Messages in this thread | | | Date | Thu, 2 Nov 2017 17:45:27 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t |
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On Thu, Nov 02, 2017 at 05:02:37PM +0100, Peter Zijlstra wrote: > On Thu, Nov 02, 2017 at 11:40:35AM -0400, Alan Stern wrote: > > On Thu, 2 Nov 2017, Peter Zijlstra wrote: > > > > > > Lock functions such as refcount_dec_and_lock() & > > > > refcount_dec_and_mutex_lock() Provide exactly the same guarantees as > > > > they atomic counterparts. > > > > > > Nope. The atomic_dec_and_lock() provides smp_mb() while > > > refcount_dec_and_lock() merely orders all prior load/store's against all > > > later load/store's. > > > > In fact there is no guaranteed ordering when refcount_dec_and_lock() > > returns false; > > It should provide a release: > > - if !=1, dec_not_one will provide release > - if ==1, dec_not_one will no-op, but then we'll acquire the lock and > dec_and_test will provide the release, even if the test fails and we > unlock again it should still dec. > > The one exception is when the counter is saturated, but in that case > we'll never free the object and the ordering is moot in any case. > > > it provides ordering only if the return value is true. > > In which case it provides acquire ordering (thanks to the spin_lock), > > and both release ordering and a control dependency (thanks to the > > refcount_dec_and_test). > > > > > The difference is subtle and involves at least 3 CPUs. I can't seem to > > > write up anything simple, keeps turning into monsters :/ Will, Paul, > > > have you got anything simple around? > > > > The combination of acquire + release is not the same as smp_mb, because > > acquire+release is nothing, its release+acquire that I meant which > should order things locally, but now that you've got me looking at it > again, we don't in fact do that. > > So refcount_dec_and_lock() will provide a release, irrespective of the > return value (assuming we're not saturated). If it returns true, it also > does an acquire for the lock. > > But combined they're acquire+release, which is unfortunate.. it means > the lock section and the refcount stuff overlaps, but I don't suppose > that's actually a problem. Need to consider more.
Right, so in that case we have refcount==0 and are guaranteed no concurrency. So its fine.
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