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rcu: Move propagation of ->completed from rcu_start_gp() to rcu_report_qs_rsp()

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It is possible for the CPU that noted the end of the prior grace period
to not need a new one, and therefore to decide to propagate ->completed
throughout the rcu_node tree without starting another grace period.
However, in so doing, it releases the root rcu_node structure's lock,
which can allow some other CPU to start another grace period.  The first
CPU will be propagating ->completed in parallel with the second CPU
initializing the rcu_node tree for the new grace period.  In theory
this is harmless, but in practice we need to keep things simple.

This commit therefore moves the propagation of ->completed to
rcu_report_qs_rsp(), and refrains from marking the old grace period
as having been completed until it has finished doing this.  This
prevents anyone from starting a new grace period concurrently with
marking the old grace period as having been completed.

Of course, the optimization where a CPU needing a new grace period
doesn't bother marking the old one completed is still in effect:
In that case, the marking happens implicitly as part of initializing
the new grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2011-08-24 16:52:09 -07:00
parent e90c53d3e2
commit afe24b122e
1 changed files with 52 additions and 21 deletions
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73
kernel/rcutree.c
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@ -842,28 +842,24 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
struct rcu_node *rnp = rcu_get_root(rsp);
if (!rcu_scheduler_fully_active ||
!cpu_needs_another_gp(rsp, rdp) ||
rsp->fqs_active) {
if (rcu_scheduler_fully_active &&
cpu_needs_another_gp(rsp, rdp))
rsp->fqs_need_gp = 1;
if (rnp->completed == rsp->completed) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
!cpu_needs_another_gp(rsp, rdp)) {
/*
* Propagate new ->completed value to rcu_node structures
* so that other CPUs don't have to wait until the start
* of the next grace period to process their callbacks.
* Either the scheduler hasn't yet spawned the first
* non-idle task or this CPU does not need another
* grace period. Either way, don't start a new grace
* period.
*/
rcu_for_each_node_breadth_first(rsp, rnp) {
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
rnp->completed = rsp->completed;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
local_irq_restore(flags);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
if (rsp->fqs_active) {
/*
* This CPU needs a grace period, but force_quiescent_state()
* is running. Tell it to start one on this CPU's behalf.
*/
rsp->fqs_need_gp = 1;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
@ -947,6 +943,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
__releases(rcu_get_root(rsp)->lock)
{
unsigned long gp_duration;
struct rcu_node *rnp = rcu_get_root(rsp);
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
@ -958,7 +956,40 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
gp_duration = jiffies - rsp->gp_start;
if (gp_duration > rsp->gp_max)
rsp->gp_max = gp_duration;
rsp->completed = rsp->gpnum;
/*
* We know the grace period is complete, but to everyone else
* it appears to still be ongoing. But it is also the case
* that to everyone else it looks like there is nothing that
* they can do to advance the grace period. It is therefore
* safe for us to drop the lock in order to mark the grace
* period as completed in all of the rcu_node structures.
*
* But if this CPU needs another grace period, it will take
* care of this while initializing the next grace period.
* We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
* because the callbacks have not yet been advanced: Those
* callbacks are waiting on the grace period that just now
* completed.
*/
if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
/*
* Propagate new ->completed value to rcu_node structures
* so that other CPUs don't have to wait until the start
* of the next grace period to process their callbacks.
*/
rcu_for_each_node_breadth_first(rsp, rnp) {
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
rnp->completed = rsp->gpnum;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
rnp = rcu_get_root(rsp);
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
}
rsp->completed = rsp->gpnum; /* Declare the grace period complete. */
trace_rcu_grace_period(rsp->name, rsp->completed, "end");
rsp->signaled = RCU_GP_IDLE;
rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */