From c64ac3ce06558e534aec62b1fadeb0a3f111dac1 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 10 Nov 2009 13:37:22 -0800 Subject: rcu: Simplify association of quiescent states with grace periods 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 Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12578890421011-git-send-email-> Signed-off-by: Ingo Molnar --- kernel/rcutree.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'kernel/rcutree.c') diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 26fc7807761d..d8024192c73b 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; } -- cgit v1.2.3