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From:   Vlastimil Babka <vbabka@suse.cz>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mike Galbraith <efault@gmx.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Jann Horn <jannh@google.com>, Vlastimil Babka <vbabka@suse.cz>
Subject: [PATCH v4 32/35] mm, slub: make slab_lock() disable irqs with PREEMPT_RT
Date:   Thu,  5 Aug 2021 17:19:57 +0200
Message-Id: <20210805152000.12817-33-vbabka@suse.cz>
In-Reply-To: <20210805152000.12817-1-vbabka@suse.cz>
References: <20210805152000.12817-1-vbabka@suse.cz>
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We need to disable irqs around slab_lock() (a bit spinlock) to make it
irq-safe. The calls to slab_lock() are nested under spin_lock_irqsave() which
doesn't disable irqs on PREEMPT_RT, so add explicit disabling with PREEMPT_RT.

We also distinguish cmpxchg_double_slab() where we do the disabling explicitly
and __cmpxchg_double_slab() for contexts with already disabled irqs.  However
these context are also typically spin_lock_irqsave() thus insufficient on
PREEMPT_RT. Thus, change __cmpxchg_double_slab() to be same as
cmpxchg_double_slab() on PREEMPT_RT.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 9208020f72d5..252421ff1d5f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -380,12 +380,12 @@ __slab_unlock(struct page *page, unsigned long *flags, bool disable_irqs)
 static __always_inline void
 slab_lock(struct page *page, unsigned long *flags)
 {
-	__slab_lock(page, flags, false);
+	__slab_lock(page, flags, IS_ENABLED(CONFIG_PREEMPT_RT));
 }
 
 static __always_inline void slab_unlock(struct page *page, unsigned long *flags)
 {
-	__slab_unlock(page, flags, false);
+	__slab_unlock(page, flags, IS_ENABLED(CONFIG_PREEMPT_RT));
 }
 
 static inline bool ___cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
@@ -429,14 +429,19 @@ static inline bool ___cmpxchg_double_slab(struct kmem_cache *s, struct page *pag
 	return false;
 }
 
-/* Interrupts must be disabled (for the fallback code to work right) */
+/*
+ * Interrupts must be disabled (for the fallback code to work right), typically
+ * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
+ * so we disable interrupts explicitly here.
+ */
 static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
 		void *freelist_old, unsigned long counters_old,
 		void *freelist_new, unsigned long counters_new,
 		const char *n)
 {
 	return ___cmpxchg_double_slab(s, page, freelist_old, counters_old,
-				      freelist_new, counters_new, n, false);
+				      freelist_new, counters_new, n,
+				      IS_ENABLED(CONFIG_PREEMPT_RT));
 }
 
 static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
-- 
2.32.0