From 28f00767e3db933cacc3030f4d9736acd037be2c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 25 Jun 2015 15:00:58 -0700 Subject: [PATCH] rcu: Abstract sequence counting from synchronize_sched_expedited() This commit creates rcu_exp_gp_seq_start() and rcu_exp_gp_seq_end() to bracket an expedited grace period, rcu_exp_gp_seq_snap() to snapshot the sequence counter, and rcu_exp_gp_seq_done() to check to see if a full expedited grace period has elapsed since the snapshot. These will be applied to synchronize_rcu_expedited(). These are defined in terms of underlying rcu_seq_start(), rcu_seq_end(), rcu_seq_snap(), rcu_seq_done(), which will be applied to _rcu_barrier(). One reason that this commit doesn't use the seqcount primitives themselves is that the smp_wmb() in those primitive is insufficient due to the fact that expedited grace periods do reads as well as writes. In addition, the read-side seqcount primitives detect a potentially partial change, where the expedited primitives instead need a guaranteed full change. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 68 ++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 58 insertions(+), 10 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index c5c8509054ef..67fe75725486 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3255,6 +3255,60 @@ void cond_synchronize_rcu(unsigned long oldstate) } EXPORT_SYMBOL_GPL(cond_synchronize_rcu); +/* Adjust sequence number for start of update-side operation. */ +static void rcu_seq_start(unsigned long *sp) +{ + WRITE_ONCE(*sp, *sp + 1); + smp_mb(); /* Ensure update-side operation after counter increment. */ + WARN_ON_ONCE(!(*sp & 0x1)); +} + +/* Adjust sequence number for end of update-side operation. */ +static void rcu_seq_end(unsigned long *sp) +{ + smp_mb(); /* Ensure update-side operation before counter increment. */ + WRITE_ONCE(*sp, *sp + 1); + WARN_ON_ONCE(*sp & 0x1); +} + +/* Take a snapshot of the update side's sequence number. */ +static unsigned long rcu_seq_snap(unsigned long *sp) +{ + unsigned long s; + + smp_mb(); /* Caller's modifications seen first by other CPUs. */ + s = (READ_ONCE(*sp) + 3) & ~0x1; + smp_mb(); /* Above access must not bleed into critical section. */ + return s; +} + +/* + * Given a snapshot from rcu_seq_snap(), determine whether or not a + * full update-side operation has occurred. + */ +static bool rcu_seq_done(unsigned long *sp, unsigned long s) +{ + return ULONG_CMP_GE(READ_ONCE(*sp), s); +} + +/* Wrapper functions for expedited grace periods. */ +static void rcu_exp_gp_seq_start(struct rcu_state *rsp) +{ + rcu_seq_start(&rsp->expedited_sequence); +} +static void rcu_exp_gp_seq_end(struct rcu_state *rsp) +{ + rcu_seq_end(&rsp->expedited_sequence); +} +static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp) +{ + return rcu_seq_snap(&rsp->expedited_sequence); +} +static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s) +{ + return rcu_seq_done(&rsp->expedited_sequence, s); +} + static int synchronize_sched_expedited_cpu_stop(void *data) { struct rcu_state *rsp = data; @@ -3269,7 +3323,7 @@ static int synchronize_sched_expedited_cpu_stop(void *data) static bool sync_sched_exp_wd(struct rcu_state *rsp, struct rcu_node *rnp, atomic_long_t *stat, unsigned long s) { - if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) { + if (rcu_exp_gp_seq_done(rsp, s)) { if (rnp) mutex_unlock(&rnp->exp_funnel_mutex); /* Ensure test happens before caller kfree(). */ @@ -3306,9 +3360,7 @@ void synchronize_sched_expedited(void) struct rcu_state *rsp = &rcu_sched_state; /* Take a snapshot of the sequence number. */ - smp_mb(); /* Caller's modifications seen first by other CPUs. */ - s = (READ_ONCE(rsp->expedited_sequence) + 3) & ~0x1; - smp_mb(); /* Above access must not bleed into critical section. */ + s = rcu_exp_gp_seq_snap(rsp); if (!try_get_online_cpus()) { /* CPU hotplug operation in flight, fall back to normal GP. */ @@ -3339,9 +3391,7 @@ void synchronize_sched_expedited(void) if (sync_sched_exp_wd(rsp, rnp0, &rsp->expedited_workdone2, s)) return; - WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1); - smp_mb(); /* Ensure expedited GP seen after counter increment. */ - WARN_ON_ONCE(!(rsp->expedited_sequence & 0x1)); + rcu_exp_gp_seq_start(rsp); /* Stop each CPU that is online, non-idle, and not us. */ init_waitqueue_head(&rsp->expedited_wq); @@ -3364,9 +3414,7 @@ void synchronize_sched_expedited(void) wait_event(rsp->expedited_wq, !atomic_read(&rsp->expedited_need_qs)); - smp_mb(); /* Ensure expedited GP seen before counter increment. */ - WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1); - WARN_ON_ONCE(rsp->expedited_sequence & 0x1); + rcu_exp_gp_seq_end(rsp); mutex_unlock(&rnp0->exp_funnel_mutex); smp_mb(); /* ensure subsequent action seen after grace period. */ -- 2.30.2