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runtime: use rwmutex for execLock #20738
CL 43713 introduces a mutex around thread creation. We wanted to use rwmutex to avoid serializing thread creation, but rwmutex depends on semaphores, which depend on sudogs, which require allocation and write barriers, so we can't use them from
We should try to figure out a way to fix this, possibly by rewriting rwmutex to not depend on semaphores.
Currently runtime.rwmutex is written to block the calling goroutine rather than the calling thread. However, rwmutex was intended to be used in the scheduler, which means it needs to be a thread-level synchronization primitive. Hence, this modifies rwmutex to synchronize threads instead of goroutines. This has the consequence of making it write-barrier-free, which is also important for using it in the scheduler. The implementation makes three changes: it replaces the "w" semaphore with a mutex, since this was all it was being used for anyway; it replaces "writerSem" with a single pending M that parks on its note; and it replaces "readerSem" with a list of Ms that park on their notes plus a pass count that together emulate a counting semaphore. I model-checked the safety and liveness of this implementation through >1 billion schedules. For #20738. Change-Id: I3cf5a18c266a96a3f38165083812803510217787 Reviewed-on: https://go-review.googlesource.com/47071 Run-TryBot: Austin Clements <email@example.com> TryBot-Result: Gobot Gobot <firstname.lastname@example.org> Reviewed-by: Ian Lance Taylor <email@example.com>