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@oracle.com>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250512092736.229935-16-bigeasy@linutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/rds/page.c b/net/rds/page.c
index 58a8548a915a9aa9d07c4b20ce9c82044ed5f380..afb151eac271c04ef3714823a84e7047c8ebc0b0 100644 (file)
--- 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,