RE: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash bucket.

[NeilBrown <neilb@suse.com>]

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On Wed, Apr 03 2019, David Laight wrote:

> From: NeilBrown
>> Sent: 02 April 2019 22:11
>> 
>> On Tue, Apr 02 2019, David Laight wrote:
>> 
>> > From: NeilBrown
>> >> Sent: 02 April 2019 00:08
>> >>
>> >> This patch changes rhashtables to use a bit_spin_lock on BIT(1) of the
>> >> bucket pointer to lock the hash chain for that bucket.
>> > ...
>> >> To enhance type checking, a new struct is introduced to represent the
>> >>   pointer plus lock-bit
>> >> that is stored in the bucket-table.  This is "struct rhash_lock_head"
>> >> and is empty.  A pointer to this needs to be cast to either an
>> >> unsigned lock, or a "struct rhash_head *" to be useful.
>> >> Variables of this type are most often called "bkt".
>> >
>> > Did you try using a union of the pointer and an 'unsigned long' ?
>> > Should remove a lot of the casts.
>> 
>> It might, but I'm not sure it is what we want.
>> The value is not an unsigned long OR a pointer, it is both blended
>> together.  So it really isn't a union.
>> We *want* it to require casts to access, so that it is clear that
>> something unusual is happening, and care is needed.
>
> Right, but you also want to make it hard to forget to do it properly.
> Using a union to access the memory as either a pointer or a long
> is perfectly valid (and is valid with 'strict aliasing' enabled).
> (Rather than the other use of a union to just save space.)

Agreed.... I personally think that a union make it easy to forget.

>
> An interesting thought....
> Are the only valid actions 'lock and read, and 'unlock with optional update' ?

No, there is also "read without locking" - use for lookups with RCU
protection.  But yes: the set of valid actions is quite limited.

> If so there are only 2 bits of code that need to understand the encoding.
> If you make the bit number(s) and polarity properties of the architecture
> you should be able to make the stored value an invalid pointer (locked
> and unlocked) on at least some architectures.

I'd rather avoid writing arch-specific code if I can avoid it.  It isn't
clear that the value of your proposal justifies the cost.
Over-loading the low-order bits of a pointer is (I think) a well
understood pattern.  I'd rather stick with such patterns.

Thanks,
NeilBrown


>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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