workqueue: Fix workqueue_cancel() not waiting for job to finish

Previously, workqueue_cancel() would remove the job from the work queue.

However, when returning it gave absolutely no guarantee that the
cancelled job was done processing. This is a problem, as if the job has
been dynamically allocated, we have no indication about when it is safe
to free it.

Address this issue by waiting for the workqueue's thread to be done if
trying to cancel the job that's currently being processed.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Author: pcercuei

kernel/thread/workqueue.c

@@ -17,6 +17,7 @@
 
 typedef struct workqueue {
     STAILQ_HEAD(workqueue_jobs, workqueue_job) jobs;
+    workqueue_job_t *curr_job;
     kthread_t *thd;
     mutex_t lock;
     condvar_t cond;
@@ -32,6 +33,10 @@ static void *workqueue_thread(void *d) {
     while(!wq->quit) {
         mutex_lock(&wq->lock);
 
+        /* Signal that we're done with the previous job */
+        wq->curr_job = NULL;
+        cond_signal(&wq->cond);
+
         job = STAILQ_FIRST(&wq->jobs);
         if(job)
             now = timer_ms_gettime64();
@@ -50,6 +55,7 @@ static void *workqueue_thread(void *d) {
         /* Remove the job from the queue */
         STAILQ_REMOVE(&wq->jobs, job, workqueue_job, entry);
 
+        wq->curr_job = job;
         mutex_unlock(&wq->lock);
 
         job->cb(d, job);
@@ -112,6 +118,14 @@ void workqueue_cancel(workqueue_t *wq, workqueue_job_t *job) {
     mutex_lock_scoped(&wq->lock);
 
     STAILQ_REMOVE(&wq->jobs, job, workqueue_job, entry);
+
+    if(wq->curr_job == job) {
+        /* The job's callback is being executed. Wait until it's done. */
+        cond_wait(&wq->cond, &wq->lock);
+    }
+
+    /* We modified the queue, so notify the thread that it should parse the
+     * list once again. */
     cond_signal(&wq->cond);
 }