linux-kernel - Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

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Date:	Tue, 23 Oct 2012 13:29:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

On Tue, Oct 23, 2012 at 08:41:23PM +0200, Oleg Nesterov wrote:
> On 10/23, Paul E. McKenney wrote:
> >
> >  * Note that this guarantee implies a further memory-ordering guarantee.
> >  * On systems with more than one CPU, when synchronize_sched() returns,
> >  * each CPU is guaranteed to have executed a full memory barrier since
> >  * the end of its last RCU read-side critical section
>          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Ah wait... I misread this comment.

And I miswrote it.  It should say "since the end of its last RCU-sched
read-side critical section."  So, for example, RCU-sched need not force
a CPU that is idle, offline, or (eventually) executing in user mode to
execute a memory barrier.  Fixed this.

> But this patch needs more? Or I misunderstood. There is no RCU unlock
> in percpu_up_read().
> 
> IOW. Suppose the code does
> 
> 	percpu_down_read();
> 	x = PROTECTED_BY_THIS_RW_SEM;
> 	percpu_up_read();
> 
> Withoit mb() the load above can be reordered with this_cpu_dec() in
> percpu_up_read().
> 
> However, we do not care if we can guarantee that the next
> percpu_down_write() can not return (iow, the next "write" section can
> not start) until this load is complete.
> 
> And I _think_ that another synchronize_sched() in percpu_down_write()
> added by this patch should work.
> 
> But, "since the end of its last  RCU read-side critical section"
> does not look enough.
> 
> Or I misundersood you/Mikulas/both ?

I clearly need to look more carefully at Mikulas's code...

							Thanx, Paul

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