linux-kernel - Re: [PATCH v4 35/35] mm, slub: convert kmem_cpu_slab protection to local

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Message-ID: <3da13f0a-2720-a38c-33a7-744668013390@suse.cz>
Date:   Tue, 17 Aug 2021 11:31:56 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Sven Eckelmann <sven@...fation.org>, akpm@...ux-foundation.org,
        brouer@...hat.com, cl@...ux.com, efault@....de,
        iamjoonsoo.kim@....com, jannh@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mgorman@...hsingularity.net, penberg@...nel.org,
        rientjes@...gle.com, tglx@...utronix.de,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 35/35] mm, slub: convert kmem_cpu_slab protection to
 local_lock

On 8/17/21 11:12 AM, Sebastian Andrzej Siewior wrote:
> On 2021-08-17 10:37:48 [+0200], Vlastimil Babka wrote:
>> OK reproduced. Thanks, will investigate.
> 
> With the local_lock at the top, the needed alignment gets broken for dbl
> cmpxchg.

Right. I wondered why the checks in __pcpu_double_call_return_bool
didn't trigger. They are VM_BUG_ON() so they did trigger after enabling
DEBUG_VM.

> 
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index b5bcac29b979c..cd14aa1f9bc3c 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -42,9 +42,9 @@ enum stat_item {
>  	NR_SLUB_STAT_ITEMS };
>  
>  struct kmem_cache_cpu {
> -	local_lock_t lock;	/* Protects the fields below except stat */
>  	void **freelist;	/* Pointer to next available object */
>  	unsigned long tid;	/* Globally unique transaction id */
> +	local_lock_t lock;	/* Protects the fields below except stat */
>  	struct page *page;	/* The slab from which we are allocating */
>  #ifdef CONFIG_SLUB_CPU_PARTIAL
>  	struct page *partial;	/* Partially allocated frozen slabs */
> 
> Sebastian
>