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

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Date:	Tue, 23 Oct 2012 20:41:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.

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 ?

Oleg.

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