From: Dmitry Vyukov <dvyukov@google.com>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
Will Deacon <will@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
bsingharora@gmail.com, Marco Elver <elver@google.com>,
stable <stable@vger.kernel.org>,
syzbot <syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [PATCH v6] taskstats: fix data-race
Date: Thu, 24 Oct 2019 13:51:20 +0200 [thread overview]
Message-ID: <CACT4Y+Z2-mm6Qk0cecJdiA5B_VsQ1v8k2z+RWrDQv6dTNFXFog@mail.gmail.com> (raw)
In-Reply-To: <20191024113155.GA7406@andrea.guest.corp.microsoft.com>
On Thu, Oct 24, 2019 at 1:32 PM Andrea Parri <parri.andrea@gmail.com> wrote:
>
> > How these later loads can be completely independent of the pointer
> > value? They need to obtain the pointer value from somewhere. And this
> > can only be done by loaded it. And if a thread loads a pointer and
> > then dereferences that pointer, that's a data/address dependency and
> > we assume this is now covered by READ_ONCE.
>
> The "dependency" I was considering here is a dependency _between the
> load of sig->stats in taskstats_tgid_alloc() and the (program-order)
> later loads of *(sig->stats) in taskstats_exit(). Roughly speaking,
> such a dependency should correspond to a dependency chain at the asm
> or registers level from the first load to the later loads; e.g., in:
>
> Thread [register r0 contains the address of sig->stats]
>
> A: LOAD r1,[r0] // LOAD_ACQUIRE sig->stats
> ...
> B: LOAD r2,[r0] // LOAD *(sig->stats)
> C: LOAD r3,[r2]
>
> there would be no such dependency from A to C. Compare, e.g., with:
>
> Thread [register r0 contains the address of sig->stats]
>
> A: LOAD r1,[r0] // LOAD_ACQUIRE sig->stats
> ...
> C: LOAD r3,[r1] // LOAD *(sig->stats)
>
> AFAICT, there's no guarantee that the compilers will generate such a
> dependency from the code under discussion.
Fixing this by making A ACQUIRE looks like somewhat weird code pattern
to me (though correct). B is what loads the address used to read
indirect data, so B ought to be ACQUIRE (or LOAD-DEPENDS which we get
from READ_ONCE).
What you are suggesting is:
addr = ptr.load(memory_order_acquire);
if (addr) {
addr = ptr.load(memory_order_relaxed);
data = *addr;
}
whereas the canonical/non-convoluted form of this pattern is:
addr = ptr.load(memory_order_consume);
if (addr)
data = *addr;
Moreover the second load of ptr is not even atomic in our case, so it
is a subject to another data race?
next prev parent reply other threads:[~2019-10-24 11:51 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20191007104039.GA16085@andrea.guest.corp.microsoft.com>
2019-10-07 11:01 ` [PATCH v2] taskstats: fix data-race Christian Brauner
2019-10-07 13:18 ` Andrea Parri
2019-10-07 13:28 ` Christian Brauner
2019-10-07 13:50 ` Dmitry Vyukov
2019-10-07 13:55 ` Christian Brauner
2019-10-07 14:08 ` Dmitry Vyukov
2019-10-07 14:10 ` Christian Brauner
2019-10-07 14:14 ` Andrea Parri
2019-10-07 14:18 ` Dmitry Vyukov
2019-10-08 14:20 ` Andrea Parri
2019-10-08 14:24 ` Christian Brauner
2019-10-08 15:26 ` Andrea Parri
2019-10-08 15:35 ` Christian Brauner
2019-10-08 15:44 ` Andrea Parri
2019-10-09 11:31 ` [PATCH] " Christian Brauner
2019-10-09 11:40 ` Christian Brauner
2019-10-09 11:48 ` [PATCH v5] " Christian Brauner
2019-10-09 12:08 ` Andrea Parri
2019-10-09 13:26 ` Christian Brauner
2019-10-21 11:33 ` [PATCH v6] " Christian Brauner
2019-10-21 12:19 ` Rasmus Villemoes
2019-10-21 13:04 ` Christian Brauner
2019-11-29 17:56 ` Will Deacon
2019-11-30 15:08 ` Christian Brauner
2019-10-23 12:16 ` Andrea Parri
2019-10-23 12:39 ` Dmitry Vyukov
2019-10-23 13:11 ` Christian Brauner
2019-10-23 13:20 ` Dmitry Vyukov
2019-10-24 11:31 ` Andrea Parri
2019-10-24 11:51 ` Dmitry Vyukov [this message]
2019-10-24 13:05 ` Andrea Parri
2019-10-24 13:13 ` Dmitry Vyukov
2019-10-24 13:21 ` Christian Brauner
2019-10-24 13:34 ` Dmitry Vyukov
2019-10-24 13:43 ` Andrea Parri
2019-10-24 13:58 ` Dmitry Vyukov
2019-10-24 14:40 ` Andrea Parri
2019-10-24 14:49 ` Dmitry Vyukov
2019-11-29 17:57 ` Will Deacon
2019-10-09 11:48 ` [PATCH] " Marco Elver
2019-10-09 11:53 ` Christian Brauner
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