Subject: Re: WARNING in __mmdrop

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 24 Jul 2019 14:25:15 -0400

On Wed, Jul 24, 2019 at 01:53:17PM -0300, Jason Gunthorpe wrote:
> 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:
> > > 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

Oh right. So prefetch holds the spinlock the whole time.
Sorry about the noise.

-- 
MST

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