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

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Message-ID: <Y8iom6Mjz9rCX42A@rowland.harvard.edu>
Date:   Wed, 18 Jan 2023 21:19:07 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     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 18, 2023 at 04:02:14PM -0800, Paul E. McKenney wrote:
> On Wed, Jan 18, 2023 at 03:54:47PM -0500, Alan Stern wrote:
> > How does this differ from srcu_read_lock() and srcu_read_unlock()?  And 
> > how do the "up" and "down" parts figure into it? -- what is going up or 
> > down?
> 
> Functionally and from a performance/scalability viewpoint, they
> are identical to srcu_read_lock() and srcu_read_unlock().  The only
> difference is that srcu_down_read() and srcu_up_read() lack the lockdep
> machinery that complains when a matching pair of srcu_read_lock() and
> srcu_read_unlock() are used from different tasks.

This makes me wonder if there's any need for srcu_down_read and 
srcu_up_read at all.  Why not just use srcu_read_lock and 
srcu_read_unlock, and remove the lockdep check?

> Within the implementation, nothing ever goes down, it is all
> this_cpu_inc().  The "down" and "up" are by analogy to down() and up(),
> where "down()" says acquire some rights to a resource and "up()" says
> release those rights.

Another reason not to use those names.  If you insist on making these 
operations distinct from srcu_read_lock and srcu_read_unlock, why not 
borrow the "_get" and "_put" nomenclature used by the device core?  I 
suspect more people would associate them with acquiring and releasing 
rights to a resource.  (Although in this case it might be so clear 
exactly what that resource is.)

> Wait, I can make "down" work.
> 
> A call to srcu_down_read() reduces the quantity computed by summing the
> unlocks then subtracting the sum of the locks.  A call to srcu_up_read()
> increases that same quantity.  ;-)

I can't honestly call that a resoundingly convincing argument.  :-)

Alan