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rcu: Make offline-CPU checking allow for indefinite delays 

The rcu_implicit_offline_qs() function implicitly assumed that execution
would progress predictably when interrupts are disabled, which is of course
not guaranteed when running on a hypervisor.  Furthermore, this function
is short, and is called from one place only in a short function.

This commit therefore ensures that the timing is checked before
checking the condition, which guarantees correct behavior even given
indefinite delays.  It also inlines rcu_implicit_offline_qs() into
rcu_implicit_dynticks_qs().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2012-08-01 14:29:20 -07:00 committed by Paul E. McKenney
parent 5cc900cf55
commit a82dcc7602
1 changed files with 21 additions and 32 deletions
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53
kernel/rcutree.c
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@ -318,35 +318,6 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
return &rsp->node[0];
}
/*
* If the specified CPU is offline, tell the caller that it is in
* a quiescent state. Otherwise, whack it with a reschedule IPI.
* Grace periods can end up waiting on an offline CPU when that
* CPU is in the process of coming online -- it will be added to the
* rcu_node bitmasks before it actually makes it online. The same thing
* can happen while a CPU is in the process of coming online. Because this
* race is quite rare, we check for it after detecting that the grace
* period has been delayed rather than checking each and every CPU
* each and every time we start a new grace period.
*/
static int rcu_implicit_offline_qs(struct rcu_data *rdp)
{
/*
* If the CPU is offline for more than a jiffy, it is in a quiescent
* state. We can trust its state not to change because interrupts
* are disabled. The reason for the jiffy's worth of slack is to
* handle CPUs initializing on the way up and finding their way
* to the idle loop on the way down.
*/
if (cpu_is_offline(rdp->cpu) &&
ULONG_CMP_LT(rdp->rsp->gp_start + 2, jiffies)) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
rdp->offline_fqs++;
return 1;
}
return 0;
}
/*
* rcu_idle_enter_common - inform RCU that current CPU is moving towards idle
*
@ -675,7 +646,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
* Return true if the specified CPU has passed through a quiescent
* state by virtue of being in or having passed through an dynticks
* idle state since the last call to dyntick_save_progress_counter()
* for this same CPU.
* for this same CPU, or by virtue of having been offline.
*/
static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
{
@ -699,8 +670,26 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
return 1;
}
/* Go check for the CPU being offline. */
return rcu_implicit_offline_qs(rdp);
/*
* Check for the CPU being offline, but only if the grace period
* is old enough. We don't need to worry about the CPU changing
* state: If we see it offline even once, it has been through a
* quiescent state.
*
* The reason for insisting that the grace period be at least
* one jiffy old is that CPUs that are not quite online and that
* have just gone offline can still execute RCU read-side critical
* sections.
*/
if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
return 0; /* Grace period is not old enough. */
barrier();
if (cpu_is_offline(rdp->cpu)) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
rdp->offline_fqs++;
return 1;
}
return 0;
}
static int jiffies_till_stall_check(void)