From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
LKML <linux-kernel@vger.kernel.org>,
Netdev <netdev@vger.kernel.org>,
"David S. Miller" <davem@davemloft.net>
Subject: Re: rcu_read_lock lost its compiler barrier
Date: Mon, 3 Jun 2019 01:42:14 -0700 [thread overview]
Message-ID: <20190603084214.GA1496@linux.ibm.com> (raw)
In-Reply-To: <20190603072339.GH28207@linux.ibm.com>
On Mon, Jun 03, 2019 at 12:23:39AM -0700, Paul E. McKenney wrote:
> On Mon, Jun 03, 2019 at 12:01:14PM +0800, Herbert Xu wrote:
> > On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote:
> > >
> > > CPU2: if (b != 1)
> > > CPU2: b = 1;
> >
> > Stop right there. The kernel is full of code that assumes that
> > assignment to an int/long is atomic. If your compiler breaks this
> > assumption that we can kiss the kernel good-bye.
>
> Here you go:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56028
>
> TL;DR: On x86, of you are doing a plain store of a 32-bit constant
> that has bits set only in the lower few bits of each 16-bit half of
> that constant, the compiler is plenty happy to use a pair of 16-bit
> store-immediate instructions to carry out that store. This is also
> known as "store tearing".
>
> The two bugs were filed (and after some back and forth, fixed) because
> someone forgot to exclude C11 atomics and volatile accesses from this
> store tearing.
I should hasten to add that I have not seen load tearing, nor have I seen
store tearing when storing a value unknown to the compiler. However,
plain C-language loads and stores can be invented, fused, and a few other
"interesting" optimization can be applied.
On kissing the kernel goodbye, a reasonable strategy might be to
identify the transformations that are actually occuring (like the
stores of certain constants called out above) and fix those. We do
occasionally use READ_ONCE() to prevent load-fusing optimizations that
would otherwise cause the compiler to turn while-loops into if-statements
guarding infinite loops. There is also the possibility of having the
compiler guys give us more command-line arguments.
Thanx, Paul
next prev parent reply other threads:[~2019-06-03 8:42 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-10 0:57 [rcu] kernel BUG at include/linux/pagemap.h:149! Fengguang Wu
2015-09-10 10:25 ` Boqun Feng
2015-09-10 17:16 ` Paul E. McKenney
2015-09-11 2:19 ` Boqun Feng
[not found] ` <CAJzB8QG=1iZW3dQEie6ZSTLv8GZ3YSut0aL1VU7LLmiHQ1B1DQ@mail.gmail.com>
2015-09-11 21:59 ` Paul E. McKenney
2015-09-12 5:46 ` Boqun Feng
2015-09-21 19:30 ` Frederic Weisbecker
2015-09-21 20:43 ` Paul E. McKenney
2019-06-02 5:56 ` rcu_read_lock lost its compiler barrier Herbert Xu
2019-06-02 20:54 ` Linus Torvalds
2019-06-03 2:46 ` Herbert Xu
2019-06-03 3:47 ` Paul E. McKenney
2019-06-03 4:01 ` Herbert Xu
2019-06-03 4:17 ` Herbert Xu
2019-06-03 7:23 ` Paul E. McKenney
2019-06-03 8:42 ` Paul E. McKenney [this message]
2019-06-03 15:26 ` David Laight
2019-06-03 15:40 ` Linus Torvalds
2019-06-03 5:26 ` Herbert Xu
2019-06-03 6:42 ` Boqun Feng
2019-06-03 20:03 ` Paul E. McKenney
2019-06-04 14:44 ` Alan Stern
2019-06-04 16:04 ` Linus Torvalds
2019-06-04 17:00 ` Alan Stern
2019-06-04 17:29 ` Linus Torvalds
2019-06-07 14:09 ` inet: frags: Turn fqdir->dead into an int for old Alphas Herbert Xu
2019-06-07 15:26 ` Eric Dumazet
2019-06-07 15:32 ` Herbert Xu
2019-06-07 16:13 ` Eric Dumazet
2019-06-07 16:19 ` Linus Torvalds
2019-06-08 15:27 ` Paul E. McKenney
2019-06-08 17:42 ` Linus Torvalds
2019-06-08 17:50 ` Linus Torvalds
2019-06-08 18:50 ` Paul E. McKenney
2019-06-08 18:14 ` Paul E. McKenney
2019-06-06 4:51 ` rcu_read_lock lost its compiler barrier Herbert Xu
2019-06-06 6:05 ` Paul E. McKenney
2019-06-06 6:14 ` Herbert Xu
2019-06-06 9:06 ` Paul E. McKenney
2019-06-06 9:28 ` Herbert Xu
2019-06-06 10:58 ` Paul E. McKenney
2019-06-06 13:38 ` Herbert Xu
2019-06-06 13:48 ` Paul E. McKenney
2019-06-06 8:16 ` Andrea Parri
2019-06-06 14:19 ` Alan Stern
2019-06-08 15:19 ` Paul E. McKenney
2019-06-08 15:56 ` Alan Stern
2019-06-08 16:31 ` Paul E. McKenney
2019-06-03 9:35 ` Paul E. McKenney
2019-06-06 8:38 ` Andrea Parri
2019-06-06 9:32 ` Herbert Xu
2019-06-03 0:06 ` Paul E. McKenney
2019-06-03 3:03 ` Herbert Xu
2019-06-03 9:27 ` Paul E. McKenney
2019-06-03 15:55 ` Linus Torvalds
2019-06-03 16:07 ` Linus Torvalds
2019-06-03 19:53 ` Paul E. McKenney
2019-06-03 20:24 ` Linus Torvalds
2019-06-04 21:14 ` Paul E. McKenney
2019-06-05 2:21 ` Herbert Xu
2019-06-05 3:30 ` Paul E. McKenney
2019-06-06 4:37 ` Herbert Xu
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