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Subject: Re: XDP socket rings, and LKMM litmus tests
From:   maranget <luc.maranget@inria.fr>
In-Reply-To: <20210303202246.GC1582185@rowland.harvard.edu>
Date:   Thu, 4 Mar 2021 16:44:34 +0100
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        "Alglave, Jade" <j.alglave@ucl.ac.uk>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>, joel@joelfernandes.org,
        =?utf-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>,
        "Karlsson, Magnus" <magnus.karlsson@intel.com>
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To:     Alan Stern <stern@rowland.harvard.edu>
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> On 3 Mar 2021, at 21:22, Alan Stern <stern@rowland.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.

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

—Luc