[ltt-dev] [PATCH urcu 5/5] Document the new call_rcu() primitives.

Paul E. McKenney paulmck at linux.vnet.ibm.com
Thu Jun 2 19:45:04 EDT 2011

Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
 API.txt |  120 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 README  |   16 ++++++++
 2 files changed, 136 insertions(+), 0 deletions(-)
 create mode 100644 API.txt

diff --git a/API.txt b/API.txt
new file mode 100644
index 0000000..162ca93
--- /dev/null
+++ b/API.txt
@@ -0,0 +1,120 @@
+Userspace RCU API
+by Mathieu Desnoyers and Paul E. McKenney
+void rcu_init(void);
+	This must be called before any of the following functions
+	are invoked.
+void rcu_read_lock(void);
+	Begin an RCU read-side critical section.  These critical
+	sections may be nested.
+void rcu_read_unlock(void);
+	End an RCU read-side critical section.
+void rcu_register_thread(void)
+	Each thread must invoke this function before its first call to
+	rcu_read_lock().  Threads that never call rcu_read_lock() need
+	not invoke this function.  In addition, rcu-bp ("bullet proof"
+	RCU) does not require any thread to invoke rcu_register_thread().
+void rcu_unregister_thread(void)
+	Each thread that invokes rcu_register_thread() must invoke
+	rcu_unregister_thread() before invoking pthread_exit()
+	or before returning from its top-level function.
+void synchronize_rcu(void);
+	Wait until every pre-existing RCU read-side critical section
+	has completed.  Note that this primitive will not necessarily
+	wait for RCU read-side critical sections that have not yet
+	started: this is not a reader-writer lock.  The duration
+	actually waited is called an RCU grace period.
+void call_rcu(struct rcu_head *head,
+	      void (*func)(struct rcu_head *head));
+	Registers the callback indicated by "head".  This means
+	that "func" will be invoked after the end of a future
+	RCU grace period.  The rcu_head structure referenced
+	by "head" will normally be a field in a larger RCU-protected
+	structure.  A typical implementation of "func" is as
+	follows:
+		void func(struct rcu_head *head)
+		{
+			struct foo *p = container_of(head, struct foo, rcu);
+			free(p);
+		}
+	This RCU callback function can be registered as follows
+	given a pointer "p" to the enclosing structure:
+		call_rcu(&p->rcu, func);
+struct call_rcu_data *create_call_rcu_data(unsigned long flags);
+	Returns a handle that can be passed to the following
+	primitives.  The "flags" argument can be zero, or can be
+	URCU_CALL_RCU_RT if the worker threads associated with the
+	new helper thread are to get real-time response.
+struct call_rcu_data *get_default_call_rcu_data(void);
+	Returns the handle of the default call_rcu() helper thread.
+struct call_rcu_data *get_call_rcu_data(void);
+	Returns the handle of the current thread's call_rcu() helper
+	thread, which might well be the default helper thread.
+struct call_rcu_data *get_thread_call_rcu_data(void);
+	Returns the handle for the current thread's hard-assigned
+	call_rcu() helper thread, or NULL if the current thread is
+	instead using a per-CPU or the default helper thread.
+void set_thread_call_rcu_data(struct call_rcu_data *crdp);
+	Sets the current thread's hard-assigned call_rcu() helper to the
+	handle specified by "crdp".  Note that "crdp" can be NULL to
+	disassociate this thread from its helper.  Once a thread is
+	disassociated from its helper, further call_rcu() invocations
+	use the current CPU's helper if there is one and the default
+	helper otherwise.
+int set_cpu_call_rcu_data(int cpu, struct call_rcu_data *crdp);
+	Sets the specified CPU's call_rcu() helper to the handle
+	specified by "crdp".  Again, "crdp" can be NULL to disassociate
+	this CPU from its helper thread.  Once a CPU has been
+	disassociated from its helper, further call_rcu() invocations
+	that would otherwise have used this CPU's helper will instead
+	use the default helper.
+int create_all_cpu_call_rcu_data(unsigned long flags)
+	Creates a separate call_rcu() helper thread for each CPU.
+	After this primitive is invoked, the global default call_rcu()
+	helper thread will not be called.
+	The set_thread_call_rcu_data(), set_cpu_call_rcu_data(), and
+	create_all_cpu_call_rcu_data() functions may be combined to set up
+	pretty much any desired association between worker and call_rcu()
+	helper threads.  If a given executable calls only call_rcu(),
+	then that executable will have only the single global default
+	call_rcu() helper thread.  This will suffice in most cases.
+void call_rcu_data_free(struct call_rcu_data *crdp)
+	Terminates a call_rcu() helper thread and frees its associated
+	data.  The caller must have ensured that this thread is no longer
+	in use, for example, by passing NULL to set_thread_call_rcu_data()
+	and set_cpu_call_rcu_data() as required.
diff --git a/README b/README
index 7d97f19..a6e29c8 100644
--- a/README
+++ b/README
@@ -139,6 +139,22 @@ Usage of liburcu-defer
 	* Its API is currently experimental. It may change in future library
+Usage of urcu-call-rcu
+	* Follow instructions for either liburcu, liburcu-qsbr,
+	  liburcu-mb, liburcu-signal, or liburcu-bp above.
+	  The urcu-call-rcu functionality is provided for each of
+	  these library modules.
+	* Provides the call_rcu() primitive to enqueue delayed callbacks
+	  in a manner similar to defer_rcu(), but without ever delaying
+	  for a grace period.  On the other hand, call_rcu()'s best-case
+	  overhead is not quite as good as that of defer_rcu().
+	* Provides call_rcu() to allow asynchronous handling of RCU
+	  grace periods.  A number of additional functions are provided
+	  to manage the helper threads used by call_rcu(), but reasonable
+	  defaults are used if these additional functions are not invoked.
+	  See API.txt for more details.
 Being careful with signals
 	The liburcu library uses signals internally. The signal handler is

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