Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH tip/core/rcu 4/4] rcu: simplify association of quiescent states with grace periods | Date | Tue, 10 Nov 2009 13:37:22 -0800 |
| |
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rdp->passed_quiesc_completed fields are used to properly associate the recorded quiescent state with a grace period. It is OK to wrongly associate a given quiescent state with a preceding grace period, but it is fatal to associate a given quiescent state with a grace period that begins after the quiescent state occurred. Grace periods are numbered, and the following fields track them:
o ->gpnum is the number of the grace period currently in progress, or the number of the last grace period to complete if no grace period is currently in progress.
o ->completed is the number of the last grace period to have completed.
These two fields are equal if there is no grace period in progress, otherwise ->gpnum is one greater than ->completed. But the rdp->passed_quiesc_completed field compared against ->completed, and if equal, the quiescent state is presumed to count against the current grace period.
The earlier code copied rdp->completed to rdp->passed_quiesc_completed, which has been made to work, but is error-prone. In contrast, copying one less than rdp->gpnum is guaranteed safe, because rdp->gpnum is not incremented until after the start of the corresponding grace period. At the end of the grace period, when ->completed has incremented, then any quiescent periods recorded previously will be discarded.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> --- kernel/rcutree.c | 4 ++-- kernel/rcutree_plugin.h | 2 +- 2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 4026560..2f5d8c5 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -100,7 +100,7 @@ void rcu_sched_qs(int cpu) struct rcu_data *rdp; rdp = &per_cpu(rcu_sched_data, cpu); - rdp->passed_quiesc_completed = rdp->completed; + rdp->passed_quiesc_completed = rdp->gpnum - 1; barrier(); rdp->passed_quiesc = 1; rcu_preempt_note_context_switch(cpu); @@ -111,7 +111,7 @@ void rcu_bh_qs(int cpu) struct rcu_data *rdp; rdp = &per_cpu(rcu_bh_data, cpu); - rdp->passed_quiesc_completed = rdp->completed; + rdp->passed_quiesc_completed = rdp->gpnum - 1; barrier(); rdp->passed_quiesc = 1; } diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index 9744dfc..b7dc0c2 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -67,7 +67,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed); static void rcu_preempt_qs(int cpu) { struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu); - rdp->passed_quiesc_completed = rdp->completed; + rdp->passed_quiesc_completed = rdp->gpnum - 1; barrier(); rdp->passed_quiesc = 1; } -- 1.5.2.5
| |