Messages in this thread | | | Date | Tue, 24 Jan 2023 12:22:11 -0500 | From | Alan Stern <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Tue, Jan 24, 2023 at 04:54:42PM +0100, Jonas Oberhauser wrote: > > > 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).
That's true. It would be nice if there was a class of write-like events which couldn't be read from and didn't contribute to the coherence ordering.
Alan
> Anyways, not much to be done about it right now. > > best wishes, jonas >
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