Implement race-free signals using futexes #4586

aykevl · 2024-11-07T13:39:47Z

Use a futex instead of the previous rather complicated code with signal masking. It was race free (I hope!) on Linux but not on MacOS. This PR makes it race-free on MacOS too.

Sleeping is very simple: just use a futex with a timeout and if the futex value changed we know there was a signal. There shouldn't be any race conditions with this approach.

This also adds an internal/futex package which I intend to reuse in #4559.

github-actions · 2024-11-07T13:56:50Z

Size difference with the dev branch:

Binary size difference

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    tinygo build -size short -o ./build/test.hex -target=feather-rp2040 ./examples/adafruit4650
    go: downloading tinygo.org/x/tinyfont v0.3.0
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    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
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    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
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    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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  65816   65816      0   0.00%    4784    4784      0   0.00% tinygo build -size short -o ./build/test.uf2 -target=pico ./examples/mcp9808/main.go
  85924   85924      0   0.00%    5140    5140      0   0.00% tinygo build -size short -o ./build/test.hex -target=challenger-rp2040 ./examples/net/ntpclient/
 296644  296644      0   0.00%   13788   13788      0   0.00% tinygo build -size short -o ./build/test.hex -target=pyportal -stack-size 8kb ./examples/net/http-get/
 119376  119376      0   0.00%    7848    7848      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-nano33 -stack-size 8kb ./examples/net/tcpclient/
 245516  245516      0   0.00%   10488   10488      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/
 103112  103112      0   0.00%    9816    9816      0   0.00% tinygo build -size short -o ./build/test.hex -target=metro-m4-airlift -stack-size 8kb ./examples/net/socket/
 338452  338452      0   0.00%   15396   15396      0   0.00% tinygo build -size short -o ./build/test.hex -target=matrixportal-m4 -stack-size 8kb ./examples/net/webstatic/
 111588  111588      0   0.00%    7772    7772      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-mkrwifi1010 -stack-size 8kb ./examples/net/tlsclient/
 154216  154216      0   0.00%    6540    6540      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/
 117048  117048      0   0.00%   13132   13132      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webclient/
 286136  286136      0   0.00%   17812   17812      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webserver/
 287132  287132      0   0.00%   18212   18212      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/
5332918 5332918      0   0.00%  811790  811790      0   0.00%

eliasnaur

Nice improvement. A few comments from the peanut gallery.

eliasnaur · 2024-11-09T13:54:22Z

src/internal/futex/futex_linux.c

+#define FUTEX_WAIT         0
+#define FUTEX_WAKE         1
+#define FUTEX_PRIVATE_FLAG 128


Why not #include <linux/futex.h>?

Because this header file is not available in musl, which we use.

eliasnaur · 2024-11-09T14:02:45Z

src/internal/futex/futex_darwin.c

+int __ulock_wait(uint32_t operation, void *addr, uint64_t value, uint32_t timeout_us);
+int __ulock_wait2(uint32_t operation, void *addr, uint64_t value, uint64_t timeout_ns, uint64_t value2);
+int __ulock_wake(uint32_t operation, void *addr, uint64_t wake_value);


Link to documentation, if it exists somewhere (semi-)stable.

Done. There's no real documentation, but someone has figured out these calls: https://outerproduct.net/futex-dictionary.html

eliasnaur · 2024-11-09T14:10:02Z

src/internal/futex/futex.go

+// to sleep. Return true if we were definitely awoken by a call to Wake or
+// WakeAll, and false if we can't be sure of that.
+func (f *Futex) Wait(cmp uint32) bool {
+	tinygo_futex_wait((*uint32)(unsafe.Pointer(&f.Uint32)), cmp)


It's probably ok to assume the layout of atomic.Uint32 here and below, but it seems to me you can avoid it by

type Futex struct { v uint32 }

and adding methods that forwards to atomic.(Load|And|Store...)Uint32 functions.

I'd prefer to keep using atomic.Uint32. I don't expect atomic.Uint32 to ever change to something else, and if it does we can always update this code.

eliasnaur · 2024-11-09T14:17:55Z

src/internal/futex/futex.go

+
+// Like Wait, but times out after the number of nanoseconds in timeout.
+// If timeout is 0, it may or may not be treated as Wait with infinite timeout.
+// Therefore, make sure the timeout value is non-zero.


Given #4568 et al can a zero timeout be tightened to always do the least surprising thing, even if it means timeout = max(timeout, 1) here or in the platform implementations?

Done. I don't see a way this can happen currently (the two places that call sleepTicks make sure it's called with a positive timeout) but since this is MacOS and not some constrainted microcontroller it doesn't really hurt to put in an extra check.

eliasnaur · 2024-11-09T14:23:28Z

src/internal/futex/futex_darwin.c

+    // Note: __ulock_wait2 is available since MacOS 11.
+    // I think that's fine, since the version before that (MacOS 10.15) is EOL
+    // since 2022. Though if needed, we could certainly use __ulock_wait instead
+    // and deal with the smaller timeout value.


Nit: I would put (most of) this note in the commit message. Go 1.23 also requires macOS 11 and it doesn't seem reasonable to do better in TinyGo.

eliasnaur · 2024-11-09T14:41:52Z

src/runtime/runtime_unix.go

+	if !hasScheduler {
+		until = ticks() + d


I don't understand why this code depends on hasScheduler.

Yeah, that's a bug. Thanks for spotting it! (I probably modified the code a few times and it probably made sense to not calculate until without a scheduler before).
I'll push a fix.

eliasnaur · 2024-11-09T14:42:48Z

src/runtime/runtime_unix.go

+		// Set duration (in next loop iteration) to the remaining time.
+		d = until - ticks()
+		if d <= 0 {


If hasScheduler, d will always be less than zero, no?

Not necessarily! There are cases where Wait and WaitUntil might return due to spurious wakeups and not due to a signal. I'm not entirely sure whether this is possible, but the futex documentation on Linux is a bit vague on it (see src/internal/futex/futex.go) and since there's no real documentation for the MacOS syscalls I think it's better to defensively assume they can return spuriously. (I know that on Windows, the equivalent syscalls do return spuriously sometimes).

dgryski · 2024-11-19T00:50:36Z

Can you rebase so we get the fix for the wasm failure hitting the linux-assert build?

This really is the only sane way to register a signal. If this flag is not set, many syscalls will return EINTR (and not complete their operation) which will be a massive source of hard-to-debug bugs.

This requires an API introduced in MacOS 11. I think that's fine, since the version before that (MacOS 10.15) is EOL since 2022. Though if needed, we could certainly work around it by using an older and slightly less nice API.

aykevl · 2024-11-19T06:00:02Z

Rebased!

aykevl force-pushed the signal-races branch from 6e1fbef to eb6f2da Compare November 7, 2024 14:45

aykevl marked this pull request as ready for review November 7, 2024 14:45

aykevl force-pushed the signal-races branch 3 times, most recently from 7d64d51 to 7a88512 Compare November 7, 2024 15:31

eliasnaur reviewed Nov 9, 2024

View reviewed changes

aykevl force-pushed the signal-races branch from 7a88512 to 7748811 Compare November 12, 2024 12:02

aykevl added 2 commits November 19, 2024 06:59

runtime: use SA_RESTART when registering a signal

445ca0e

This really is the only sane way to register a signal. If this flag is not set, many syscalls will return EINTR (and not complete their operation) which will be a massive source of hard-to-debug bugs.

runtime: implement race-free signals using futexes

bb98375

This requires an API introduced in MacOS 11. I think that's fine, since the version before that (MacOS 10.15) is EOL since 2022. Though if needed, we could certainly work around it by using an older and slightly less nice API.

aykevl force-pushed the signal-races branch from 7748811 to bb98375 Compare November 19, 2024 05:59

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Implement race-free signals using futexes #4586

Implement race-free signals using futexes #4586

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Implement race-free signals using futexes #4586

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dgryski commented Nov 19, 2024

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