when publicizing a pointer to a structure that can
be traversed by an RCU read-side critical section.
-5. If call_rcu(), or a related primitive such as call_rcu_bh() or
- call_rcu_sched(), is used, the callback function must be
- written to be called from softirq context. In particular,
+5. If call_rcu(), or a related primitive such as call_rcu_bh(),
+ call_rcu_sched(), or call_srcu() is used, the callback function
+ must be written to be called from softirq context. In particular,
it cannot block.
6. Since synchronize_rcu() can block, it cannot be called from
updater uses call_rcu_sched() or synchronize_sched(), then
the corresponding readers must disable preemption, possibly
by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
- If the updater uses synchronize_srcu(), the the corresponding
- readers must use srcu_read_lock() and srcu_read_unlock(),
- and with the same srcu_struct. The rules for the expedited
- primitives are the same as for their non-expedited counterparts.
- Mixing things up will result in confusion and broken kernels.
+ If the updater uses synchronize_srcu() or call_srcu(),
+ the the corresponding readers must use srcu_read_lock() and
+ srcu_read_unlock(), and with the same srcu_struct. The rules for
+ the expedited primitives are the same as for their non-expedited
+ counterparts. Mixing things up will result in confusion and
+ broken kernels.
One exception to this rule: rcu_read_lock() and rcu_read_unlock()
may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
victim CPU from ever going offline.)
14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
- synchronize_srcu(), and synchronize_srcu_expedited()) may only
- be invoked from process context. Unlike other forms of RCU, it
- -is- permissible to block in an SRCU read-side critical section
- (demarked by srcu_read_lock() and srcu_read_unlock()), hence the
- "SRCU": "sleepable RCU". Please note that if you don't need
- to sleep in read-side critical sections, you should be using
- RCU rather than SRCU, because RCU is almost always faster and
- easier to use than is SRCU.
+ synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
+ may only be invoked from process context. Unlike other forms of
+ RCU, it -is- permissible to block in an SRCU read-side critical
+ section (demarked by srcu_read_lock() and srcu_read_unlock()),
+ hence the "SRCU": "sleepable RCU". Please note that if you
+ don't need to sleep in read-side critical sections, you should be
+ using RCU rather than SRCU, because RCU is almost always faster
+ and easier to use than is SRCU.
If you need to enter your read-side critical section in a
hardirq or exception handler, and then exit that same read-side
cleanup_srcu_struct(). These are passed a "struct srcu_struct"
that defines the scope of a given SRCU domain. Once initialized,
the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
- synchronize_srcu(), and synchronize_srcu_expedited(). A given
- synchronize_srcu() waits only for SRCU read-side critical
+ synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
+ A given synchronize_srcu() waits only for SRCU read-side critical
sections governed by srcu_read_lock() and srcu_read_unlock()
calls that have been passed the same srcu_struct. This property
is what makes sleeping read-side critical sections tolerable --
requiring SRCU's read-side deadlock immunity or low read-side
realtime latency.
- Note that, rcu_assign_pointer() relates to SRCU just as they do
+ Note that, rcu_assign_pointer() relates to SRCU just as it does
to other forms of RCU.
15. The whole point of call_rcu(), synchronize_rcu(), and friends
2. Execute rcu_barrier().
3. Allow the module to be unloaded.
-Quick Quiz #1: Why is there no srcu_barrier()?
-
The rcutorture module makes use of rcu_barrier in its exit function
as follows:
Then lines 55-62 print status and do operation-specific cleanup, and
then return, permitting the module-unload operation to be completed.
-Quick Quiz #2: Is there any other situation where rcu_barrier() might
+Quick Quiz #1: Is there any other situation where rcu_barrier() might
be required?
Your module might have additional complications. For example, if your
4 complete(&rcu_barrier_completion);
5 }
-Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
+Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
are delayed for a full grace period? Couldn't this result in
Answers to Quick Quizzes
-Quick Quiz #1: Why is there no srcu_barrier()?
-
-Answer: Since there is no call_srcu(), there can be no outstanding SRCU
- callbacks. Therefore, there is no need to wait for them.
-
-Quick Quiz #2: Is there any other situation where rcu_barrier() might
+Quick Quiz #1: Is there any other situation where rcu_barrier() might
be required?
Answer: Interestingly enough, rcu_barrier() was not originally
implementing rcutorture, and found that rcu_barrier() solves
this problem as well.
-Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
+Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
are delayed for a full grace period? Couldn't this result in
projects / linux-beck.git / commitdiff