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From:   Dexuan Cui <decui@microsoft.com>
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
        "hch@lst.de" <hch@lst.de>,
        "m.szyprowski@samsung.com" <m.szyprowski@samsung.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water mark
 in debugfs
Thread-Topic: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water
 mark in debugfs
Thread-Index: AQHZSsqBAEWh9nqUUUmkQXhDk78K2q8Go2MAgABfLoCAAHbNkA==
Date:   Wed, 22 Mar 2023 21:53:58 +0000
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> From: Michael Kelley (LINUX) <mikelley@microsoft.com>
> Sent: Wednesday, March 22, 2023 7:39 AM
> > >  ...
> > > +	new_used =3D atomic_long_add_return(nslots, &total_used);
> > > +	old_hiwater =3D atomic_long_read(&used_hiwater);
> > > +	do {
> > > +		if (new_used <=3D old_hiwater)
> > > +			break;
> > > +	} while (!atomic_long_try_cmpxchg(&used_hiwater, &old_hiwater,
> new_used));
> >
> > Here 'old_hiwater' is not updated in the loop, which looks suspicious t=
o me.
>=20
> Actually, it *is* updated in the loop. The atomic_long_try_cmpxchg()
> function updates old_hiwater to the current value if the exchange
> fails.

Yes, you're correct. Thanks for the explanation!