linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Message-ID: <20230126015330.GX2948950@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 25 Jan 2023 17:53:30 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Wed, Jan 25, 2023 at 08:45:44PM -0500, Alan Stern wrote:
> On Wed, Jan 25, 2023 at 03:33:08PM -0800, Paul E. McKenney wrote:
> > Ah, and returning to the earlier question as to whether srcu_read_unlock()
> > can use release semantics instead of smp_mb(), at the very least, this
> > portion of the synchronize_srcu() function's header comment must change:
> > 
> > 	On systems with more than one CPU, when synchronize_srcu()
> > 	returns, each CPU is guaranteed to have executed a full
> > 	memory barrier since the end of its last corresponding SRCU
> > 	read-side critical section whose beginning preceded the call
> > 	to synchronize_srcu().
> 
> Yes, that would not be true.  But on the other hand, it would be true 
> that each CPU is guaranteed to have executed a release memory barrier 
> since the end of its last corresponding SRCU read-side critical section 
> whose beginning preceded the call to synchronize_srcu(), _and_ the CPU 
> executing synchronize_srcu() is guaranteed to have executed a full 
> memory barrier after seeing the values from all those release stores.
> This is not quite the same thing but it ought to be just as good.

Here is hoping!

> > I don't know of any SRCU code that relies on this, but it would be good to
> > check.	There used to (and might still) be RCU code relying on this, which
> > is why this sentence was added to the header comment in the first place.
> 
> If there is code relying on that guarantee, it ought to work just as 
> well by relying on the modified guarantee.

Again, here is hoping!

> Of course, there might be code relying on a guarantee that 
> srcu_read_unlock() executes a full memory barrier.  This guarantee would 
> certainly no longer hold.  But as I understand it, this guarantee was 
> never promised by the SRCU subsystem.

That indented sentence was copied from the synchronize_srcu() function's
header comment, which might be interpreted by some as a promise by the
SRCU subsystem.

							Thanx, Paul