Re: [openib-general] Re: [git patch review 1/4] IPoIB: Don't start send-only joins while multicast thread is stopped

[Michael S. Tsirkin]

Quoting r. Roland Dreier <rdreier@xxxxxxxxx>:
> Subject: Re: [openib-general] Re: [git patch review 1/4] IPoIB: Don't start send-only joins while multicast thread is stopped
> 
>     Michael> Basically, its as Andrew said: the lock around clear_bit
>     Michael> is there to ensure that ipoib_mcast_send isnt running
>     Michael> already when we stop the thread.  Thats why test_bit has
>     Michael> to be inside the lock, too.
> 
> Makes sense I guess.  If I'm understanding correctly, the lock isn't
> really there to serialize the bit ops, but rather to make sure
> ipoib_mcast_send() won't do anything after we clear the bit.

Right. Thats one way to put it.

> Does that mean that there's no reason to take the lock around the set_bit()?

Ugh, sorry, I dont really remember why I put it there.

I guess I just have easier time reasoning about locks than barriers and atomic
operations. "bit is protected by priv->lock" is a simple rule, and we are not on
data path here. The fact that the race went unnoticed for a while validates
this approach in my eyes.

I guess longer term we will replace mcast_mutex with priv->lock anyway, so it
doesnt matter much.

-- 
Michael S. Tsirkin
Staff Engineer, Mellanox Technologies
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