linux-kernel - [PATCH RT 2/2 v2] list_bl: avoid BUG when the list is not locked

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Date:   Tue, 12 Nov 2019 11:16:29 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     tglx@...utronix.de, linux-rt-users@...r.kernel.org
cc:     Mike Snitzer <msnitzer@...hat.com>,
        Nikos Tsironis <ntsironis@...ikto.com>,
        Scott Wood <swood@...hat.com>,
        Ilias Tsitsimpis <iliastsi@...ikto.com>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Daniel Wagner <dwagner@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH RT 2/2 v2] list_bl: avoid BUG when the list is not locked

list_bl would crash with BUG() if we used it without locking. dm-snapshot 
uses its own locking on realtime kernels (it can't use list_bl because 
list_bl uses raw spinlock and dm-snapshot takes other non-raw spinlocks 
while holding bl_lock).

To avoid this BUG, we must set LIST_BL_LOCKMASK = 0.

This patch is intended only for the realtime kernel patchset, not for the 
upstream kernel.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

Index: linux-rt-devel/include/linux/list_bl.h
===================================================================
--- linux-rt-devel.orig/include/linux/list_bl.h	2019-11-07 14:01:51.000000000 +0100
+++ linux-rt-devel/include/linux/list_bl.h	2019-11-08 10:12:49.000000000 +0100
@@ -19,7 +19,7 @@
  * some fast and compact auxiliary data.
  */
 
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
+#if (defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)) && !defined(CONFIG_PREEMPT_RT_BASE)
 #define LIST_BL_LOCKMASK	1UL
 #else
 #define LIST_BL_LOCKMASK	0UL
@@ -161,9 +161,6 @@ static inline void hlist_bl_lock(struct
 	bit_spin_lock(0, (unsigned long *)b);
 #else
 	raw_spin_lock(&b->lock);
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
-	__set_bit(0, (unsigned long *)b);
-#endif
 #endif
 }
 
@@ -172,9 +169,6 @@ static inline void hlist_bl_unlock(struc
 #ifndef CONFIG_PREEMPT_RT_BASE
 	__bit_spin_unlock(0, (unsigned long *)b);
 #else
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
-	__clear_bit(0, (unsigned long *)b);
-#endif
 	raw_spin_unlock(&b->lock);
 #endif
 }