On 10/05/21 19:45, Sean Christopherson wrote:
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Currently, rmaps are always allocated and published together with a new
memslot, so the srcu_dereference for the memslots array already ensures that
the memory pointed to by slots->arch.rmap is zero at the time
slots->arch.rmap. However, they still need to be accessed in an SRCU
read-side critical section, as the whole memslot can be deleted outside
SRCU.
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I disagree, sprinkling random and unnecessary __rcu/SRCU annotations does more
harm than good. Adding the unnecessary tag could be quite misleading as it
would imply the rmap pointers can_change_ independent of the memslots.
Similary, adding rcu_assign_pointer() in alloc_memslot_rmap() implies that its
safe to access the rmap after its pointer is assigned, and that's simply not
true since an rmap array can be freed if rmap allocation for a different memslot
fails. Accessing the rmap is safe if and only if all rmaps are allocated, i.e.
if arch.memslots_have_rmaps is true, as you pointed out.
This about freeing is a very good point.
Furthermore, to actually gain any protection from SRCU, there would have to be
an synchronize_srcu() call after assigning the pointers, and that _does_ have an
associated.
... but this is incorrect (I was almost going to point out the below in
my reply to Ben, then decided I was pointing out the obvious; lesson
learned).
synchronize_srcu() is only needed after *deleting* something, which in
this case is done as part of deleting the memslots---it's perfectly fine
to batch multiple synchronize_*() calls given how expensive some of them
are.
(BTW an associated what?)
So they still count as RCU-protected in my opinion, just because reading
them outside SRCU is a big no and ought to warn (it's unlikely that it
happens with rmaps, but then we just had 2-3 bugs like this being
reported in a short time for memslots so never say never). However,
rcu_assign_pointer is not needed because the visibility of the rmaps is
further protected by the have-rmaps flag (to be accessed with
load-acquire/store-release) and not just by the pointer being there and
non-NULL.
Paolo
Not to mention that to truly respect the __rcu annotation, deleting
the rmaps would also have to be done "independently" with the correct
rcu_assign_pointer() and synchronize_srcu() logic.
