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Date: Tue, 23 Oct 2012 20:41:23 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers
Message-ID: <20121023184123.GB24055@redhat.com>
References: <20121017165902.GB9872@redhat.com> <20121017224430.GC2518@linux.vnet.ibm.com> <20121018162409.GA28504@redhat.com> <20121018163833.GK2518@linux.vnet.ibm.com> <20121018175747.GA30691@redhat.com> <20121019192838.GM2518@linux.vnet.ibm.com> <Pine.LNX.4.64.1210221910150.25995@file.rdu.redhat.com> <Pine.LNX.4.64.1210221936570.25995@file.rdu.redhat.com> <20121023165912.GA18712@redhat.com> <20121023180558.GF2585@linux.vnet.ibm.com>
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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.