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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2012-10-23 13:47:01 -0700 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2012-11-13 14:08:23 -0800 |
commit | f0a0e6f282c72247e7c8ec17c68d528c1bb4d49e (patch) | |
tree | 22b66fc8ac9b95586866ddb447dcc8712d441c14 /kernel/rcutree.c | |
parent | 67afeed2cab0e59712b4ebf1aef9a2e555a188ce (diff) | |
download | linux-stable-f0a0e6f282c72247e7c8ec17c68d528c1bb4d49e.tar.gz linux-stable-f0a0e6f282c72247e7c8ec17c68d528c1bb4d49e.tar.bz2 linux-stable-f0a0e6f282c72247e7c8ec17c68d528c1bb4d49e.zip |
rcu: Clarify memory-ordering properties of grace-period primitives
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>
Diffstat (limited to 'kernel/rcutree.c')
-rw-r--r-- | kernel/rcutree.c | 29 |
1 files changed, 25 insertions, 4 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c index e4c2192b47c8..15a2beec320f 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -2228,10 +2228,28 @@ static inline int rcu_blocking_is_gp(void) * rcu_read_lock_sched(). * * This means that all preempt_disable code sequences, including NMI and - * hardware-interrupt handlers, in progress on entry will have completed - * before this primitive returns. However, this does not guarantee that - * softirq handlers will have completed, since in some kernels, these - * handlers can run in process context, and can block. + * non-threaded hardware-interrupt handlers, in progress on entry will + * have completed before this primitive returns. However, this does not + * guarantee that softirq handlers will have completed, since in some + * kernels, these handlers can run in process context, and can block. + * + * Note that this guarantee implies further memory-ordering guarantees. + * On systems with more than one CPU, when synchronize_sched() returns, + * each CPU is guaranteed to have executed a full memory barrier since the + * end of its last RCU-sched read-side critical section whose beginning + * preceded the call to synchronize_sched(). In addition, each CPU having + * an RCU read-side critical section that extends beyond the return from + * synchronize_sched() is guaranteed to have executed a full memory barrier + * after the beginning of synchronize_sched() and 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 synchronize_sched(), which returned + * to its caller on CPU B, then both CPU A and CPU B are guaranteed + * to have executed a full memory barrier during the execution of + * synchronize_sched() -- even if CPU A and CPU B are the same CPU (but + * again only if the system has more than one CPU). * * This primitive provides the guarantees made by the (now removed) * synchronize_kernel() API. In contrast, synchronize_rcu() only @@ -2259,6 +2277,9 @@ EXPORT_SYMBOL_GPL(synchronize_sched); * read-side critical sections have completed. RCU read-side critical * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(), * and may be nested. + * + * See the description of synchronize_sched() for more detailed information + * on memory ordering guarantees. */ void synchronize_rcu_bh(void) { |