Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

[Steven Rostedt <rostedt@goodmis.org>]

[ Removed npiggen@kernel.dk as I keep getting bounces from that addr ]

On Tue, 17 Mar 2015 01:45:25 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> ----- Original Message -----
> > From: "Peter Zijlstra" <peterz@infradead.org>
> > To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
> > Cc: linux-kernel@vger.kernel.org, "KOSAKI Motohiro" <kosaki.motohiro@jp.fujitsu.com>, "Steven Rostedt"
> > <rostedt@goodmis.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "Nicholas Miell" <nmiell@comcast.net>,
> > "Linus Torvalds" <torvalds@linux-foundation.org>, "Ingo Molnar" <mingo@redhat.com>, "Alan Cox"
> > <gnomes@lxorguk.ukuu.org.uk>, "Lai Jiangshan" <laijs@cn.fujitsu.com>, "Stephen Hemminger"
> > <stephen@networkplumber.org>, "Andrew Morton" <akpm@linux-foundation.org>, "Josh Triplett" <josh@joshtriplett.org>,
> > "Thomas Gleixner" <tglx@linutronix.de>, "David Howells" <dhowells@redhat.com>, "Nick Piggin" <npiggin@kernel.dk>
> > Sent: Monday, March 16, 2015 4:54:35 PM
> > Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

Can you please fix your mail client to not include the entire header in
your replies please.

> Let's consider the following memory barrier scenario performed in
> user-space on an architecture with very relaxed ordering. PowerPC comes
> to mind.
> 
> https://lwn.net/Articles/573436/
> scenario 12:
> 
> CPU 0                   CPU 1
> CAO(x) = 1;             r3 = CAO(y);
> cmm_smp_wmb();          cmm_smp_rmb();
> CAO(y) = 1;             r4 = CAO(x);
> 
> BUG_ON(r3 == 1 && r4 == 0)
> 
> 
> We tweak it to use sys_membarrier on CPU 1, and a simple compiler
> barrier() on CPU 0:
> 
> CPU 0                   CPU 1
> CAO(x) = 1;             r3 = CAO(y);
> barrier();              sys_membarrier();
> CAO(y) = 1;             r4 = CAO(x);
> 
> BUG_ON(r3 == 1 && r4 == 0)
> 
> Now if CPU 1 executes sys_membarrier while CPU 0 is preempted after both
> stores, we have:
> 
> CPU 0                           CPU 1
> CAO(x) = 1;
>   [1st store is slow to
>    reach other cores]
> CAO(y) = 1;
>   [2nd store reaches other
>    cores more quickly]
> [preempted]
>                                 r3 = CAO(y)
>                                   (may see y = 1)
>                                 sys_membarrier()
> Scheduler changes rq->curr.
>                                 skips CPU 0, because rq->curr has
>                                   been updated.
>                                 [return to userspace]
>                                 r4 = CAO(x)
>                                   (may see x = 0)
>                                 BUG_ON(r3 == 1 && r4 == 0) -> fails.
> load_cr3, with implied
>   memory barrier, comes
>   after CPU 1 has read "x".
> 
> The only way to make this scenario work is if a memory barrier is added
> before updating rq->curr. (we could also do a similar scenario for the
> needed barrier after store to rq->curr).

Hmm, I wonder if anything were to break if rq->curr was updated after
the context_switch() call?

Would that help?

	this_cpu_write(saved_next, next);
	rq = context_switch(rq, prev, next);
	rq->curr = this_cpu_read(saved_next);

As I recently found out that this_cpu_read/write() is not that nice on
all architectures, something else may need to be updated. Or we can add
a temp variable on the rq.

	rq->saved_next = next;
	rq = context_switch(rq, prev, next);
	rq->curr = rq->saved_next;

-- Steve