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Subject: Re: [rfc/patch] mm/slub: restore/expand unfreeze_partials() local
 exclusion scope
To:     Mike Galbraith <efault@gmx.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     linux-rt-users@vger.kernel.org,
        Mel Gorman <mgorman@techsingularity.net>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <5be1a703-9a0a-4115-1d69-634e5e8ecefd@suse.cz>
Date:   Sat, 24 Jul 2021 00:39:20 +0200
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On 7/21/21 11:33 AM, Mike Galbraith wrote:
> On Wed, 2021-07-21 at 10:44 +0200, Vlastimil Babka wrote:
>>
>> So this doesn't look like our put_cpu_partial() preempted a
>> __slab_alloc() on the same cpu, right?
> 
> No, likely it was the one preempted by someone long gone, but we'll
> never know without setting a trap.
> 
>> BTW did my ugly patch work?
> 
> Nope.  I guess you missed my reporting it to have been a -ENOBOOT, and

Indeed, I misunderstood it as you talking about your patch.

> that cutting it in half, ie snagging only __slab_free() does boot, and
> seems to cure all of the RT fireworks.

OK, so depending on drain=1 makes this apply only to put_cpu_partial()
called from __slab_free and not get_partial_node(). One notable
difference is that in __slab_free we don't have n->list_lock locked and
in get_partial_node() we do. I guess in case your list_lock is made raw
again by another patch, that explains a local_lock can't nest under it.
If not, then I would expect this to work (I don't think they ever nest
in the opposite order, also lockdep should tell us instead of
-ENOBOOT?), but might be missing something...

I'd rather not nest those locks in any case. I just need to convince
myself that the scenario the half-fix fixes is indeed the only one
that's needed and we're not leaving there other races that are just
harder to trigger...

> (chainsaw noises...)
> 
> ---
>  mm/slub.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2551,6 +2551,8 @@ static void put_cpu_partial(struct kmem_
>  	int pobjects;
> 
>  	slub_get_cpu_ptr(s->cpu_slab);
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
> +		local_lock(&s->cpu_slab->lock);
>  	do {
>  		pages = 0;
>  		pobjects = 0;
> @@ -2564,7 +2566,13 @@ static void put_cpu_partial(struct kmem_
>  				 * partial array is full. Move the existing
>  				 * set to the per node partial list.
>  				 */
> -				unfreeze_partials(s);
> +				this_cpu_write(s->cpu_slab->partial, NULL);
> +				if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
> +					local_unlock(&s->cpu_slab->lock);
> +				__unfreeze_partials(s, oldpage);
> +				if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
> +					local_lock(&s->cpu_slab->lock);
> +
>  				oldpage = NULL;
>  				pobjects = 0;
>  				pages = 0;
> @@ -2581,6 +2589,8 @@ static void put_cpu_partial(struct kmem_
> 
>  	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
>  								!= oldpage);
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && drain)
> +		local_unlock(&s->cpu_slab->lock);
>  	slub_put_cpu_ptr(s->cpu_slab);
>  #endif	/* CONFIG_SLUB_CPU_PARTIAL */
>  }
> 
>