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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 <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2015-06-25 15:00:58 -07:00
parent 3a6d7c64d7
commit 28f00767e3
1 changed files with 58 additions and 10 deletions
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68
kernel/rcu/tree.c
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@ -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. */