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Date: Tue, 24 May 2016 18:07:55 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160524220755.GA25193@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v5 13/18] qht: support parallel writes
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To: Sergey Fedorov <serge.fdrv@gmail.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On Mon, May 23, 2016 at 23:28:27 +0300, Sergey Fedorov wrote:
> What if we turn qht::lock into a mutex and change the function as follows:
> 
>     static inline
>     struct qht_bucket *qht_bucket_lock__no_stale(struct qht *ht,
>     uint32_t hash,
>                                                  struct qht_map
>     **pmap)       
>     {
>         struct qht_bucket *b;
>         struct qht_map *map;
> 
>         map = atomic_rcu_read(&ht->map);
>         b = qht_map_to_bucket(map, hash);
> 
>         qemu_spin_lock(&b->lock);
>         /* 'ht->map' access is serialized by 'b->lock' here */
>         if (likely(map == ht->map)) {
>             /* no resize in progress; we're done */
>             *pmap = map;
>             return b;
>         }
>         qemu_spin_unlock(&b->lock);
> 
>         /* resize in progress; retry grabbing 'ht->lock' */
>         qemu_mutex_lock(&ht->lock);
>         b = qht_map_to_bucket(ht->map, hash);
>         *pmap = ht->map;
>         qemu_spin_lock(&b->lock);
>         qemu_mutex_unlock(&ht->lock);
> 
>         return b;
>     }
> 
> 
> With this implementation we could:
>  (1) get rid of qht_map::stale
>  (2) don't waste cycles waiting for resize to complete

I'll include this in v6.

Thanks,

		Emilio