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Perry Kivolowitz 2023-04-21 09:04:46 -05:00
parent c2fe010bc1
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3 changed files with 25 additions and 21 deletions
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41
more/spin-lock/README.md
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@ -105,25 +105,24 @@ Bummer.
Here is the source code to the spin-lock for ARM V8. Here is the source code to the spin-lock for ARM V8.
```text ```text
#if defined(__APPLE__) // 1 #if defined(__APPLE__) // 1
_Lock: // 2 _Lock: // 2
#else // 3 #else // 3
Lock: // 4 Lock: // 4
#endif // 5 #endif // 5
START_PROC // 6 START_PROC // 6
1: ldaxr w1, [x0] // 7 mov w3, 1 // 7
cbnz w1, 1b // lock taken - spin. // 8 1: ldaxr w1, [x0] // 8
add w1, w1, 1 // 9 cbnz w1, 1b // lock taken - spin. // 9
stlxr w2, w1, [x0] // 10 stlxr w2, w3, [x0] // 10
cbnz w2, 1b // shucks - somebody meddled. // 11 cbnz w2, 1b // shucks - somebody meddled. // 11
// considered using dmb here // 12 ret // 12
ret // 13 END_PROC // 13
END_PROC // 14
``` ```
Once again, line 7 does a `ldaxr` dereferencing the lock itself (once Line 8 does a `ldaxr` dereferencing the lock itself (once again an
again an `int32_t`) and marks the location of the lock as being `int32_t`) and marks the location of the lock as being hopefully,
hopefully, exclusive. exclusive.
Having gotten the value of the lock, on line 8, its value is inspected Having gotten the value of the lock, on line 8, its value is inspected
and if found to be non-zero, we branch back to attempting to get it and if found to be non-zero, we branch back to attempting to get it
@ -149,7 +148,7 @@ Unlock: // 4
#endif // 5 #endif // 5
START_PROC // 6 START_PROC // 6
str wzr, [x0] // 7 str wzr, [x0] // 7
// considered using dmb here // 8 dmb ish // 8
ret // 9 ret // 9
END_PROC // 10 END_PROC // 10
``` ```
@ -159,5 +158,11 @@ of the lock requires that no bad actor simply stomps on the lock by
calling `Unlock` without first owning the lock. Just say no to lock calling `Unlock` without first owning the lock. Just say no to lock
stompers. stompers.
Line 8 sets up a data memory barrier across each processor - it makes
sure threads running on different cores see the update correctly. This
code seemed to work without this line but intuition suggests it could
be important. In `Lock()` the `stlxr` instruction has an implied data
memory barrier.
Please see the source code located [here](./spin_lock.S) for some Please see the source code located [here](./spin_lock.S) for some
additional comments regarding the implementation. additional comments regarding the implementation.

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more/spin-lock/README.pdf
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5
more/spin-lock/spin_lock.S
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@ -33,7 +33,6 @@ Lock:
cbnz w1, 1b // lock taken - spin. cbnz w1, 1b // lock taken - spin.
stlxr w2, w3, [x0] stlxr w2, w3, [x0]
cbnz w2, 1b // shucks - somebody meddled. cbnz w2, 1b // shucks - somebody meddled.
// considered using dmb here
ret ret
END_PROC END_PROC
@ -44,8 +43,8 @@ Unlock:
#endif #endif
START_PROC START_PROC
str wzr, [x0] str wzr, [x0]
// considered using dmb here dmb ish // ensure all cores are aware.
ret ret
END_PROC END_PROC
.end .end