Message-ID: <280fb8f6-f1fd-76ce-7851-cf720820c44e@huaweicloud.com>
Date:   Tue, 24 Jan 2023 16:54:42 +0100
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Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     "paulmck@kernel.org" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "parri.andrea" <parri.andrea@gmail.com>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
References: <20220921173109.GA1214281@paulmck-ThinkPad-P17-Gen-1>
 <YytfFiMT2Xsdwowf@rowland.harvard.edu>
 <YywXuzZ/922LHfjI@hirez.programming.kicks-ass.net>
 <114ECED5-FED1-4361-94F7-8D9BC02449B7> <Y8lynRI35cFeuqb5@rowland.harvard.edu>
From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
In-Reply-To: <Y8lynRI35cFeuqb5@rowland.harvard.edu>
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On 1/19/2023 5:41 PM, Alan Stern wrote:
> On Thu, Jan 19, 2023 at 12:22:50PM +0100, Jonas Oberhauser wrote:
>>
>> On 1/19/2023 3:28 AM, Alan Stern wrote:
>>>> This is a permanent error; I've given up. Sorry it didn't
>>> work out.
>> [It seems the e-mail still reached me through the mailing list]
> [For everyone else, Jonas is referring to the fact that the last two
> emails I sent to his huaweicloud.com address could not be delivered, so
> I copied them off-list to his huawei.com address.]
>
>>>> I consider that a hack though and don't like it.
>>> It _is_ a bit of a hack, but not a huge one.  srcu_read_lock() really
>>> is a lot like a load, in that it returns a value obtained by reading
>>> something from memory (along with some other operations, though, so it
>>> isn't a simple straightforward read -- perhaps more like an
>>> atomic_inc_return_relaxed).
>> The issue I have with this is that it might create accidental ordering. How
>> does it behave when you throw fences in the mix?
> I think this isn't going to be a problem.  Certainly any real
> implementation of scru_read_lock() is going to involve some actual load
> operations, so any unintentional ordering caused by fences will also
> apply to real executions.  Likewise for srcu_read_unlock and store
> operations.

Note that there may indeed be reads in the implementation, but most 
likely not from the srcu_read_unlock()s of other threads. Most probably 
from the synchronize_srcu() calls. So the rfe edges being added are 
probably not corresponding to any rfe edges in the implementation.

That said, I believe there may indeed not be any restrictions in 
behavior caused by this, because any code that relies on the order being 
a certain thing would need to use some other ordering mechanism, and 
that would probably restrict the behavior anyways.

It does have the negative side-effect of creating an explosion of 
permutations though, by ordering all unlocks() in a total way and also 
sometimes allowing multiple options for each lock() (e.g., 
lock();unlock() || lock();unlock()  has 4 executions instead of 1).

Anyways, not much to be done about it right now.

best wishes, jonas