[master] f42141f2d drop the probe lock while scheduling a probe

[Nils Goroll]

commit f42141f2d99a3a6e1c34641a3778058afdf61e56
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Tue Mar 10 10:51:20 2020 +0100

    drop the probe lock while scheduling a probe
    
    The great witness test introduced by @bsdphk and reworked relatively
    recently by @Dridi reported a hypothetical lock cycle here.
    
    This was not a real issue, because the scheduling code would not re-enter
    the probe code, yet we might still drop the probe lock while
    scheduling.

diff --git a/bin/varnishd/cache/cache_backend_probe.c b/bin/varnishd/cache/cache_backend_probe.c
index 8d0bc41f7..52b1871ba 100644
--- a/bin/varnishd/cache/cache_backend_probe.c
+++ b/bin/varnishd/cache/cache_backend_probe.c
@@ -465,6 +465,7 @@ vbp_thread(struct worker *wrk, void *priv)
 {
 	vtim_real now, nxt;
 	struct vbp_target *vt;
+	int r;
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
 	AZ(priv);
@@ -486,7 +487,10 @@ vbp_thread(struct worker *wrk, void *priv)
 				vt->running = 1;
 				vt->task.func = vbp_task;
 				vt->task.priv = vt;
-				if (Pool_Task_Any(&vt->task, TASK_QUEUE_REQ))
+				Lck_Unlock(&vbp_mtx);
+				r = Pool_Task_Any(&vt->task, TASK_QUEUE_REQ);
+				Lck_Lock(&vbp_mtx);
+				if (r)
 					vt->running = 0;
 			}
 			binheap_insert(vbp_heap, vt);