[tip: core/rcu] srcu: Fix __call_srcu()/srcu_get_delay() datarace

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     8c9e0cb32315835825ea1ad725a858a2d2ce4a8e
Gitweb:        https://git.kernel.org/tip/8c9e0cb32315835825ea1ad725a858a2d2ce4a8e
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Sun, 22 Dec 2019 19:36:33 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 16:01:11 -08:00

srcu: Fix __call_srcu()/srcu_get_delay() datarace

The srcu_struct structure's ->srcu_gp_seq_needed_exp field is accessed
locklessly, so updates must use WRITE_ONCE().  This commit therefore
adds the needed WRITE_ONCE() invocations.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index b1edac9..79848f7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -534,7 +534,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	rcu_seq_end(&ssp->srcu_gp_seq);
 	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
 	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
-		ssp->srcu_gp_seq_needed_exp = gpseq;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, gpseq);
 	spin_unlock_irq_rcu_node(ssp);
 	mutex_unlock(&ssp->srcu_gp_mutex);
 	/* A new grace period can start at this point.  But only one. */
@@ -614,7 +614,7 @@ static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp
 	}
 	spin_lock_irqsave_rcu_node(ssp, flags);
 	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
-		ssp->srcu_gp_seq_needed_exp = s;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 	spin_unlock_irqrestore_rcu_node(ssp, flags);
 }
 
@@ -674,7 +674,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 		smp_store_release(&ssp->srcu_gp_seq_needed, s); /*^^^*/
 	}
 	if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
-		ssp->srcu_gp_seq_needed_exp = s;
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
 
 	/* If grace period not already done and none in progress, start it. */
 	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&