linux-kernel - RE: [PATCH 1/1] swiotlb: Track and report io_tlb

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Message-ID: <SA1PR21MB13358B4721F407781725B1F3BF869@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Wed, 22 Mar 2023 21:53:58 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "hch@....de" <hch@....de>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] swiotlb: Track and report io_tlb_used high water mark
 in debugfs

> From: Michael Kelley (LINUX) <mikelley@...rosoft.com>
> Sent: Wednesday, March 22, 2023 7:39 AM
> > >  ...
> > > +	new_used = atomic_long_add_return(nslots, &total_used);
> > > +	old_hiwater = atomic_long_read(&used_hiwater);
> > > +	do {
> > > +		if (new_used <= 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 to me.
> 
> 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!