Currently, for non-preempt kernels (with CONFIG_PREEMPTION=n),
rcu_blocking_is_gp() checks (with preemption disabled), whether
there is only one cpu online. It uses num_online_cpus() to
decide whether only one cpu is online. If there is only single
cpu online, synchronize_rcu() is optimized to return without
doing all the work to wait for grace period. However, there are
few issues with the num_online_cpus() check used, for transition
of __num_online_cpus from 2 -> 1 for cpu down path and 1 -> 2
for cpu up path.
1. For CPU path, num_online_cpus() does a atomic_read(&__num_online_cpus).
As hotplug locks are not held, this does not ensure that
new incoming cpus update of the count is visible. This can
result in read side section on new incoming cpu, observe
updates which should not be visible beyond the grace period
corresponding to synchronize_rcu().
For e.g. below litmus test, where P0 process corresponds to
synchronize_rcu() and P1 corresponds to new online cpu, has
positive witnesses; confirming the possibility of read side
section to extend before and after the grace period, thereby
breaking guarantees provided by synchronize_rcu().
{
int x = 0;
atomic_t numonline = ATOMIC_INIT(1);
}
P0(int *x, atomic_t *numonline)
{
int r0;
WRITE_ONCE(*x, 1);
r0 = atomic_read(numonline);
if (r0 == 1) {
smp_mb();
} else {
synchronize_rcu();
}
WRITE_ONCE(*x, 2);
}
P1(int *x, atomic_t *numonline)
{
int r0; int r1;
atomic_inc(numonline);
smp_mb();
rcu_read_lock();
r0 = READ_ONCE(*x);
smp_rmb();
r1 = READ_ONCE(*x);
rcu_read_unlock();
}
locations [x;numonline;]
exists (1:r0=0 /\ 1:r1=2)
2. Second problem is, the same early exit, from synchronize_rcu()
does not provide full ordering, memory barrier, w.r.t. memory
accesses after synchronize_rcu() call.
3. Third, more important issue is related to outgoing cpu. Checking
only for __num_online_cpus with preemotion disabled isn't sufficient
for RCU, as, on completion of CPUHP_TEARDOWN_CPU stop machine (which
clears outgoing cpu from __num_online_cpus, the CPU switches to idle
task. So, checking only for __num_online_cpus does not consider
RCU read side sections in scheduler code (before switching to idle
task) and any potential read side sections in idle task, before final
RCU-quiesce entry into cpuhp_report_idle_dead() -> rcu_report_dead().
The problem here is, changes to number of online cpus happen from places
that are not helpful for lockless access from synchronize_rcu() :
for offlining cpu, it happens from stop machine on that cpu, before
cpu is marked offline in rcu bookkeeping, resulting in unwatched RCU
critical sections; and for onlining cpu, it happens on the new cpu,
after it is online, resulting in visibility issues.
To handle these issues, add a new rcu_state member n_online_cpus, to keep
count of the current number of online cpus. This counter is incremented
early in the CPU-hotplug CPU-online process, that is, on the control CPU,
prior to the new CPU coming online. For outgoing cpu, counter is
decremented on the control cpu, after the offlining cpu is long gone.
Update of this counter from control cpu means that, if rcu_state.
n_online_cpus is equal to one in rcu_blocking_is_gp(), below is
guaranteed:
a. There is only one CPU, and that CPU sees all prior accesses made by
any CPU that was online at the time of its access. Furthermore,
rcu_state.n_online_cpus value cannot change, as CONFIG_PREEMPTION=n.
b. All later CPUs (both this one and any that come online later on) are
guaranteed to see all accesses by any CPU prior to this point in the
code. The CPU-hotplug code provides the required memory barriers to
ensure this.
This eliminates the need for any additional memory barriers for the single
cpu online case; it also eliminates the early counter update and visibility
problems with __num_online_cpus value. For multiple cpu online case,
synchronize_rcu() does all the work of grace period initiation and wait
and that provides the required ordering gurantees. rcu_state.n_online_cpus
update from control cpu would result in unnecessary calls to
synchronize_rcu() slow path during the CPU-online process, but that
should have negligible impact.
Signed-off-by: Neeraj Upadhyay <[email protected]>
---
Changes in V2:
- Make rcu_state.n_online_cpus int, instead of atomic_t.
- Move counter updates to rcutree_prepare_cpu() and rcutree_dead_cpu().
- Update commit log and code comments to highlight single-cpu behavior.
kernel/rcu/tree.c | 21 ++++++++++++++++++++-
kernel/rcu/tree.h | 1 +
2 files changed, 21 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2424e2a..facdec9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2405,6 +2405,7 @@ int rcutree_dead_cpu(unsigned int cpu)
if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
return 0;
+ WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
/* Adjust any no-longer-needed kthreads. */
rcu_boost_kthread_setaffinity(rnp, -1);
/* Do any needed no-CB deferred wakeups from this CPU. */
@@ -3610,7 +3611,20 @@ static int rcu_blocking_is_gp(void)
return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
might_sleep(); /* Check for RCU read-side critical section. */
preempt_disable();
- ret = num_online_cpus() <= 1;
+ /*
+ * If rcu_state.n_online_cpus is equal to one, there is only one CPU,
+ * and that CPU sees all prior accesses made by any CPU that was
+ * online at the time of its access. Furthermore, if rcu_state.
+ * n_online_cpus is equal to one, its value cannot change until
+ * after the preempt_enable() below.
+ *
+ * Furthermore, if n_online_cpus is equal to one here, all later CPUs
+ * (both this one and any that come online later on) are guaranteed to
+ * see all accesses by any CPU prior to this point in the code,
+ * without the need for additional memory barriers. Those memory
+ * barriers are provided by CPU-hotplug code.
+ */
+ ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
preempt_enable();
return ret;
}
@@ -3655,6 +3669,10 @@ void synchronize_rcu(void)
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu() in RCU read-side critical section");
if (rcu_blocking_is_gp())
+ /*
+ * Single cpu-state (see comment in rcu_blocking_is_gp())
+ * guarantees memory ordering.
+ */
return;
if (rcu_gp_is_expedited())
synchronize_rcu_expedited();
@@ -3996,6 +4014,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
rcu_prepare_kthreads(cpu);
rcu_spawn_cpu_nocb_kthread(cpu);
+ WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
return 0;
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e4f66b8..805c9eb 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -298,6 +298,7 @@ struct rcu_state {
/* Hierarchy levels (+1 to */
/* shut bogus gcc warning) */
int ncpus; /* # CPUs seen so far. */
+ int n_online_cpus; /* # CPUs online for RCU. */
/* The following fields are guarded by the root rcu_node's lock. */
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
On Wed, Sep 23, 2020 at 12:59:33PM +0530, Neeraj Upadhyay wrote:
> Currently, for non-preempt kernels (with CONFIG_PREEMPTION=n),
> rcu_blocking_is_gp() checks (with preemption disabled), whether
> there is only one cpu online. It uses num_online_cpus() to
> decide whether only one cpu is online. If there is only single
> cpu online, synchronize_rcu() is optimized to return without
> doing all the work to wait for grace period. However, there are
> few issues with the num_online_cpus() check used, for transition
> of __num_online_cpus from 2 -> 1 for cpu down path and 1 -> 2
> for cpu up path.
Again, good catch!
As usual, I could not resist editing the commit log and comments, so
could you please look the following over to make sure that I did not
mess anything up?
Thanx, Paul
------------------------------------------------------------------------
commit 7af8c1c8d13c6c9c7013fbfef77f843dbc430c4b
Author: Neeraj Upadhyay <[email protected]>
Date: Wed Sep 23 12:59:33 2020 +0530
rcu: Fix single-CPU check in rcu_blocking_is_gp()
Currently, for CONFIG_PREEMPTION=n kernels, rcu_blocking_is_gp() uses
num_online_cpus() to determine whether there is only one CPU online. When
there is only single CPU online, the simple fact that synchronize_rcu()
could be legally called implies that a full grace period has elapsed.
Therefore, in the single-CPU case, synchronize_rcu() simply returns
immediately. Unfortunately, num_online_cpus() is unreliable while a
CPU-hotplug operation is transitioning to or from single-CPU operation
because:
1. num_online_cpus() uses atomic_read(&__num_online_cpus) to
locklessly sample the number of online CPUs. The hotplug locks
are not held, which means that an incoming CPU can concurrently
update this count. This in turn means that an RCU read-side
critical section on the incoming CPU might observe updates
prior to the grace period, but also that this critical section
might extend beyond the end of the optimized synchronize_rcu().
This breaks RCU's fundamental guarantee.
2. In addition, num_online_cpus() does no ordering, thus providing
another way that RCU's fundamental guarantee can be broken by
the current code.
3. The most probable failure mode happens on outgoing CPUs.
The outgoing CPU updates the count of online CPUs in the
CPUHP_TEARDOWN_CPU stop-machine handler, which is fine in
and of itself due to preemption being disabled at the call
to num_online_cpus(). Unfortunately, after that stop-machine
handler returns, the CPU takes one last trip through the
scheduler (which has RCU readers) and, after the resulting
context switch, one final dive into the idle loop. During this
time, RCU needs to keep track of two CPUs, but num_online_cpus()
will say that there is only one, which in turn means that the
surviving CPU will incorrectly ignore the outgoing CPU's RCU
read-side critical sections.
This problem is illustrated by the following litmus test in which P0()
corresponds to synchronize_rcu() and P1() corresponds to the incoming CPU.
The herd7 tool confirms that the "exists" clause can be satisfied,
thus demonstrating that this breakage can happen according to the Linux
kernel memory model.
{
int x = 0;
atomic_t numonline = ATOMIC_INIT(1);
}
P0(int *x, atomic_t *numonline)
{
int r0;
WRITE_ONCE(*x, 1);
r0 = atomic_read(numonline);
if (r0 == 1) {
smp_mb();
} else {
synchronize_rcu();
}
WRITE_ONCE(*x, 2);
}
P1(int *x, atomic_t *numonline)
{
int r0; int r1;
atomic_inc(numonline);
smp_mb();
rcu_read_lock();
r0 = READ_ONCE(*x);
smp_rmb();
r1 = READ_ONCE(*x);
rcu_read_unlock();
}
locations [x;numonline;]
exists (1:r0=0 /\ 1:r1=2)
It is important to note that these problems arise only when the system
is transitioning to or from single-CPU operation.
One solution would be to hold the CPU-hotplug locks while sampling
num_online_cpus(), which was in fact the intent of the (redundant)
preempt_disable() and preempt_enable() surrounding this call to
num_online_cpus(). Actually blocking CPU hotplug would not only result
in excessive overhead, but would also unnecessarily impede CPU-hotplug
operations.
This commit therefore follows long-standing RCU tradition by maintaining
a separate RCU-specific set of CPU-hotplug books.
This separate set of books is implemented by a new ->n_online_cpus field
in the rcu_state structure that maintains RCU's count of the online CPUs.
This count is incremented early in the CPU-online process, so that
the critical transition away from single-CPU operation will occur when
there is only a single CPU. Similarly for the critical transition to
single-CPU operation, the counter is decremented late in the CPU-offline
process, again while there is only a single CPU. Because there is only
ever a single CPU when the ->n_online_cpus field undergoes the critical
1->2 and 2->1 transitions, full memory ordering and mutual exclusion is
provided implicitly and, better yet, for free.
In the case where the CPU is coming online, nothing will happen until
the current CPU helps it come online. Therefore, the new CPU will see
all accesses prior to the optimized grace period, which means that RCU
does not need to further delay this new CPU. In the case where the CPU
is going offline, the outgoing CPU is totally out of the picture before
the optimized grace period starts, which means that this outgoing CPU
cannot see any of the accesses following that grace period. Again,
RCU needs no further interaction with the outgoing CPU.
This does mean that synchronize_rcu() will unnecessarily do a few grace
periods the hard way just before the second CPU comes online and just
after the second-to-last CPU goes offline, but it is not worth optimizing
this uncommon case.
Signed-off-by: Neeraj Upadhyay <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6d9ec8e..9c56b63 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2402,6 +2402,7 @@ int rcutree_dead_cpu(unsigned int cpu)
if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
return 0;
+ WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
/* Adjust any no-longer-needed kthreads. */
rcu_boost_kthread_setaffinity(rnp, -1);
/* Do any needed no-CB deferred wakeups from this CPU. */
@@ -3604,7 +3605,20 @@ static int rcu_blocking_is_gp(void)
return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
might_sleep(); /* Check for RCU read-side critical section. */
preempt_disable();
- ret = num_online_cpus() <= 1;
+ /*
+ * If the rcu_state.n_online_cpus counter is equal to one,
+ * there is only one CPU, and that CPU sees all prior accesses
+ * made by any CPU that was online at the time of its access.
+ * Furthermore, if this counter is equal to one, its value cannot
+ * change until after the preempt_enable() below.
+ *
+ * Furthermore, if rcu_state.n_online_cpus is equal to one here,
+ * all later CPUs (both this one and any that come online later
+ * on) are guaranteed to see all accesses prior to this point
+ * in the code, without the need for additional memory barriers.
+ * Those memory barriers are provided by CPU-hotplug code.
+ */
+ ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
preempt_enable();
return ret;
}
@@ -3649,7 +3663,7 @@ void synchronize_rcu(void)
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu() in RCU read-side critical section");
if (rcu_blocking_is_gp())
- return;
+ return; // Context allows vacuous grace periods.
if (rcu_gp_is_expedited())
synchronize_rcu_expedited();
else
@@ -3989,6 +4003,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
rcu_prepare_kthreads(cpu);
rcu_spawn_cpu_nocb_kthread(cpu);
+ WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
return 0;
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e4f66b8..805c9eb 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -298,6 +298,7 @@ struct rcu_state {
/* Hierarchy levels (+1 to */
/* shut bogus gcc warning) */
int ncpus; /* # CPUs seen so far. */
+ int n_online_cpus; /* # CPUs online for RCU. */
/* The following fields are guarded by the root rcu_node's lock. */
Hi Paul,
On 9/24/2020 2:33 AM, Paul E. McKenney wrote:
> On Wed, Sep 23, 2020 at 12:59:33PM +0530, Neeraj Upadhyay wrote:
>> Currently, for non-preempt kernels (with CONFIG_PREEMPTION=n),
>> rcu_blocking_is_gp() checks (with preemption disabled), whether
>> there is only one cpu online. It uses num_online_cpus() to
>> decide whether only one cpu is online. If there is only single
>> cpu online, synchronize_rcu() is optimized to return without
>> doing all the work to wait for grace period. However, there are
>> few issues with the num_online_cpus() check used, for transition
>> of __num_online_cpus from 2 -> 1 for cpu down path and 1 -> 2
>> for cpu up path.
>
> Again, good catch!
>
> As usual, I could not resist editing the commit log and comments, so
> could you please look the following over to make sure that I did not
> mess anything up?
>
The commit log and comments look very good! Thanks!
Thanks
Neeraj
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 7af8c1c8d13c6c9c7013fbfef77f843dbc430c4b
> Author: Neeraj Upadhyay <[email protected]>
> Date: Wed Sep 23 12:59:33 2020 +0530
>
> rcu: Fix single-CPU check in rcu_blocking_is_gp()
>
> Currently, for CONFIG_PREEMPTION=n kernels, rcu_blocking_is_gp() uses
> num_online_cpus() to determine whether there is only one CPU online. When
> there is only single CPU online, the simple fact that synchronize_rcu()
> could be legally called implies that a full grace period has elapsed.
> Therefore, in the single-CPU case, synchronize_rcu() simply returns
> immediately. Unfortunately, num_online_cpus() is unreliable while a
> CPU-hotplug operation is transitioning to or from single-CPU operation
> because:
>
> 1. num_online_cpus() uses atomic_read(&__num_online_cpus) to
> locklessly sample the number of online CPUs. The hotplug locks
> are not held, which means that an incoming CPU can concurrently
> update this count. This in turn means that an RCU read-side
> critical section on the incoming CPU might observe updates
> prior to the grace period, but also that this critical section
> might extend beyond the end of the optimized synchronize_rcu().
> This breaks RCU's fundamental guarantee.
>
> 2. In addition, num_online_cpus() does no ordering, thus providing
> another way that RCU's fundamental guarantee can be broken by
> the current code.
>
> 3. The most probable failure mode happens on outgoing CPUs.
> The outgoing CPU updates the count of online CPUs in the
> CPUHP_TEARDOWN_CPU stop-machine handler, which is fine in
> and of itself due to preemption being disabled at the call
> to num_online_cpus(). Unfortunately, after that stop-machine
> handler returns, the CPU takes one last trip through the
> scheduler (which has RCU readers) and, after the resulting
> context switch, one final dive into the idle loop. During this
> time, RCU needs to keep track of two CPUs, but num_online_cpus()
> will say that there is only one, which in turn means that the
> surviving CPU will incorrectly ignore the outgoing CPU's RCU
> read-side critical sections.
>
> This problem is illustrated by the following litmus test in which P0()
> corresponds to synchronize_rcu() and P1() corresponds to the incoming CPU.
> The herd7 tool confirms that the "exists" clause can be satisfied,
> thus demonstrating that this breakage can happen according to the Linux
> kernel memory model.
>
> {
> int x = 0;
> atomic_t numonline = ATOMIC_INIT(1);
> }
>
> P0(int *x, atomic_t *numonline)
> {
> int r0;
> WRITE_ONCE(*x, 1);
> r0 = atomic_read(numonline);
> if (r0 == 1) {
> smp_mb();
> } else {
> synchronize_rcu();
> }
> WRITE_ONCE(*x, 2);
> }
>
> P1(int *x, atomic_t *numonline)
> {
> int r0; int r1;
>
> atomic_inc(numonline);
> smp_mb();
> rcu_read_lock();
> r0 = READ_ONCE(*x);
> smp_rmb();
> r1 = READ_ONCE(*x);
> rcu_read_unlock();
> }
>
> locations [x;numonline;]
>
> exists (1:r0=0 /\ 1:r1=2)
>
> It is important to note that these problems arise only when the system
> is transitioning to or from single-CPU operation.
>
> One solution would be to hold the CPU-hotplug locks while sampling
> num_online_cpus(), which was in fact the intent of the (redundant)
> preempt_disable() and preempt_enable() surrounding this call to
> num_online_cpus(). Actually blocking CPU hotplug would not only result
> in excessive overhead, but would also unnecessarily impede CPU-hotplug
> operations.
>
> This commit therefore follows long-standing RCU tradition by maintaining
> a separate RCU-specific set of CPU-hotplug books.
>
> This separate set of books is implemented by a new ->n_online_cpus field
> in the rcu_state structure that maintains RCU's count of the online CPUs.
> This count is incremented early in the CPU-online process, so that
> the critical transition away from single-CPU operation will occur when
> there is only a single CPU. Similarly for the critical transition to
> single-CPU operation, the counter is decremented late in the CPU-offline
> process, again while there is only a single CPU. Because there is only
> ever a single CPU when the ->n_online_cpus field undergoes the critical
> 1->2 and 2->1 transitions, full memory ordering and mutual exclusion is
> provided implicitly and, better yet, for free.
>
> In the case where the CPU is coming online, nothing will happen until
> the current CPU helps it come online. Therefore, the new CPU will see
> all accesses prior to the optimized grace period, which means that RCU
> does not need to further delay this new CPU. In the case where the CPU
> is going offline, the outgoing CPU is totally out of the picture before
> the optimized grace period starts, which means that this outgoing CPU
> cannot see any of the accesses following that grace period. Again,
> RCU needs no further interaction with the outgoing CPU.
>
> This does mean that synchronize_rcu() will unnecessarily do a few grace
> periods the hard way just before the second CPU comes online and just
> after the second-to-last CPU goes offline, but it is not worth optimizing
> this uncommon case.
>
> Signed-off-by: Neeraj Upadhyay <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 6d9ec8e..9c56b63 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2402,6 +2402,7 @@ int rcutree_dead_cpu(unsigned int cpu)
> if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
> return 0;
>
> + WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> /* Adjust any no-longer-needed kthreads. */
> rcu_boost_kthread_setaffinity(rnp, -1);
> /* Do any needed no-CB deferred wakeups from this CPU. */
> @@ -3604,7 +3605,20 @@ static int rcu_blocking_is_gp(void)
> return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
> might_sleep(); /* Check for RCU read-side critical section. */
> preempt_disable();
> - ret = num_online_cpus() <= 1;
> + /*
> + * If the rcu_state.n_online_cpus counter is equal to one,
> + * there is only one CPU, and that CPU sees all prior accesses
> + * made by any CPU that was online at the time of its access.
> + * Furthermore, if this counter is equal to one, its value cannot
> + * change until after the preempt_enable() below.
> + *
> + * Furthermore, if rcu_state.n_online_cpus is equal to one here,
> + * all later CPUs (both this one and any that come online later
> + * on) are guaranteed to see all accesses prior to this point
> + * in the code, without the need for additional memory barriers.
> + * Those memory barriers are provided by CPU-hotplug code.
> + */
> + ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
> preempt_enable();
> return ret;
> }
> @@ -3649,7 +3663,7 @@ void synchronize_rcu(void)
> lock_is_held(&rcu_sched_lock_map),
> "Illegal synchronize_rcu() in RCU read-side critical section");
> if (rcu_blocking_is_gp())
> - return;
> + return; // Context allows vacuous grace periods.
> if (rcu_gp_is_expedited())
> synchronize_rcu_expedited();
> else
> @@ -3989,6 +4003,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> rcu_prepare_kthreads(cpu);
> rcu_spawn_cpu_nocb_kthread(cpu);
> + WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
>
> return 0;
> }
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index e4f66b8..805c9eb 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -298,6 +298,7 @@ struct rcu_state {
> /* Hierarchy levels (+1 to */
> /* shut bogus gcc warning) */
> int ncpus; /* # CPUs seen so far. */
> + int n_online_cpus; /* # CPUs online for RCU. */
>
> /* The following fields are guarded by the root rcu_node's lock. */
>
>
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation