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AVR: Fix interrupt bombing during a context switch

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TCB_RESTORE macro has a problem when restoring Status Register
and returning from the function (in up_fullcontextrestore()) as non-atomic action.

If there is some frequently occurring interrupt, chances are that we will
enter the interrupt handler just before ret is called.
The handler may cause a context switch which, when unrolled, will
execute up_fullcontextrestore() function that employs TCB_RESTORE.
It will be interrupted again just before return, leaving part of context
switch content un-popped again, etc... Thus, chances are that the stack will
eventually blow.

Note that this is not some edge condition fix. This bug was discovered when
testing AVR with UART configured to work on 115200 baud rate.
This commit is contained in:
Dimitry Kloper 2015-12-26 21:55:40 +02:00
parent 6b1f3da01a
commit 0998876ef6
2 changed files with 32 additions and 5 deletions
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32
arch/avr/src/avr/excptmacros.h
View File

@ -514,9 +514,22 @@
ld r0, x+
/* Restore the status register (probably re-enabling interrupts) */
/* The following control flow split is required to eliminate non-atomic
interrupt_enable - return sequence.
NOTE: since actual returning is handled by this macro it has been removed
from up_fullcontextrestore function (up_switchcontext.S)
*/
/* if interrupts shall be enabled go to 'restore remaining and reti' code
otherwise just do 'restore remaining and ret' */
ld r24, x+
bst r24, SREG_I
brts go_reti
/* Restore the status register, interrupts are disabled */
out _SFR_IO_ADDR(SREG), r24
/* Restore r24-r25 - The temporary and IRQ number registers */
@ -531,6 +544,23 @@
pop r27 /* R27 then R26 */
pop r26
ret
go_reti:
/* restore the Status Register with interrupts disabled
and exit with reti (that will set the Interrupt Enable) */
andi r24, ~(1 << SREG_I)
out _SFR_IO_ADDR(SREG), r24
ld r25, x+
ld r24, x+
pop r27
pop r26
reti
.endm
/********************************************************************************************

5
arch/avr/src/avr/up_switchcontext.S
View File

@ -130,11 +130,8 @@ up_fullcontextrestore:
TCB_RESTORE
/* And "return" to the new task (using ret not reti so that the interrupt
* state that we just restored will be preserved).
*/
/* Retruning from the function is handled in TCB_RESTORE */
ret
.endfunc
.end