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sync: don't use volatile in Mutex #4558

Merged
merged 1 commit into from
Oct 28, 2024
Merged

sync: don't use volatile in Mutex #4558

merged 1 commit into from
Oct 28, 2024

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aykevl
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@aykevl aykevl commented Oct 28, 2024

Volatile loads/stors are only useful for communication with interrupts or for memory-mapped I/O. They do not provide any sort of safety for sync.Mutex, while making it appear as if it is more safe.

  • sync.Mutex cannot be used safely inside interrupts, because any blocking calls (including Lock) will cause a runtime panic.
  • For multithreading, volatile is also the wrong choice. Atomic operations should be used instead, and the current code would not work for multithreaded programs anyway.

Looking at the original PR in #3983 I don't see why volatile is needed here. It makes it look like it's doing something safe, while it is not (the default without volatile is already safe for the current scheduler because there's no threading).
I'm making this PR in advance because it will make it easier to implement a sync.Mutex that does in fact support multithreading (using atomics and futex).

The only thing that this would provide, is to make the following unconventional pattern somewhat safe:

var mutex sync.Mutex

func startTransfer() {
    mutex.Lock()
    startSomeInterruptHandler()

    // wait until the interrupt unlocks the mutex
    mutex.Lock()
}

func handleInterrupt() {
    // do something with the interrupt

    // signal to the main goroutine that we're done
    mutex.Unlock()
}

I'm not aware of anybody doing this, and I don't think it's documented anywhere. And in any case, we'd be better off providing a proper primitive for this feature of waiting for an interrupt to finish (a TODO for me for years already).

@aykevl
sync: don't use volatile in Mutex
4a03cb3 
Volatile loads/stors are only useful for communication with interrupts
or for memory-mapped I/O. They do not provide any sort of safety for
sync.Mutex, while making it *appear* as if it is more safe.

  * `sync.Mutex` cannot be used safely inside interrupts, because any
    blocking calls (including `Lock`) will cause a runtime panic.
  * For multithreading, `volatile` is also the wrong choice. Atomic
    operations should be used instead, and the current code would not
    work for multithreaded programs anyway.
@aykevl aykevl requested review from dgryski and soypat October 28, 2024 13:59
@dgryski
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dgryski commented Oct 28, 2024

My guess is I had probably just been looking at a tinygo runtime but where llvm was doing something unexpected and so decided to take a more defensive route.

dgryski
dgryski approved these changes Oct 28, 2024
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LGTM

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Size difference with the dev branch:

Binary size difference 
not the same command!
    tinygo build -size short -o ./build/test.hex -target=feather-rp2040 ./examples/adafruit4650
    go: downloading tinygo.org/x/tinyfont v0.3.0
not the same command!
    tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/
    go: downloading golang.org/x/net v0.7.0
not the same command!
    tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/
    go: downloading github.com/soypat/natiu-mqtt v0.5.1
not the same command!
    tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/
    go: downloading github.com/eclipse/paho.mqtt.golang v1.2.0
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aykevl commented Oct 28, 2024

My guess is I had probably just been looking at a tinygo runtime but where llvm was doing something unexpected and so decided to take a more defensive route.

There are most certainly valid use cases for volatile! I just don't see it in this case - the code is basically entirely synchronous (stack switching is possible, but that also happens at defined points in the program).

soypat
soypat approved these changes Oct 28, 2024
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I honestly don't recall why I stuck volatile in there 🫠. Looks and sounds good to me!

@aykevl aykevl merged commit 76d5b3d into dev Oct 28, 2024
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@aykevl aykevl deleted the sync-mutex-volatile branch October 28, 2024 15:43
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aykevl commented Oct 28, 2024 •
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I will in fact be adding atomic instructions in the future, see #4559 (commit name "sync: implement futex-based Mutex"). That's how I found the volatile instructions and was wondering how they got there - hence this PR.

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@soypat soypat soypat approved these changes

@dgryski dgryski dgryski approved these changes

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@aykevl @dgryski @soypat