Why will it deadlock if a goroutine acquire a mutex while pinned to its P?

Cholerae Hu

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Apr 8, 2019, 2:31:07 AM4/8/19

to golang-nuts

I'm reading this commit https://github.com/golang/go/commit/d5fd2dd6a17a816b7dfd99d4df70a85f1bf0de31 . Inside runtime_procPin we only increases the m.lock count, why will it cause deadlock when acquiring a mutex after pin?

Ian Lance Taylor

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Apr 8, 2019, 2:25:06 PM4/8/19

to Cholerae Hu, Austin Clements, golang-nuts

[ + austin ]

On Sun, Apr 7, 2019 at 11:31 PM Cholerae Hu <chole...@gmail.com> wrote:
>
> I'm reading this commit https://github.com/golang/go/commit/d5fd2dd6a17a816b7dfd99d4df70a85f1bf0de31 . Inside runtime_procPin we only increases the m.lock count, why will it cause deadlock when acquiring a mutex after pin?
>

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Robert Engels

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Apr 8, 2019, 5:34:22 PM4/8/19

to Ian Lance Taylor, Cholerae Hu, Austin Clements, golang-nuts

Apologies not warranted. I was only pointing out that it didn't seem correct to me.

Austin Clements

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Apr 9, 2019, 10:59:37 AM4/9/19

to Cholerae Hu, golang-nuts

Acquiring a mutex while pinned can cause deadlock because pinning prevents a stop-the-world. For example, the following sequence could result in a deadlock:

M1: Acquires mutex l.

M2: Pins the M.

M2: Attempts to acquire mutex l.

M3: Initiates stop-the-world

M3: Stops M1

M3: Attempts to stop M2, but can't because M2 is pinned.

At this point, M1 can't make progress to release mutex l because M3 stopped it, which means M2 won't be able to finish acquiring the mutex (so it will never release the pin), which means M3 won't be able to finish stopping the world (so it will never start M1 back up).

On Mon, Apr 8, 2019 at 2:31 AM Cholerae Hu <chole...@gmail.com> wrote:

I'm reading this commit https://github.com/golang/go/commit/d5fd2dd6a17a816b7dfd99d4df70a85f1bf0de31 . Inside runtime_procPin we only increases the m.lock count, why will it cause deadlock when acquiring a mutex after pin?

--

Cholerae Hu

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Apr 10, 2019, 3:42:10 AM4/10/19

to golang-nuts

Thanks austin! I totally understand.

在 2019年4月9日星期二 UTC+8下午10:59:37，Austin Clements写道：

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