Re: [Qemu-devel] [PATCH v3 01/11] util/qht: Document memory ordering assumptions

[Sergey Fedorov]

On 15/07/16 15:51, Paolo Bonzini wrote:
>
> On 15/07/2016 14:37, Sergey Fedorov wrote:
>> I understand why an implied wmb can be expected after the entry is
>> removed: so that the caller can trash the contents of the object
>> removed. However that would require double-check on lookup side to make
>> sure the entry hadn't been removed after the first lookup (something
>> like seqlock read side). But I have no idea why we might like to promise
>> wmb before the removal. Could you please share your point regarding this?
> The reasoning was to make qht_remove() "look to be ordered" with respect
> to writes.  So anything after remove wouldn't bleed into it, nor would
> anything before.
>
> In the case of this series, it would let you remove the smp_wmb() after
> tb_mark_invalid().  However, it's also okay to only handle qht_insert()
> and qht_lookup(), and keep the memory barrier after the TB is marked
> invalid (though it is unnecessary).
>

I'm pretty sure the smp_wmb() after tb_mark_invalid() is unnecessary
anyway. We don't rely on it at all because we're to recheck for
tb_is_invalid() under tb_lock before tb_add_jump(). However we have to
invalidate the CPU state atomically since we're going to check for it
out of tb_lock.

Kind regards,
Sergey