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Message-ID: <20190724165317.GD28493@ziepe.ca>
Date: Wed, 24 Jul 2019 13:53:17 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>,
syzbot <syzbot+e58112d71f77113ddb7b@...kaller.appspotmail.com>,
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linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
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Subject: Re: WARNING in __mmdrop
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
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