netdev - [PATCH net-next 18/18] rds: Use nested-BH locking for rds_page

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Message-ID: <20250309144653.825351-19-bigeasy@linutronix.de>
Date: Sun,  9 Mar 2025 15:46:53 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: netdev@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Allison Henderson <allison.henderson@...cle.com>,
	linux-rdma@...r.kernel.org
Subject: [PATCH net-next 18/18] rds: Use nested-BH locking for rds_page_remainder.

rds_page_remainder is a per-CPU variable and relies on disabled BH for its
locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Add a local_lock_t to the data structure and use
local_lock_nested_bh() for locking. This change adds only lockdep
coverage and does not alter the functional behaviour for !PREEMPT_RT.

Cc: Allison Henderson <allison.henderson@...cle.com>
Cc: linux-rdma@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 net/rds/page.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/rds/page.c b/net/rds/page.c
index 58a8548a915a9..afb151eac271c 100644
--- a/net/rds/page.c
+++ b/net/rds/page.c
@@ -40,10 +40,12 @@
 struct rds_page_remainder {
 	struct page	*r_page;
 	unsigned long	r_offset;
+	local_lock_t	bh_lock;
 };
 
-static
-DEFINE_PER_CPU_SHARED_ALIGNED(struct rds_page_remainder, rds_page_remainders);
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct rds_page_remainder, rds_page_remainders) = {
+	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};
 
 /**
  * rds_page_remainder_alloc - build up regions of a message.
@@ -87,6 +89,7 @@ int rds_page_remainder_alloc(struct scatterlist *scat, unsigned long bytes,
 	}
 
 	local_bh_disable();
+	local_lock_nested_bh(&rds_page_remainders.bh_lock);
 	rem = this_cpu_ptr(&rds_page_remainders);
 
 	while (1) {
@@ -115,11 +118,13 @@ int rds_page_remainder_alloc(struct scatterlist *scat, unsigned long bytes,
 		}
 
 		/* alloc if there is nothing for us to use */
+		local_unlock_nested_bh(&rds_page_remainders.bh_lock);
 		local_bh_enable();
 
 		page = alloc_page(gfp);
 
 		local_bh_disable();
+		local_lock_nested_bh(&rds_page_remainders.bh_lock);
 		rem = this_cpu_ptr(&rds_page_remainders);
 
 		if (!page) {
@@ -138,6 +143,7 @@ int rds_page_remainder_alloc(struct scatterlist *scat, unsigned long bytes,
 		rem->r_offset = 0;
 	}
 
+	local_unlock_nested_bh(&rds_page_remainders.bh_lock);
 	local_bh_enable();
 out:
 	rdsdebug("bytes %lu ret %d %p %u %u\n", bytes, ret,
-- 
2.47.2