linux-kernel - Re: XDP socket rings, and LKMM litmus tests

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Message-ID: <20210304190708.GT2696@paulmck-ThinkPad-P72>
Date:   Thu, 4 Mar 2021 11:07:08 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     maranget <luc.maranget@...ia.fr>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        "Alglave, Jade" <j.alglave@....ac.uk>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>, joel@...lfernandes.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: XDP socket rings, and LKMM litmus tests

On Thu, Mar 04, 2021 at 04:44:34PM +0100, maranget wrote:
> 
> 
> > On 3 Mar 2021, at 21:22, Alan Stern <stern@...land.harvard.edu> wrote:
> > 
> >>> 
> >>> Local variables absolutely should be treated just like CPU registers, if 
> >>> possible.  In fact, the compiler has the option of keeping local 
> >>> variables stored in registers.
> >>> 
> >>> (Of course, things may get complicated if anyone writes a litmus test 
> >>> that uses a pointer to a local variable,  Especially if the pointer 
> >>> could hold the address of a local variable in one execution and a 
> >>> shared variable in another!  Or if the pointer is itself a shared 
> >>> variable and is dereferenced in another thread!)
> >> 
> >> Good point!  I did miss this complication.  ;-)
> > 
> > I suspect it wouldn't be so bad if herd7 disallowed taking addresses of 
> > local variables.
> 
> Herd7 does disallow taking addresses of local variables.

Good to know, and thank you!

> However, such  tests can still be run on machine, provided function bodies are accepted by the C compiler.

True, but that would be outside of the LKMM proper, correct?

							Thanx, Paul