[merged] mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch removed from -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: mm, slub: simplify kmem_cache_cpu and tid setup
has been removed from the -mm tree.  Its filename was
     mm-slub-simplify-kmem_cache_cpu-and-tid-setup.patch

This patch was dropped because it was merged into mainline or a subsystem tree

------------------------------------------------------
From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm, slub: simplify kmem_cache_cpu and tid setup

In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee
that our kmem_cache_cpu pointer is from the same cpu as the tid value. 
Currently that's done by reading the tid first using this_cpu_read(), then
the kmem_cache_cpu pointer and verifying we read the same tid using the
pointer and plain READ_ONCE().

This can be simplified to just fetching kmem_cache_cpu pointer and then
reading tid using the pointer.  That guarantees they are from the same
cpu.  We don't need to read the tid using this_cpu_read() because the
value will be validated by this_cpu_cmpxchg_double(), making sure we are
on the correct cpu and the freelist didn't change by anyone preempting us
since reading the tid.

Link: https://lkml.kernel.org/r/20210904105003.11688-11-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Qian Cai <quic_qiancai@quicinc.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/slub.c |   22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

--- a/mm/slub.c~mm-slub-simplify-kmem_cache_cpu-and-tid-setup
+++ a/mm/slub.c
@@ -2882,15 +2882,14 @@ redo:
 	 * reading from one cpu area. That does not matter as long
 	 * as we end up on the original cpu again when doing the cmpxchg.
 	 *
-	 * We should guarantee that tid and kmem_cache are retrieved on
-	 * the same cpu. It could be different if CONFIG_PREEMPTION so we need
-	 * to check if it is matched or not.
+	 * We must guarantee that tid and kmem_cache_cpu are retrieved on the
+	 * same cpu. We read first the kmem_cache_cpu pointer and use it to read
+	 * the tid. If we are preempted and switched to another cpu between the
+	 * two reads, it's OK as the two are still associated with the same cpu
+	 * and cmpxchg later will validate the cpu.
 	 */
-	do {
-		tid = this_cpu_read(s->cpu_slab->tid);
-		c = raw_cpu_ptr(s->cpu_slab);
-	} while (IS_ENABLED(CONFIG_PREEMPTION) &&
-		 unlikely(tid != READ_ONCE(c->tid)));
+	c = raw_cpu_ptr(s->cpu_slab);
+	tid = READ_ONCE(c->tid);
 
 	/*
 	 * Irqless object alloc/free algorithm used here depends on sequence
@@ -3164,11 +3163,8 @@ redo:
 	 * data is retrieved via this pointer. If we are on the same cpu
 	 * during the cmpxchg then the free will succeed.
 	 */
-	do {
-		tid = this_cpu_read(s->cpu_slab->tid);
-		c = raw_cpu_ptr(s->cpu_slab);
-	} while (IS_ENABLED(CONFIG_PREEMPTION) &&
-		 unlikely(tid != READ_ONCE(c->tid)));
+	c = raw_cpu_ptr(s->cpu_slab);
+	tid = READ_ONCE(c->tid);
 
 	/* Same with comment on barrier() in slab_alloc_node() */
 	barrier();
_

Patches currently in -mm which might be from vbabka@suse.cz are