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Commit2923d54

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rp2: Support calling pendsv_suspend/resume from core 1.
Previously, this was subject to races incrementing/decrementingthe counter variable pendsv_lock.Technically, all that's needed here would be to make pendsv_lock an atomiccounter.This implementation fulfils a stronger guarantee: it also provides mutualexclusion for the core which calls pendsv_suspend(). This is because thecurrent use of pendsv_suspend/resume in MicroPython is to ensure exclusiveaccess to softtimer data structures, and this does require mutualexclusion.The conceptually cleaner implementation would split the mutual exclusionpart out into a softtimer-specific spinlock, but this increases thecomplexity and doesn't seem like it makes for a better implementation inthe long run.This work was funded through GitHub Sponsors.Signed-off-by: Angus Gratton <angus@redyak.com.au>
1 parentfccab87 commit2923d54

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3 files changed

+27
-7
lines changed

3 files changed

+27
-7
lines changed

‎ports/rp2/main.c

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -76,6 +76,7 @@ int main(int argc, char **argv) {
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// This is a tickless port, interrupts should always trigger SEV.
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SCB->SCR |=SCB_SCR_SEVONPEND_Msk;
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79+
pendsv_init();
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soft_timer_init();
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#ifMICROPY_HW_ENABLE_UART_REPL

‎ports/rp2/pendsv.c

Lines changed: 25 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -25,6 +25,7 @@
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*/
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#include<assert.h>
28+
#include"pico/mutex.h"
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#include"py/mpconfig.h"
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#include"pendsv.h"
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#include"RP2040.h"
@@ -34,15 +35,21 @@
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#endif
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staticpendsv_dispatch_tpendsv_dispatch_table[PENDSV_DISPATCH_NUM_SLOTS];
37-
staticintpendsv_lock;
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staticrecursive_mutex_tpendsv_mutex;
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40+
voidpendsv_init(void) {
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recursive_mutex_init(&pendsv_mutex);
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}
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voidpendsv_suspend(void) {
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pendsv_lock++;
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// Recursive Mutex here as either core may call pendsv_suspend() and expect
46+
// both mutual exclusion (other core can't enter pendsv_suspend() at the
47+
// same time), and that no PendSV handler will run.
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recursive_mutex_enter_blocking(&pendsv_mutex);
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}
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voidpendsv_resume(void) {
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pendsv_lock--;
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assert(pendsv_lock >=0);
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recursive_mutex_exit(&pendsv_mutex);
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// Run pendsv if needed. Find an entry with a dispatch and call pendsv dispatch
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// with it. If pendsv runs it will service all slots.
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intcount=PENDSV_DISPATCH_NUM_SLOTS;
@@ -55,9 +62,11 @@ void pendsv_resume(void) {
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}
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voidpendsv_schedule_dispatch(size_tslot,pendsv_dispatch_tf) {
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assert(pendsv_lock >=0);
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pendsv_dispatch_table[slot]=f;
60-
if (pendsv_lock==0) {
66+
if (pendsv_mutex.enter_count==0) {
67+
// There is a race here where other core calls pendsv_suspend() before
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// ISR can execute, but dispatch will happen later when other core
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// calls pendsv_resume().
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SCB->ICSR=SCB_ICSR_PENDSVSET_Msk;
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}else {
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#ifMICROPY_PY_NETWORK_CYW43
@@ -68,7 +77,14 @@ void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
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// PendSV interrupt handler to perform background processing.
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voidPendSV_Handler(void) {
71-
assert(pendsv_lock==0);
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if (!recursive_mutex_try_enter(&pendsv_mutex,NULL)) {
82+
// Failure here means core 1 holds pendsv_mutex. ISR will
83+
// run again after core 1 calls pendsv_resume().
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return;
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}
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// Core 0 should not already have locked pendsv_mutex
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assert(pensv_mutex.enter_count==1);
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#ifMICROPY_PY_NETWORK_CYW43
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CYW43_STAT_INC(PENDSV_RUN_COUNT);
@@ -81,4 +97,6 @@ void PendSV_Handler(void) {
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f();
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}
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}
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recursive_mutex_exit(&pendsv_mutex);
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}

‎ports/rp2/pendsv.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -47,6 +47,7 @@ enum {
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typedefvoid (*pendsv_dispatch_t)(void);
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voidpendsv_init(void);
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voidpendsv_suspend(void);
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voidpendsv_resume(void);
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voidpendsv_schedule_dispatch(size_tslot,pendsv_dispatch_tf);

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