fix(rp2350): add software spinlocks #5034

mikesmitty · 2025-09-10T01:17:45Z

As it turns out, the RP2350 has hardware spinlocks that can be unlocked by writes to nearby addresses, the lower spinlocks currently in use in TinyGo happen to be unlocked by writes to the doorbell interrupt registers used to signal between cores, very possibly leading to some unexpected unlocks. This was not corrected in the A3 or A4 steppings and instead software spinlocks are used by default on RP2350 in pico-sdk:

RP2350 Warning. Due to erratum RP2350-E2, writes to new SIO registers above an offset of +0x180 alias the spinlocks, causing spurious lock releases. This SDK by default use atomic memory accesses to implement the hardware_sync_spin_lock API, as a workaround on RP2350 A2.

https://www.raspberrypi.com/documentation/pico-sdk/hardware.html#group_hardware_sync

Another thing I noticed in that section, they also always disable interrupts when the spinlocks are being held, we may want to do the same:

[...] the default spinlock related methods here (e.g. spin_lock_blocking) always disable interrupts while the lock is held as use by IRQ handlers and user code is common/desirable, and spin locks are only expected to be held for brief periods.

These are the software spinlock macros ported over:
https://github.com/raspberrypi/pico-sdk/blob/2.2.0/src/rp2_common/hardware_sync_spin_lock/include/hardware/sync/spin_lock.h#L112
https://github.com/raspberrypi/pico-sdk/blob/2.2.0/src/rp2_common/hardware_sync_spin_lock/include/hardware/sync/spin_lock.h#L197

mikesmitty · 2025-09-10T02:08:06Z

Oh, whoops. My go fmt extension has been flaking out on me. Will have the missing rp2040 imports updated in a moment. Here's the lock/unlock disassembled output with inlining disabled:

   0x10001178 <(*runtime.spinLock).Lock+0>:     cbz     r0, 0x10001192 <(*runtime.spinLock).Lock+26>
   0x1000117a <(*runtime.spinLock).Lock+2>:     ldaexb  r2, [r0]
   0x1000117e <(*runtime.spinLock).Lock+6>:     movs    r1, #1
   0x10001180 <(*runtime.spinLock).Lock+8>:     cmp     r2, #0
   0x10001182 <(*runtime.spinLock).Lock+10>:    bne.n   0x1000117a <(*runtime.spinLock).Lock+2>
   0x10001184 <(*runtime.spinLock).Lock+12>:    strexb  r2, r1, [r0]
   0x10001188 <(*runtime.spinLock).Lock+16>:    cmp     r2, #0
   0x1000118a <(*runtime.spinLock).Lock+18>:    bne.n   0x1000117a <(*runtime.spinLock).Lock+2>
   0x1000118c <(*runtime.spinLock).Lock+20>:    dmb     sy
   0x10001190 <(*runtime.spinLock).Lock+24>:    bx      lr
   0x10001192 <(*runtime.spinLock).Lock+26>:    bl      0x100013f4 <runtime.nilPanic>

   0x100013e4 <(*runtime.spinLock).Unlock+0>:   cbz     r0, 0x100013ee <(*runtime.spinLock).Unlock+10>
   0x100013e6 <(*runtime.spinLock).Unlock+2>:   movs    r1, #0
   0x100013e8 <(*runtime.spinLock).Unlock+4>:   stlb    r1, [r0]
   0x100013ec <(*runtime.spinLock).Unlock+8>:   bx      lr
   0x100013ee <(*runtime.spinLock).Unlock+10>:  bl      0x100013f4 <runtime.nilPanic>

eliasnaur

I support the switch to atomic instructions, but if you need something that works right away, RP2350-E2 mentions that some spinlocks are not affected:

The following SIO spinlocks can be used normally because they don’t alias with writable registers: 5, 6, 7,
10, 11, and 18 through 31. Some of the other lock addresses may be used safely depending on which of
the high-addressed SIO registers are in use.
Locks 18 through 24 alias with some read-only TMDS encoder registers, which is safe as only writes are
mis-decoded.

src/runtime/runtime_rp2350.go

aykevl

See my comment below.

Also,

Another thing I noticed in that section, they also always disable interrupts when the spinlocks are being held, we may want to do the same: [...]

The runtime does this in various places if needed. The spinlock implementation doesn't need to disable interrupts too.

src/runtime/runtime_rp2350.go

mikesmitty · 2025-09-10T23:21:55Z

I support the switch to atomic instructions, but if you need something that works right away, RP2350-E2 mentions that some spinlocks are not affected:

The following SIO spinlocks can be used normally because they don’t alias with writable registers: 5, 6, 7,
10, 11, and 18 through 31. Some of the other lock addresses may be used safely depending on which of
the high-addressed SIO registers are in use.
Locks 18 through 24 alias with some read-only TMDS encoder registers, which is safe as only writes are
mis-decoded.

Thank you for tracking those down. I figured there would probably be some, but I couldn't find them

eliasnaur

Other than a nit, this looks good to me. @aykevl WDYT?

eliasnaur · 2025-09-17T14:40:17Z

src/runtime/runtime_rp2.go

 	printLock     = spinLock{id: 0}
 	schedulerLock = spinLock{id: 1}
 	atomicsLock   = spinLock{id: 2}
 	futexLock     = spinLock{id: 3}


id is an implementation detail of rp2040 spinlocks, whereas on rp2350 ids have some meaning but are unusued. I suggest moving the spinlock variables to the rpXXXX.go files and avoid the id field on rp2350.

eliasnaur · 2025-09-17T14:41:40Z

src/runtime/runtime_rp2350.go

+
+type spinLock struct {
+	atomic.Uint32
+	id uint8


Delete field.

That one's still needed for compatibility with how the rp2040 hardware spinlocks are initialized here:

tinygo/src/runtime/runtime_rp2.go

Lines 295 to 298 in 109e076

printLock = spinLock{id: 20}

schedulerLock = spinLock{id: 21}

atomicsLock = spinLock{id: 22}

futexLock = spinLock{id: 23}

Indeed. Above, I suggested moving that var () block to the runtime_rp2xxx.go files to get rid of that dependency.

Ah yep, I missed that

mikesmitty · 2025-09-18T01:37:49Z

After doing some testing, I think this might not actually be locking properly. Going to do some more thorough testing

aykevl · 2025-09-23T12:17:49Z

Yes, this looks good. @mikesmitty can you apply the changes proposed by @eliasnaur?

Also, you may want to rebase on the dev branch to hopefully fix the assert-test-linux CI failure.

mikesmitty · 2025-09-24T12:58:24Z

Sure, here's the rebase. I haven't had time to test or diagnose it yet, been stuck working on a project that's consumed all my free time, but I'm pretty sure it's not functioning properly as confirmed by that CI failure

mikesmitty · 2025-09-24T13:11:44Z

Hmm, no I guess it is actually locking. I'm not sure what's happening with that CI test though

aykevl

LGTM, thank you!

aykevl · 2025-09-29T13:02:42Z

Restarting CI, these are probably flaky tests.

eliasnaur · 2025-09-29T13:21:14Z

src/runtime/runtime_rp2350.go


+// #include <stdint.h>
+//
+// static void spinlock_lock(void *lock) {


What was wrong with the sync/atomic operations? Custom assembly for just one target (rp2350) seems to me like a maintenance headache for little gain.

I forget, but I'll take a second look at them. One reason was the size of them, but I think there was another reason also. In hindsight, the size difference is fairly small all things considered

mikesmitty · 2025-10-03T13:29:20Z

Ok, finally coming back to take a look at this again. I still have some reservations about the stability of this one. I'm getting mysterious panics in various runtime locations that I haven't tracked down yet. I suspect some race conditions are going on that I haven't narrowed down yet

For example:

[2025-10-03T13:28:06.865Z, +005191ms] panipanic: runtime  ee[2025-10-03T13:28:06.877Z, +005203ms] rrrroorr  aatt  0x100036ed0x10003a5d: unsafe.Slice/String: len out of range

aykevl

Just realized there are some issues with the assembly code.

aykevl · 2025-10-03T13:41:28Z

src/runtime/runtime_rp2350.go

+//     "cmp %1, #0\n"                                             \
+//     /* Claim failed due to intervening write, so retry */      \
+//     "bne 1b\n"                                                 \
+//     : "=&r" (_tmp0), "=&r" (_tmp1) : "r" (lock)                \


Just realized this is incomplete. You also need to clobber flags and memory at least.

aykevl · 2025-10-03T13:43:27Z

src/runtime/runtime_rp2350.go

+//     uint32_t zero = 0;                                                      \
+//     __asm volatile (                                                        \
+//         "stlb %0, [%1]\n"                                                   \
+//         : : "r" (zero), "r" (lock)                                          \


Same here. It modifies memory, though I don't think it updates any flags.

eliasnaur reviewed Sep 10, 2025

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mikesmitty requested review from aykevl and eliasnaur September 10, 2025 23:24

eliasnaur approved these changes Sep 17, 2025

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mikesmitty added 6 commits September 24, 2025 08:39

fix(rp2350): add software spinlocks

d1eca84

chore: clarify comments

fc81160

chore: use atomics instead of artisinal assembly

31696c2

chore: comment cleanup

09fbdc0

fix: add id field for hw spinlock compatibility

ab8bf89

chore: switch to cgo inline asm

cee4537

mikesmitty force-pushed the ms/rp2350-sw-mutex branch from 1f2642c to cee4537 Compare September 24, 2025 12:43

aykevl changed the base branch from release to dev September 29, 2025 12:59

aykevl approved these changes Sep 29, 2025

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eliasnaur reviewed Sep 29, 2025

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aykevl requested changes Oct 3, 2025

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	printLock = spinLock{id: 20}
	schedulerLock = spinLock{id: 21}
	atomicsLock = spinLock{id: 22}
	futexLock = spinLock{id: 23}

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fix(rp2350): add software spinlocks #5034

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