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avr: fix race condition in stack write

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If an interrupt happens between the writes to SPL and SPH, the stack
pointer is inconsistent and terrible things will happen. Therefore,
disable interrupts while updating the stack pointer.

Interrupts are restored _before_ the write to SPH. This is safe, because
interrupts are re-enabled with a one cycle delay. The avr-gcc and Clang
compilers do the same thing when they need to update the stack pointer.

It's almost impossible to test for this bug, but it should make firmware
just a little bit more reliable.
This commit is contained in:
Ayke van Laethem
2022-06-17 17:09:17 +02:00
committed by Ron Evans
parent bcf58c0840
commit 9294141d70
1 changed files with 3 additions and 0 deletions
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src/internal/task/task_stack_avr.S
+3
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@@ -72,7 +72,10 @@ tinygo_swapTask:
std Y+1, r3
// Switch to the new stack pointer.
in r0, 0x3f ; SREG
cli
out 0x3d, r24; SPL
out 0x3f, r0 ; SREG, restore interrupts (after the next instruction)
out 0x3e, r25; SPH
// Load saved register from the new stack.