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Date:   Wed, 13 Nov 2019 23:48:08 +0100
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
Message-ID: <20191113224808.GA3960@google.com>
References: <20191113213336.GA20665@google.com>
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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__ */
 
 /*