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rcu: Accelerate callbacks for CPU initiating a grace period 

Because grace-period initialization is carried out by a separate
kthread, it might happen on a different CPU than the one that
had the callback needing a grace period -- which is where the
callback acceleration needs to happen.

Fortunately, rcu_start_gp() holds the root rcu_node structure's
->lock, which prevents a new grace period from starting.  This
allows this function to safely determine that a grace period has
not yet started, which in turn allows it to fully accelerate any
callbacks that it has pending.  This commit adds this acceleration.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2012-09-20 16:02:49 -07:00
parent 6f0c0580b7
commit 62da192129
1 changed files with 24 additions and 2 deletions
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26
kernel/rcutree.c
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@ -1404,15 +1404,37 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
!cpu_needs_another_gp(rsp, rdp)) {
/*
* Either we have not yet spawned the grace-period
* task or this CPU does not need another grace period.
* task, this CPU does not need another grace period,
* or a grace period is already in progress.
* Either way, don't start a new grace period.
*/
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
/*
* Because there is no grace period in progress right now,
* any callbacks we have up to this point will be satisfied
* by the next grace period. So promote all callbacks to be
* handled after the end of the next grace period. If the
* CPU is not yet aware of the end of the previous grace period,
* we need to allow for the callback advancement that will
* occur when it does become aware. Deadlock prevents us from
* making it aware at this point: We cannot acquire a leaf
* rcu_node ->lock while holding the root rcu_node ->lock.
*/
rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
if (rdp->completed == rsp->completed)
rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
rsp->gp_flags = RCU_GP_FLAG_INIT;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
raw_spin_unlock(&rnp->lock); /* Interrupts remain disabled. */
/* Ensure that CPU is aware of completion of last grace period. */
rcu_process_gp_end(rsp, rdp);
local_irq_restore(flags);
/* Wake up rcu_gp_kthread() to start the grace period. */
wake_up(&rsp->gp_wq);
}