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runtime: use rwmutex for execLock #20738

aclements opened this issue Jun 20, 2017 · 3 comments


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commented Jun 20, 2017

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 runtime.newm.

We should try to figure out a way to fix this, possibly by rewriting rwmutex to not depend on semaphores.

/cc @ianlancetaylor @chipaca

@aclements aclements added this to the Go1.9 milestone Jun 20, 2017

@aclements aclements self-assigned this Jun 20, 2017


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commented Jun 22, 2017

Broken link: CL 43713


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commented Jun 28, 2017

CL mentions this issue.


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commented Jun 28, 2017

CL mentions this issue.

gopherbot pushed a commit that referenced this issue Jun 28, 2017

runtime: make rwmutex work on Ms instead of Gs
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
Run-TryBot: Austin Clements <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Ian Lance Taylor <>

@gopherbot gopherbot closed this in 9745e88 Jun 28, 2017

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