From: "Michael S. Tsirkin" <mst@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
linux-kernel@vger.kernel.org, mingo@kernel.org,
jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
oleg@redhat.com, Jason Wang <jasowang@redhat.com>,
kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org
Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()
Date: Tue, 5 Dec 2017 20:57:46 +0200 [thread overview]
Message-ID: <20171205204928-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20171205183946.GP3165@worktop.lehotels.local>
On Tue, Dec 05, 2017 at 07:39:46PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 05, 2017 at 08:31:20PM +0200, Michael S. Tsirkin wrote:
>
> > Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE.
> >
> > I can read a pointer with READ_ONCE and be sure the value
> > is sane, but only if I also remember to put in smp_wmb before
> > WRITE_ONCE. Otherwise the pointer is ok but no guarantees
> > about the data pointed to.
>
> That was already the case on everything except Alpha. And the canonical
> match do the data dependency is store_release, not wmb.
Oh, interesting
static __always_inline void __write_once_size(volatile void *p, void *res, int size)
{
switch (size) {
case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
default:
barrier();
__builtin_memcpy((void *)p, (const void *)res, size);
barrier();
}
}
#define WRITE_ONCE(x, val) \
({ \
union { typeof(x) __val; char __c[1]; } __u = \
{ .__val = (__force typeof(x)) (val) }; \
__write_once_size(&(x), __u.__c, sizeof(x)); \
__u.__val; \
})
I don't see WRITE_ONCE inserting any barriers, release or
write.
So it seems that on an architecture where writes can be reordered,
if I do
*pointer = 0xa;
WRITE_ONCE(array[x], pointer);
array write might bypass the pointer write,
and readers will read a stale value.
--
MST
next prev parent reply other threads:[~2017-12-05 18:57 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-01 19:50 [PATCH tip/core/rcu 0/21] De-emphasize {smp_,}read_barrier_depends Paul E. McKenney
2017-12-01 19:50 ` [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies smp_barrier_depends() Paul E. McKenney
2017-12-01 19:50 ` [PATCH tip/core/rcu 02/21] mn10300: READ_ONCE() now implies smp_read_barrier_depends() Paul E. McKenney
2017-12-01 19:50 ` [PATCH tip/core/rcu 03/21] drivers/net/ethernet/qlogic/qed: Fix __qed_spq_block() ordering Paul E. McKenney
2017-12-01 19:50 ` [PATCH tip/core/rcu 04/21] fs/dcache: Use release-acquire for name/length update Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 05/21] percpu: READ_ONCE() now implies smp_read_barrier_depends() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 06/21] rcu: Adjust read-side accessor comments for READ_ONCE() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 07/21] rtnetlink: Update now-misleading smp_read_barrier_depends() comment Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 08/21] seqlock: Remove now-redundant smp_read_barrier_depends() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 09/21] uprobes: " Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 10/21] locking: Remove smp_read_barrier_depends() from queued_spin_lock_slowpath() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 11/21] tracepoint: Remove smp_read_barrier_depends() from comment Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 12/21] lib/assoc_array: Remove smp_read_barrier_depends() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 13/21] mm/ksm: Remove now-redundant smp_read_barrier_depends() Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 14/21] netfilter: " Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 15/21] keyring: " Paul E. McKenney
2017-12-04 0:59 ` James Morris
2017-12-04 18:54 ` Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 16/21] drivers/infiniband: " Paul E. McKenney
2017-12-02 0:11 ` Jason Gunthorpe
2017-12-02 1:08 ` Paul E. McKenney
2017-12-05 15:08 ` Jason Gunthorpe
2017-12-05 20:06 ` Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 17/21] doc: De-emphasize smp_read_barrier_depends Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 18/21] genetlink: Remove smp_read_barrier_depends() from comment Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 19/21] netlink: " Paul E. McKenney
2017-12-01 19:51 ` [PATCH tip/core/rcu 20/21] checkpatch: Add warnings for {smp_,}read_barrier_depends() Paul E. McKenney
2017-12-01 20:14 ` Joe Perches
2017-12-01 21:44 ` Paul E. McKenney
2017-12-02 4:45 ` Joe Perches
2017-12-04 19:06 ` Paul E. McKenney
2017-12-04 19:12 ` Joe Perches
2017-12-01 19:51 ` [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends() Paul E. McKenney
2017-12-05 18:31 ` Michael S. Tsirkin
2017-12-05 18:39 ` Peter Zijlstra
2017-12-05 18:57 ` Michael S. Tsirkin [this message]
2017-12-05 19:17 ` Peter Zijlstra
2017-12-05 19:24 ` Michael S. Tsirkin
2017-12-05 19:33 ` Paul E. McKenney
2017-12-05 19:51 ` Michael S. Tsirkin
2017-12-05 19:57 ` Peter Zijlstra
2017-12-05 20:28 ` Michael S. Tsirkin
2017-12-05 21:17 ` Peter Zijlstra
2017-12-05 21:42 ` Michael S. Tsirkin
2017-12-05 20:08 ` Paul E. McKenney
2017-12-05 21:24 ` Michael S. Tsirkin
2017-12-05 21:36 ` Paul E. McKenney
2017-12-05 21:43 ` Michael S. Tsirkin
2017-12-05 22:02 ` Paul E. McKenney
2017-12-05 22:09 ` Peter Zijlstra
2017-12-05 21:57 ` Peter Zijlstra
2017-12-05 22:09 ` Michael S. Tsirkin
2017-12-05 23:39 ` Paul E. McKenney
2017-12-05 19:55 ` Peter Zijlstra
2017-12-04 15:38 ` [PATCH tip/core/rcu 01/21] doc: READ_ONCE() now implies smp_barrier_depends() David Howells
2017-12-04 18:52 ` Paul E. McKenney
2017-12-04 21:54 ` Peter Zijlstra
2017-12-04 22:15 ` Paul E. McKenney
2017-12-04 22:39 ` David Howells
2017-12-04 22:57 ` Peter Zijlstra
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