[refer #PGPRO-6599]: Avoid race conditions while processing PROFILE_REQUEST and

rzharkov · rzharkov · commit e17ab38f7f45 · 2022-07-04T18:19:02.000+07:00
PROFILE_RESET requests.

After initialization of "request" variable in collector.c main loop another
client backend could change the value of "collector_hdr-&gt;request" variable.
Changing this value from "PROFILE_RESET" to "PROFILE_REQUEST" could cause
deadlock: "collector" processed "PROFILE_RESET" query while client backend
waits data from the "collector_mq" shared memory message queue. Now we read
and write "collector_hdr-&gt;request" variable only using "PGWS_COLLECTOR_LOCK"
lock.

Removed obsolete "read_current_wait()" function definition from
pg_wait_sampling.h.

tags: pg_wait_sampling
diff --git a/collector.c b/collector.c
@@ -446,11 +446,12 @@ collector_main(Datum main_arg)
 		if (collector_hdr->request != NO_REQUEST)
 		{
 			LOCKTAG		tag;
-			SHMRequest	request = collector_hdr->request;
+			SHMRequest	request;
 
 			init_lock_tag(&tag, PGWS_COLLECTOR_LOCK);
 
 			LockAcquire(&tag, ExclusiveLock, false, false);
+			request = collector_hdr->request;
 			collector_hdr->request = NO_REQUEST;
 
 			if (request == HISTORY_REQUEST || request == PROFILE_REQUEST)
diff --git a/pg_wait_sampling.c b/pg_wait_sampling.c
@@ -590,13 +590,12 @@ receive_array(SHMRequest request, Size item_size, Size *count)
 	init_lock_tag(&queueTag, PGWS_QUEUE_LOCK);
 	LockAcquire(&queueTag, ExclusiveLock, false, false);
 
-	/* Ensure collector has processed previous request */
+	recv_mq = shm_mq_create(collector_mq, COLLECTOR_QUEUE_SIZE);
+
 	init_lock_tag(&collectorTag, PGWS_COLLECTOR_LOCK);
 	LockAcquire(&collectorTag, ExclusiveLock, false, false);
-	LockRelease(&collectorTag, ExclusiveLock, false);
-
-	recv_mq = shm_mq_create(collector_mq, COLLECTOR_QUEUE_SIZE);
 	collector_hdr->request = request;
+	LockRelease(&collectorTag, ExclusiveLock, false);
 
 	if (!collector_hdr->latch)
 		ereport(ERROR, (errcode(ERRCODE_INTERNAL_ERROR),
@@ -756,19 +755,19 @@ Datum
 pg_wait_sampling_reset_profile(PG_FUNCTION_ARGS)
 {
 	LOCKTAG		tag;
-	LOCKTAG		tagCollector;
+	LOCKTAG		collectorTag;
 
 	check_shmem();
 
 	init_lock_tag(&tag, PGWS_QUEUE_LOCK);
 
 	LockAcquire(&tag, ExclusiveLock, false, false);
 
-	init_lock_tag(&tagCollector, PGWS_COLLECTOR_LOCK);
-	LockAcquire(&tagCollector, ExclusiveLock, false, false);
-	LockRelease(&tagCollector, ExclusiveLock, false);
-
+	init_lock_tag(&collectorTag, PGWS_COLLECTOR_LOCK);
+	LockAcquire(&collectorTag, ExclusiveLock, false, false);
 	collector_hdr->request = PROFILE_RESET;
+	LockRelease(&collectorTag, ExclusiveLock, false);
+
 	SetLatch(collector_hdr->latch);
 
 	LockRelease(&tag, ExclusiveLock, false);
diff --git a/pg_wait_sampling.h b/pg_wait_sampling.h
@@ -75,7 +75,6 @@ extern void check_shmem(void);
 extern CollectorShmqHeader *collector_hdr;
 extern shm_mq			   *collector_mq;
 extern uint64			   *proc_queryids;
-extern void read_current_wait(PGPROC *proc, HistoryItem *item);
 extern void init_lock_tag(LOCKTAG *tag, uint32 lock);
 
 /* collector.c */
