[PATCH RFC v2 05/15] hazptr: add debug option to force the acquire slow path

[Kunwu Chan <kunwu.chan@gmail.com>]

Add a debug option that makes hazptr_acquire() always enter
__hazptr_acquire(), even when a per-CPU slot is available.

This allows the slow path, including its lock acquisition, to be
exercised by lockdep-heavy workloads.  It also lets hazptrtorture
exercise the slow path without requiring the per-CPU slots to be
exhausted.

Suggested-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 include/linux/hazptr.h   |  3 ++-
 kernel/rcu/Kconfig.debug | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
index 275cbe54b075..4ee02d23abc9 100644
--- a/include/linux/hazptr.h
+++ b/include/linux/hazptr.h
@@ -245,7 +245,8 @@ void *hazptr_acquire(struct hazptr_ctx *ctx, void * const *addr_p)
 	ctx->acquire_cpu = smp_processor_id();
 	ctx->acquire_caller = _THIS_IP_;
 #endif
-	if (unlikely(slot->addr))
+	if (IS_ENABLED(CONFIG_HAZPTR_ACQUIRE_FORCE_SLOWPATH) ||
+	    unlikely(slot->addr))
 		return __hazptr_acquire(ctx, addr_p);
 	WRITE_ONCE(slot->addr, READ_ONCE(hazptr_wildcard));	/* Store B */
 
diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 7c3c6017b266..3a2e9d7f58f8 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -265,4 +265,14 @@ config HAZPTR_DEBUG
 
 	  Say Y here if you want to enable those assert, N otherwise.
 
+config HAZPTR_ACQUIRE_FORCE_SLOWPATH
+	bool "Force hazard-pointer acquire into the slow path"
+	depends on DEBUG_KERNEL
+	default n
+	help
+	  Make hazptr_acquire() always enter __hazptr_acquire().
+	  This is useful for testing slow-path locking, especially
+	  with lockdep.  It also lets hazptrtorture exercise the slow
+	  path without filling the per-CPU slots.
+
 endmenu # "RCU Debugging"
-- 
2.43.0