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
operations that would not normally be undertaken while a real-time
workload is running.
+ In particular, if you find yourself invoking one of the expedited
+ primitives repeatedly in a loop, please do everyone a favor:
+ Restructure your code so that it batches the updates, allowing
+ a single non-expedited primitive to cover the entire batch.
+ This will very likely be faster than the loop containing the
+ expedited primitive, and will be much much easier on the rest
+ of the system, especially to real-time workloads running on
+ the rest of the system.
+
+ In addition, it is illegal to call the expedited forms from
+ a CPU-hotplug notifier, or while holding a lock that is acquired
+ by a CPU-hotplug notifier. Failing to observe this restriction
+ will result in deadlock.
+
7. If the updater uses call_rcu() or synchronize_rcu(), then the
corresponding readers must use rcu_read_lock() and
rcu_read_unlock(). If the updater uses call_rcu_bh() or
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
projects / deliverable / linux.git / blobdiff