On Wed, Jul 24, 2019 at 04:05:17AM -0400, Michael S. Tsirkin wrote:

On Wed, Jul 24, 2019 at 10:17:14AM +0800, Jason Wang wrote:

quoted

So even PTE is read speculatively before reading invalidate_count (only in
the case of invalidate_count is zero). The spinlock has guaranteed that we
won't read any stale PTEs.

I'm sorry I just do not get the argument.
If you want to order two reads you need an smp_rmb
or stronger between them executed on the same CPU.

No, that is only for unlocked algorithms.

In this case the spinlock provides all the 'or stronger' ordering
required.

For invalidate_count going 0->1 the spin_lock ensures that any
following PTE update during invalidation does not order before the
spin_lock()

While holding the lock and observing 1 in invalidate_count the PTE
values might be changing, but are ignored. C's rules about sequencing
make this safe.

For invalidate_count going 1->0 the spin_unlock ensures that any
preceeding PTE update during invalidation does not order after the
spin_unlock

While holding the lock and observing 0 in invalidating_count the PTE
values cannot be changing.

Jason

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