[PATCH AUTOSEL 4.20 028/304] srcu: Prevent __call_srcu() counter wrap with read-side critical section

Sasha Levin
2019-01-28T17:50:43Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

[ Upstream commit 0607ba8403c4cdb253f8c5200ecf654dfb7790cc ]

Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from
both process and interrupt context"), it has been permissible
to use SRCU read-side critical sections in interrupt context.
This allows __call_srcu() to use SRCU read-side critical sections to
prevent a new SRCU grace period from ending before the call to either
srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing
SRCU grace-period counter overflow during that time.

Note that this does not permit removal of the counter-wrap checks in
srcu_gp_end(). These check are necessary to handle the case where
a given CPU does not interact at all with SRCU for an extended time
period.

This commit therefore adds an SRCU read-side critical section to
__call_srcu() in order to prevent grace period counter wrap during
the funnel-locking process.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/srcutree.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index a180abc8c925..697a2d7e8e8a 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -860,6 +860,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
rcu_callback_t func, bool do_norm)
{
unsigned long flags;
+ int idx;
bool needexp = false;
bool needgp = false;
unsigned long s;
@@ -873,6 +874,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
return;
}
rhp->func = func;
+ idx = srcu_read_lock(sp);
local_irq_save(flags);
sdp = this_cpu_ptr(sp->sda);
spin_lock_rcu_node(sdp);
@@ -894,6 +896,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
srcu_funnel_gp_start(sp, sdp, s, do_norm);
else if (needexp)
srcu_funnel_exp_start(sp, sdp->mynode, s);
+ srcu_read_unlock(sp, idx);
}

/**
--
2.19.1