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From: Marco Elver <elver@google.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Eric Dumazet <edumazet@google.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrea Parri <parri.andrea@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Wed, 13 Nov 2019 23:48:08 +0100
Message-ID: <20191113224808.GA3960@google.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1911131648010.1558-100000@iolanthe.rowland.org>

On Wed, 13 Nov 2019, Alan Stern wrote:

> On Wed, 13 Nov 2019, Marco Elver wrote:
> 
> > An expression works fine. The below patch would work with KCSAN, and all
> > your above examples work.
> > 
> > Re name: would it make sense to more directly convey the intent?  I.e.
> > "this expression can race, and it's fine that the result is approximate
> > if it does"?
> > 
> > My vote would go to something like 'smp_lossy' or 'lossy_race' -- but
> > don't have a strong preference, and would also be fine with 'data_race'.
> > Whatever is most legible.  Comments?
> 
> Lossiness isn't really relevant.  Things like sticky writes work 
> perfectly well with data races; they don't lose anything.
> 
> My preference would be for "data_race" or something very similar
> ("racy"? "race_ok"?).  That's the whole point -- we know the
> operation can be part of a data race and we don't care.

Makes sense. Let's stick with 'data_race' then.

As Linus pointed out this won't yet work for void types and
if-statements. How frequent would that use be? Should it even be
encouraged?

I'll add this as a patch for the next version of the KCSAN patch series.

Thanks,
-- Marco

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 0b6506b9dd11..a97f323b61e3 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -308,6 +308,26 @@ unsigned long read_word_at_a_time(const void *addr)
 	__u.__val;					\
 })
 
+#include <linux/kcsan.h>
+
+/*
+ * data_race: macro to document that accesses in an expression may conflict with
+ * other concurrent accesses resulting in data races, but the resulting
+ * behaviour is deemed safe regardless.
+ *
+ * This macro *does not* affect normal code generation, but is a hint to tooling
+ * that data races here are intentional.
+ */
+#define data_race(expr)                                                      \
+	({                                                                     \
+		typeof(({ expr; })) __val;                                     \
+		kcsan_nestable_atomic_begin();                                 \
+		__val = ({ expr; });                                           \
+		kcsan_nestable_atomic_end();                                   \
+		__val;                                                         \
+	})
+#else
+
 #endif /* __KERNEL__ */
 
 /*

  reply index

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAHk-=wjB61GNmqpX0BLA5tpL4tsjWV7akaTc2Roth7uGgax+mw@mail.gmail.com>
2019-11-10 16:09 ` Alan Stern
2019-11-10 19:10   ` Marco Elver
2019-11-11 15:51     ` Alan Stern
2019-11-11 16:51       ` Linus Torvalds
2019-11-11 17:52         ` Eric Dumazet
2019-11-11 18:04           ` Linus Torvalds
2019-11-11 18:31             ` Eric Dumazet
2019-11-11 18:44               ` Eric Dumazet
2019-11-11 19:00                 ` Linus Torvalds
2019-11-11 19:13                   ` Eric Dumazet
2019-11-11 20:43                     ` Linus Torvalds
2019-11-11 20:46                       ` Linus Torvalds
2019-11-11 21:53                         ` Eric Dumazet
2019-11-11 23:51                   ` Linus Torvalds
2019-11-12 16:50                     ` Kirill Smelkov
2019-11-12 17:23                       ` Linus Torvalds
2019-11-12 17:36                         ` Linus Torvalds
2019-11-17 18:56                           ` Kirill Smelkov
2019-11-17 19:20                             ` Linus Torvalds
2019-11-11 18:50               ` Linus Torvalds
2019-11-11 18:59                 ` Marco Elver
2019-11-11 18:59                 ` Eric Dumazet
2019-11-10 19:12   ` Linus Torvalds
2019-11-10 19:20     ` Linus Torvalds
2019-11-10 20:44       ` Paul E. McKenney
2019-11-10 21:10         ` Linus Torvalds
2019-11-10 21:31           ` Paul E. McKenney
2019-11-11 14:17         ` Marco Elver
2019-11-11 14:31           ` Paul E. McKenney
2019-11-11 15:10             ` Marco Elver
2019-11-13  0:25               ` Paul E. McKenney
2019-11-12 19:14     ` Alan Stern
2019-11-12 19:47       ` Linus Torvalds
2019-11-12 20:29         ` Alan Stern
2019-11-12 20:58           ` Linus Torvalds
2019-11-12 21:13             ` Linus Torvalds
2019-11-12 22:05               ` Marco Elver
2019-11-12 21:48             ` Alan Stern
2019-11-12 22:07               ` Eric Dumazet
2019-11-12 22:44                 ` Alexei Starovoitov
2019-11-12 23:17                   ` Eric Dumazet
2019-11-12 23:40                     ` Linus Torvalds
2019-11-13 15:00                       ` Marco Elver
2019-11-13 16:57                         ` Linus Torvalds
2019-11-13 21:33                           ` Marco Elver
2019-11-13 21:50                             ` Alan Stern
2019-11-13 22:48                               ` Marco Elver [this message]
2019-11-08 13:16 syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01   ` Linus Torvalds
2019-11-08 17:22     ` Eric Dumazet
2019-11-08 17:38       ` Linus Torvalds
2019-11-08 17:53         ` Eric Dumazet
2019-11-08 17:55           ` Eric Dumazet
2019-11-08 18:02             ` Eric Dumazet
2019-11-08 18:12               ` Linus Torvalds
2019-11-08 20:30             ` Linus Torvalds
2019-11-08 20:53               ` Eric Dumazet
2019-11-08 21:36                 ` Linus Torvalds
2019-11-08 18:05           ` Linus Torvalds
2019-11-08 18:15             ` Marco Elver
2019-11-08 18:40               ` Linus Torvalds
2019-11-08 19:48                 ` Marco Elver
2019-11-08 20:26                   ` Linus Torvalds
2019-11-08 21:57                     ` Alan Stern
2019-11-08 22:06                       ` Linus Torvalds
2019-11-09 23:08                         ` Alan Stern

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