From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Laight <David.Laight@aculab.com>
Cc: "paulmck@linux.ibm.com" <paulmck@linux.ibm.com>,
Herbert Xu <herbert@gondor.apana.org.au>,
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 08:40:40 -0700 [thread overview]
Message-ID: <CAHk-=wj4pqF9bwjQHNzAKBmbAFTDWi9KwvLznj0HZLAZ+eGFpw@mail.gmail.com> (raw)
In-Reply-To: <9c0a9e2faae7404cb712f57910c8db34@AcuMS.aculab.com>
On Mon, Jun 3, 2019 at 8:26 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Paul E. McKenney
>
> > 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.
>
> In that case the variable ought to be volatile...
No.
We do not use volatile on variables.
The C people got the semantics wrong, probably because 'volatile' was
historically for IO, not for access atomicity without locking.
It's not the memory location that is volatile, it is really the
_access_ that is volatile.
The same memory location might be completely stable in other access
situations (ie when done under a lock).
In other words, we should *never* use volatile in the kernel. It's
fundamentally mis-designed for modern use.
(Of course, we then can use volatile in a cast in code, which drives
some compiler people crazy, but that's because said compiler people
don't care about reality, they care about some paperwork).
Linus
WARNING: multiple messages have this Message-ID (diff)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: lkp@lists.01.org
Subject: Re: rcu_read_lock lost its compiler barrier
Date: Mon, 03 Jun 2019 08:40:40 -0700 [thread overview]
Message-ID: <CAHk-=wj4pqF9bwjQHNzAKBmbAFTDWi9KwvLznj0HZLAZ+eGFpw@mail.gmail.com> (raw)
In-Reply-To: <9c0a9e2faae7404cb712f57910c8db34@AcuMS.aculab.com>
[-- Attachment #1: Type: text/plain, Size: 1099 bytes --]
On Mon, Jun 3, 2019 at 8:26 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Paul E. McKenney
>
> > 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.
>
> In that case the variable ought to be volatile...
No.
We do not use volatile on variables.
The C people got the semantics wrong, probably because 'volatile' was
historically for IO, not for access atomicity without locking.
It's not the memory location that is volatile, it is really the
_access_ that is volatile.
The same memory location might be completely stable in other access
situations (ie when done under a lock).
In other words, we should *never* use volatile in the kernel. It's
fundamentally mis-designed for modern use.
(Of course, we then can use volatile in a cast in code, which drives
some compiler people crazy, but that's because said compiler people
don't care about reality, they care about some paperwork).
Linus
next prev parent reply other threads:[~2019-06-03 15:41 UTC|newest]
Thread overview: 99+ 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 0:57 ` Fengguang Wu
2015-09-10 10:25 ` Boqun Feng
2015-09-10 17:16 ` Paul E. McKenney
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
2015-09-11 21:59 ` Paul E. McKenney
2015-09-12 5:46 ` Boqun Feng
2015-09-21 19:30 ` Frederic Weisbecker
2015-09-21 19:30 ` Frederic Weisbecker
2015-09-21 20:43 ` Paul E. McKenney
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 5:56 ` Herbert Xu
2019-06-02 20:54 ` Linus Torvalds
2019-06-02 20:54 ` Linus Torvalds
2019-06-03 2:46 ` Herbert Xu
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 4:01 ` Herbert Xu
2019-06-03 4:17 ` 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
2019-06-03 15:26 ` David Laight
2019-06-03 15:40 ` Linus Torvalds [this message]
2019-06-03 15:40 ` Linus Torvalds
2019-06-03 5:26 ` Herbert Xu
2019-06-03 5:26 ` Herbert Xu
2019-06-03 6:42 ` Boqun Feng
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 14:44 ` Alan Stern
2019-06-04 16:04 ` Linus Torvalds
2019-06-04 16:04 ` Linus Torvalds
2019-06-04 17:00 ` Alan Stern
2019-06-04 17:00 ` Alan Stern
2019-06-04 17:29 ` Linus Torvalds
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 14:09 ` Herbert Xu
2019-06-07 15:26 ` Eric Dumazet
2019-06-07 15:26 ` Eric Dumazet
2019-06-07 15:32 ` Herbert Xu
2019-06-07 15:32 ` Herbert Xu
2019-06-07 16:13 ` Eric Dumazet
2019-06-07 16:13 ` Eric Dumazet
2019-06-07 16:19 ` Linus Torvalds
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 17:42 ` Linus Torvalds
2019-06-08 17:50 ` 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 4:51 ` Herbert Xu
2019-06-06 6:05 ` Paul E. McKenney
2019-06-06 6:14 ` Herbert Xu
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 9:28 ` Herbert Xu
2019-06-06 10:58 ` Paul E. McKenney
2019-06-06 13:38 ` Herbert Xu
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
2019-06-06 14:19 ` Alan Stern
2019-06-08 15:19 ` Paul E. McKenney
2019-06-08 15:56 ` Alan Stern
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
2019-06-06 9:32 ` Herbert Xu
2019-06-03 0:06 ` Paul E. McKenney
2019-06-03 3:03 ` Herbert Xu
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 15:55 ` Linus Torvalds
2019-06-03 16:07 ` Linus Torvalds
2019-06-03 16:07 ` Linus Torvalds
2019-06-03 19:53 ` Paul E. McKenney
2019-06-03 20:24 ` Linus Torvalds
2019-06-03 20:24 ` Linus Torvalds
2019-06-04 21:14 ` Paul E. McKenney
2019-06-05 2:21 ` Herbert Xu
2019-06-05 2:21 ` Herbert Xu
2019-06-05 3:30 ` Paul E. McKenney
2019-06-06 4:37 ` Herbert Xu
2019-06-06 4:37 ` Herbert Xu
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