linux-kernel - Re: [PATCH 3/4] rhashtable: use bit_spin

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Date:   Wed, 27 Mar 2019 09:40:14 +1100
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash bucket.

On Tue, Mar 26 2019, Herbert Xu wrote:

> On Mon, Mar 25, 2019 at 04:05:39PM +1100, NeilBrown wrote:
>>
>> +/*
>> + * If 'p' is a bucket head and might be locked:
>> + *   rht_ptr() returns the address without the lock bit.
>> + *   rht_ptr_locked() returns the address WITH the lock bit.
>> + *   rht_is_locked() tests if the lock bit is set.
>> + */
>> +static inline struct rhash_head __rcu *rht_ptr(const struct rhash_head *p)
>> +{
>> +	return (void *)(((unsigned long)p) & ~2UL);
>> +}
>
> My preference would be to turn rhash_head * into an opaque type
> that can only be dereferenced through rht_ptr.

I can try that.
Presumably the 'next' pointer in 'struct rhash_head' can still be a
normal pointer, but the buckets in the bucket_table would need to
opaque pointers.
Maybe pointers to some new undefined struct, which gets cast and masked
as appropriate?

I'll have a go and see how it looks.

Thanks,
NeilBrown

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