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

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Message-ID: <20210817093408.zanfoybsbnrdnav7@linutronix.de>
Date:   Tue, 17 Aug 2021 11:34:08 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Vlastimil Babka <vbabka@...e.cz>
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 2021-08-17 11:31:56 [+0200], Vlastimil Babka wrote:
> 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.

Without the right debugging enabled

| typedef struct {
| #ifdef CONFIG_DEBUG_LOCK_ALLOC
|         struct lockdep_map      dep_map;
|         struct task_struct      *owner;
| #endif
| } local_lock_t;

the struct is just empty.

Sebastian