disable irq on a given acquisition of that lock will result in
deadlock as soon as the RCU callback happens to interrupt that
acquisition's critical section.
+
+13. RCU callbacks can be and are executed in parallel. In many cases,
+ the callback code simply wrappers around kfree(), so that this
+ is not an issue (or, more accurately, to the extent that it is
+ an issue, the memory-allocator locking handles it). However,
+ if the callbacks do manipulate a shared data structure, they
+ must use whatever locking or other synchronization is required
+ to safely access and/or modify that data structure.
+
+14. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_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.
+
+ Also unlike other forms of RCU, explicit initialization
+ and cleanup is required via init_srcu_struct() and
+ 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()
+ and synchronize_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 passd the same
+ srcu_struct. This property is what makes sleeping read-side
+ critical sections tolerable -- a given subsystem delays only
+ its own updates, not those of other subsystems using SRCU.
+ Therefore, SRCU is less prone to OOM the system than RCU would
+ be if RCU's read-side critical sections were permitted to
+ sleep.
+
+ The ability to sleep in read-side critical sections does not
+ come for free. First, corresponding srcu_read_lock() and
+ srcu_read_unlock() calls must be passed the same srcu_struct.
+ Second, grace-period-detection overhead is amortized only
+ over those updates sharing a given srcu_struct, rather than
+ being globally amortized as they are for other forms of RCU.
+ Therefore, SRCU should be used in preference to rw_semaphore
+ only in extremely read-intensive situations, or in situations
+ requiring SRCU's read-side deadlock immunity or low read-side
+ realtime latency.
+
+ Note that, rcu_assign_pointer() and rcu_dereference() relate to
+ SRCU just as they do to other forms of RCU.