From: Jason Gunthorpe <jgg@nvidia.com>
To: David Hildenbrand <david@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
Yang Shi <shy828301@gmail.com>,
peterx@redhat.com, kirill.shutemov@linux.intel.com,
hughd@google.com, akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse
Date: Tue, 6 Sep 2022 11:30:44 -0300 [thread overview]
Message-ID: <YxdZlCly2ad1rtcI@nvidia.com> (raw)
In-Reply-To: <4516a349-49cb-fd7b-176a-f1a9479906d9@redhat.com>
On Tue, Sep 06, 2022 at 03:57:30PM +0200, David Hildenbrand wrote:
> > READ_ONCE primarily is a marker that the data being read is unstable
> > and that the compiler must avoid all instability when reading it. eg
> > in this case the compiler could insanely double read the value, even
> > though the 'if' requires only a single read. This would result in
> > corrupt calculation.
>
> As we have a full memory barrier + compile barrier, the compiler might
> indeed do double reads and all that stuff. BUT, it has to re-read after we
> incremented the refcount, and IMHO that's the important part to detect the
> change.
Yes, it is important, but it is not the only important part.
The compiler still has to exectute "if (*a != b)" *correctly*.
This is what READ_ONCE is for. It doesn't set order, it doesn't
implement a barrier, it tells the compiler that '*a' is unstable data
and the compiler cannot make assumptions based on the idea that
reading '*a' multiple times will always return the same value.
If the compiler makes those assumptions then maybe even though 'if (*a
!= b)' is the reality, it could mis-compute '*a == b'. You enter into
undefined behavior here.
Though it is all very unlikely, the general memory model standard is
to annotate with READ_ONCE.
Jason
next prev parent reply other threads:[~2022-09-06 15:22 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-01 22:27 [PATCH] mm: gup: fix the fast GUP race against THP collapse Yang Shi
2022-09-01 23:26 ` Peter Xu
2022-09-01 23:50 ` Yang Shi
2022-09-02 6:39 ` David Hildenbrand
2022-09-02 15:23 ` Yang Shi
2022-09-02 15:59 ` Peter Xu
2022-09-02 16:04 ` Peter Xu
2022-09-02 17:30 ` Yang Shi
2022-09-02 17:45 ` Yang Shi
2022-09-02 20:33 ` Peter Xu
2022-09-05 8:56 ` Aneesh Kumar K.V
2022-09-05 8:54 ` Aneesh Kumar K.V
2022-09-06 19:07 ` Yang Shi
2022-09-07 4:50 ` Aneesh Kumar K V
2022-09-07 17:08 ` Yang Shi
2022-09-04 22:21 ` John Hubbard
2022-09-02 6:42 ` David Hildenbrand
2022-09-04 22:29 ` John Hubbard
2022-09-05 7:59 ` David Hildenbrand
2022-09-05 10:16 ` Baolin Wang
2022-09-05 10:24 ` David Hildenbrand
2022-09-05 11:11 ` David Hildenbrand
2022-09-05 14:35 ` Baolin Wang
2022-09-05 14:40 ` David Hildenbrand
2022-09-06 5:53 ` Baolin Wang
2022-09-06 2:12 ` John Hubbard
2022-09-06 12:50 ` David Hildenbrand
2022-09-06 13:47 ` Jason Gunthorpe
2022-09-06 13:57 ` David Hildenbrand
2022-09-06 14:30 ` Jason Gunthorpe [this message]
2022-09-06 14:44 ` David Hildenbrand
2022-09-06 15:33 ` Jason Gunthorpe
2022-09-06 19:11 ` Yang Shi
2022-09-06 23:16 ` John Hubbard
2022-09-06 19:01 ` Yang Shi
2022-09-05 9:03 ` Baolin Wang
2022-09-06 18:50 ` Yang Shi
2022-09-06 21:27 ` John Hubbard
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