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rcu: Clarify memory-ordering properties of grace-period primitives

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This commit explicitly states the memory-ordering properties of the
RCU grace-period primitives.  Although these properties were in some
sense implied by the fundmental property of RCU ("a grace period must
wait for all pre-existing RCU read-side critical sections to complete"),
stating it explicitly will be a great labor-saving device.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
This commit is contained in:
Paul E. McKenney 2012-10-23 13:47:01 -07:00
commit f0a0e6f282
3 changed files with 58 additions and 4 deletions
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25
include/linux/rcupdate.h
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@ -90,6 +90,25 @@ extern void do_trace_rcu_torture_read(char *rcutorturename,
* that started after call_rcu() was invoked. RCU read-side critical
* sections are delimited by rcu_read_lock() and rcu_read_unlock(),
* and may be nested.
*
* Note that all CPUs must agree that the grace period extended beyond
* all pre-existing RCU read-side critical section. On systems with more
* than one CPU, this means that when "func()" is invoked, each CPU is
* guaranteed to have executed a full memory barrier since the end of its
* last RCU read-side critical section whose beginning preceded the call
* to call_rcu(). It also means that each CPU executing an RCU read-side
* critical section that continues beyond the start of "func()" must have
* executed a memory barrier after the call_rcu() but before the beginning
* of that RCU read-side critical section. Note that these guarantees
* include CPUs that are offline, idle, or executing in user mode, as
* well as CPUs that are executing in the kernel.
*
* Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
* resulting RCU callback function "func()", then both CPU A and CPU B are
* guaranteed to execute a full memory barrier during the time interval
* between the call to call_rcu() and the invocation of "func()" -- even
* if CPU A and CPU B are the same CPU (but again only if the system has
* more than one CPU).
*/
extern void call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *head));
@ -118,6 +137,9 @@ extern void call_rcu(struct rcu_head *head,
* OR
* - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
* These may be nested.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
extern void call_rcu_bh(struct rcu_head *head,
void (*func)(struct rcu_head *head));
@ -137,6 +159,9 @@ extern void call_rcu_bh(struct rcu_head *head,
* OR
* anything that disables preemption.
* These may be nested.
*
* See the description of call_rcu() for more detailed information on
* memory ordering guarantees.
*/
extern void call_rcu_sched(struct rcu_head *head,
void (*func)(struct rcu_head *rcu));