diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 49bde428f7f25680be18772456c9b63494e5fd5b..37cc8f127cf672faafb28ecaf0f391c257817a40 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2590,6 +2590,16 @@ bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
 	 *   assignment causes no harm).
 	 */
 	new_bfqq->bic = NULL;
+	/*
+	 * If the queue is shared, the pid is the pid of one of the associated
+	 * processes. Which pid depends on the exact sequence of merge events
+	 * the queue underwent. So printing such a pid is useless and confusing
+	 * because it reports a random pid between those of the associated
+	 * processes.
+	 * We mark such a queue with a pid -1, and then print SHARED instead of
+	 * a pid in logging messages.
+	 */
+	new_bfqq->pid = -1;
 	bfqq->bic = NULL;
 	/* release process reference to bfqq */
 	bfq_put_queue(bfqq);
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 829730b96fb2cd5bb6c5a3ae5c74027465d1622b..67e63c276c7ac5c82ad73bd8a63d6b23e988416e 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -32,6 +32,8 @@
 #define BFQ_DEFAULT_GRP_IOPRIO	0
 #define BFQ_DEFAULT_GRP_CLASS	IOPRIO_CLASS_BE
 
+#define MAX_PID_STR_LENGTH 12
+
 /*
  * Soft real-time applications are extremely more latency sensitive
  * than interactive ones. Over-raise the weight of the former to
@@ -1016,13 +1018,23 @@ void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq);
 /* --------------- end of interface of B-WF2Q+ ---------------- */
 
 /* Logging facilities. */
+static inline void bfq_pid_to_str(int pid, char *str, int len)
+{
+	if (pid != -1)
+		snprintf(str, len, "%d", pid);
+	else
+		snprintf(str, len, "SHARED-");
+}
+
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
 struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
 
 #define bfq_log_bfqq(bfqd, bfqq, fmt, args...)	do {			\
+	char pid_str[MAX_PID_STR_LENGTH];	\
+	bfq_pid_to_str((bfqq)->pid, pid_str, MAX_PID_STR_LENGTH);	\
 	blk_add_cgroup_trace_msg((bfqd)->queue,				\
 			bfqg_to_blkg(bfqq_group(bfqq))->blkcg,		\
-			"bfq%d%c " fmt, (bfqq)->pid,			\
+			"bfq%s%c " fmt, pid_str,			\
 			bfq_bfqq_sync((bfqq)) ? 'S' : 'A', ##args);	\
 } while (0)
 
@@ -1033,10 +1045,13 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
 
 #else /* CONFIG_BFQ_GROUP_IOSCHED */
 
-#define bfq_log_bfqq(bfqd, bfqq, fmt, args...)	\
-	blk_add_trace_msg((bfqd)->queue, "bfq%d%c " fmt, (bfqq)->pid,	\
+#define bfq_log_bfqq(bfqd, bfqq, fmt, args...) do {	\
+	char pid_str[MAX_PID_STR_LENGTH];	\
+	bfq_pid_to_str((bfqq)->pid, pid_str, MAX_PID_STR_LENGTH);	\
+	blk_add_trace_msg((bfqd)->queue, "bfq%s%c " fmt, pid_str,	\
 			bfq_bfqq_sync((bfqq)) ? 'S' : 'A',		\
-				##args)
+				##args);	\
+} while (0)
 #define bfq_log_bfqg(bfqd, bfqg, fmt, args...)		do {} while (0)
 
 #endif /* CONFIG_BFQ_GROUP_IOSCHED */