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

From: tip-bot2 for Paul E. McKenney
Date: Tue Mar 24 2020 - 05:19:05 EST


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

Commit-ID: 7ff8b4502bc0f576450d4ecbda97fa30e8002ed1
Gitweb: https://git.kernel.org/tip/7ff8b4502bc0f576450d4ecbda97fa30e8002ed1
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate: Sun, 22 Dec 2019 19:32:54 -08:00
Committer: Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 16:01:11 -08:00

srcu: Fix __call_srcu()/process_srcu() datarace

The srcu_node 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 | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 657e6a7..b1edac9 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -550,7 +550,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
snp->srcu_have_cbs[idx] = gpseq;
rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
- snp->srcu_gp_seq_needed_exp = gpseq;
+ WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
mask = snp->srcu_data_have_cbs[idx];
snp->srcu_data_have_cbs[idx] = 0;
spin_unlock_irq_rcu_node(snp);
@@ -660,7 +660,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
if (snp == sdp->mynode)
snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
- snp->srcu_gp_seq_needed_exp = s;
+ WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
spin_unlock_irqrestore_rcu_node(snp, flags);
}