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fix semaphore bug (preempt before block/deblock)

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This commit is contained in:
Christoph Urlacher
2022-07-22 18:13:30 +02:00
parent 43477834fc
commit cde3967961
1 changed files with 7 additions and 7 deletions
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14
c_os/lib/Semaphore.cc
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@ -2,7 +2,7 @@
#include "kernel/Globals.h" #include "kernel/Globals.h"
void Semaphore::p() { void Semaphore::p() {
// Lock to allow deterministic operations on counter // Lock to allow deterministic operations on counter/queue
this->lock.acquire(); this->lock.acquire();
if (this->counter > 0) { if (this->counter > 0) {
@ -12,8 +12,10 @@ void Semaphore::p() {
} else { } else {
// Block and manage thread in semaphore queue until it's woken up by v() again // Block and manage thread in semaphore queue until it's woken up by v() again
this->wait_queue.push_back(scheduler.get_active()); this->wait_queue.push_back(scheduler.get_active());
cpu.disable_int(); // Make sure the block() comes through after releasing the lock
this->lock.release(); this->lock.release();
scheduler.block(); // Moves to next thread scheduler.block(); // Moves to next thread, enables int
} }
} }
@ -22,14 +24,12 @@ void Semaphore::v() {
if (!this->wait_queue.empty()) { if (!this->wait_queue.empty()) {
// Semaphore stays busy and unblocks next thread to work in critical section // Semaphore stays busy and unblocks next thread to work in critical section
if (this->wait_queue.empty()) {
this->lock.release();
return;
}
unsigned int tid = this->wait_queue.front(); unsigned int tid = this->wait_queue.front();
this->wait_queue.erase(wait_queue.begin()); this->wait_queue.erase(wait_queue.begin());
cpu.disable_int(); // Make sure the deblock() comes through after releasing the lock
this->lock.release(); this->lock.release();
scheduler.deblock(tid); scheduler.deblock(tid); // Enables int
} else { } else {
// No more threads want to work so free semaphore // No more threads want to work so free semaphore
this->counter = this->counter + 1; this->counter = this->counter + 1;