Messages in this thread | | | Date | Tue, 24 Jan 2023 16:54:42 +0100 | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) | From | Jonas Oberhauser <> |
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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
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