linux-kernel - Re: [RFC PATCH] iommu/amd: fix a race in fetch

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Message-ID: <20200418121022.GA6113@8bytes.org>
Date:   Sat, 18 Apr 2020 14:10:22 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Qian Cai <cai@....pw>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] iommu/amd: fix a race in fetch_pte()

On Thu, Apr 16, 2020 at 09:42:41PM -0400, Qian Cai wrote:
> So, this is still not enough that would still trigger storage driver offline under
> memory pressure for a bit longer. It looks to me that in fetch_pte() there are
> could still racy?

Yes, your patch still looks racy. You need to atomically read
domain->pt_root to a stack variable and derive the pt_root pointer and
the mode from that variable instead of domain->pt_root directly. If you
read the domain->pt_root twice there could still be an update between
the two reads.
Probably the lock in increase_address_space() can also be avoided if
pt_root is updated using cmpxchg()?

Regards,

	Joerg