- synchronize_kernel (deprecated)
- synchronize_net
- synchronize_sched
- synchronize_rcu
- call_rcu
- call_rcu_bh
+ rcu_dereference_check
+ rcu_dereference_protected
+ rcu_access_pointer
See the comment headers in the source code (or the docbook generated
from them) for more information.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff