[PATCH v3 09/30] block: Pre-allocate zone write plugs

[Christoph Hellwig]

On Thu, Mar 28, 2024 at 02:28:40PM +0900, Damien Le Moal wrote:
> That was my thinking initially as well, which is why I did not have the
> grace period. However, getting a reference on a plug is a not done under
> disk->zone_wplugs_lock and is thus racy, albeit with a super tiny time
> window: the hash table lookup may "see" a plug that has already been
> removed and has a refcount dropped to 0 already. The use of
> atomic_inc_not_zero() prevents us from trying to keep using that stale
> plug, but we *are* referencing it. So without the grace period, I think
> there is a risk (again, super tiny window) that we start reusing the
> plug, or kfree it while atomic_inc_not_zero() is executing...
> I am overthinking this ?

Well.  All the lookups fail (or should fail) when BLK_ZONE_WPLUG_UNHASHED
is set, probably even before even trying to grab a reference.  So all
the lookups for a zone that is beeing torn down will fail.  Now once
the actual final reference is dropped, we'll now need to clear
BLK_ZONE_WPLUG_UNHASHED and lookup can happe again.  We'd have a race
window there, but I guess we can plug it by checking for the right
zone number?  If we it while it already got reduce that'll still fail
the lookup.