On Wed, Jul 24, 2019 at 11:24 AM shivaram via golang-nuts <golang-nuts@googlegroups.com> wrote: > > Several sources, including this issue comment: > > https://github.com/golang/go/issues/27544#issuecomment-419265234 > > state that a buffered channel of size 1 can be used as a trylock by > select'ing on it with a default case. > > I was thinking about it and it seemed to me that with such an implementation, > it is unclear whether a failed acquisition of the trylock guarantees that the > lock is held by someone else. More precisely, I can't tell if the following > guarantee holds: a failed acquisition of the trylock cannot happen-before a > successful acquisition, unless a release happens-between them. (To put it > another way: can there be "spurious" failures of trylock acquisition?)
In any realistic program, a lock may be held by a different goroutine as the trylock starts and released as the trylock ends. So it is always possible for trylock acquisition to fail but for the lock to be unlocked after trylock completes. > In particular: > > 1. The memory model documentation states ordering guarantees for *successful* > channel receives and sends, but doesn't say anything about failed nonblocking > receives and sends, so it seems to me that in principle it can't be promising > anything about this behavior. > > 2. I can't tell whether the fast-path checks in the current implementation > allow for this: > > https://github.com/golang/go/blob/919594830f17f25c9e971934d825615463ad8a10/src/runtime/chan.go#L159 > https://github.com/golang/go/blob/919594830f17f25c9e971934d825615463ad8a10/src/runtime/chan.go#L437 > > In particular, I don't understand why `chanrecv` does an atomic read of > `qcount`, but `chansend` just does a normal read. This is explained by the comments about read ordering and order of operations. Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcX0HB6SiJNL4U1eUBn_4Vd8KET45n%3Dq3vCzTrFhn-SX%3Dw%40mail.gmail.com.
