From: Willy Tarreau <w@1wt.eu>
Date: Thu, 26 Jul 2018 05:38:54 +0000 (+0200)
Subject: MINOR: queue: use a distinct variable for the assigned server and the queue
X-Git-Tag: v1.9-dev1~22
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=88930dd364cf6f77687f8f57618d2e5810c4d29f;p=thirdparty%2Fhaproxy.git

MINOR: queue: use a distinct variable for the assigned server and the queue

The pendconn struct uses ->px and ->srv to designate where the element is
queued. There is something confusing regarding threads though, because we
have to lock the appropriate queue before inserting/removing elements, and
this queue may only be determined by looking at ->srv (if it's not NULL
it's the server, otherwise use the proxy). But pendconn_grab_from_px() and
pendconn_process_next_strm() both assign this ->srv field, making it
complicated to know what queue to lock before manipulating the element,
which is exactly why we have the pendconn_lock in the first place.

This commit introduces pendconn->target which is the target server that
the two aforementioned functions will set when assigning the server.
Thanks to this, the server pointer may always be relied on to determine
what queue to use.
---

diff --git a/include/types/queue.h b/include/types/queue.h
index 42dbbd0478..1f72a31c1c 100644
--- a/include/types/queue.h
+++ b/include/types/queue.h
@@ -34,7 +34,8 @@ struct pendconn {
 	int            strm_flags; /* stream flags */
 	struct stream *strm;
 	struct proxy  *px;
-	struct server *srv;        /* the server we are waiting for, may be NULL */
+	struct server *srv;        /* the server we are waiting for, may be NULL if don't care */
+	struct server *target;     /* the server that was assigned, = srv except if srv==NULL */
 	struct list    list;       /* next pendconn */
 	__decl_hathreads(HA_SPINLOCK_T lock);
 };
diff --git a/src/queue.c b/src/queue.c
index 2773826402..49a4011a83 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -25,8 +25,10 @@
  *     otherwise it necessarily belongs to one of the other lists ; this may
  *     not be atomically checked under threads though ;
  *   - pendconn->px is never NULL if pendconn->list is not empty
- *   - pendconn->sv is never NULL if pendconn->list is in the server's queue,
+ *   - pendconn->srv is never NULL if pendconn->list is in the server's queue,
  *     and is always NULL if pendconn->list is in the backend's queue or empty.
+ *   - pendconn->target is NULL while the element is queued, and points to the
+ *     assigned server when the pendconn is picked.
  *
  * Threads complicate the design a little bit but rules remain simple :
  *   - a lock is present in the pendconn (pendconn->lock). This lock is
@@ -65,10 +67,9 @@
  *   - no single operation except the pendconn initialisation prior to the
  *     insertion are performed without a queue lock held.
  *
- *   - pendconn_grab_from_px() and pendconn_process_next_strm() assign ->srv so
- *     that the stream knows what server to work with (via pendconn_dequeue()).
- *     It's worth noting that this assignment complicates the detection of the
- *     queue to be locked/unlocked.
+ *   - pendconn_grab_from_px() and pendconn_process_next_strm() assign ->target
+ *     so that the stream knows what server to work with (via
+ *     pendconn_dequeue() which sets it on strm->target).
  *
  *   - a pendconn doesn't switch between queues, it stays where it is.
  */
@@ -239,7 +240,7 @@ static int pendconn_process_next_strm(struct server *srv, struct proxy *px)
   pendconn_found:
 	__pendconn_unlink(p);
 	p->strm_flags |= SF_ASSIGNED;
-	p->srv = srv;
+	p->target = srv;
 
 	HA_ATOMIC_ADD(&srv->served, 1);
 	HA_ATOMIC_ADD(&srv->proxy->served, 1);
@@ -301,6 +302,7 @@ struct pendconn *pendconn_add(struct stream *strm)
 	srv = objt_server(strm->target);
 	px  = strm->be;
 
+	p->target     = NULL;
 	p->srv        = NULL;
 	p->px         = px;
 	p->strm       = strm;
@@ -393,7 +395,7 @@ int pendconn_grab_from_px(struct server *s)
 			continue;
 
 		__pendconn_unlink(p);
-		p->srv = s;
+		p->target = s;
 
 		remote |= !(p->strm->task->thread_mask & tid_bit);
 		task_wakeup(p->strm->task, TASK_WOKEN_RES);
@@ -439,8 +441,8 @@ int pendconn_dequeue(struct stream *strm)
 	}
 
 	/* the pendconn must be dequeued now */
-	if (p->srv)
-		strm->target = &p->srv->obj_type;
+	if (p->target)
+		strm->target = &p->target->obj_type;
 
 	strm->flags &= ~(SF_DIRECT | SF_ASSIGNED | SF_ADDR_SET);
 	strm->flags |= p->strm_flags & (SF_DIRECT | SF_ASSIGNED | SF_ADDR_SET);