From: David van Moolenbroek <david@minix3.org>
Date: Fri, 4 Sep 2015 05:04:17 +0000 (+0200)
Subject: Resolve boot-time VM/RS deadlock
X-Git-Url: http://zhaoyanbai.com/repos/%22http:/www.isc.org/icons/zpipe.c?a=commitdiff_plain;h=37489f8a24b995dfc498de35eba6b007cf912182;p=minix.git

Resolve boot-time VM/RS deadlock

VM used to call sendrec to send a boot-time RS_INIT reply to RS, but
RS could run into a pagefault at the same time, thus spawning a
message to VM, resulting in a deadlock.  We resolve this situation by
making VM acknowledge RS_INIT asynchronously at boot time, while
retaining the synchronous sendrec for subsequent RS_INIT responses.

Change-Id: I3cb72d7f8d6b9bfdc59a85958ada739c37fa3bde
---

diff --git a/minix/include/minix/sef.h b/minix/include/minix/sef.h
index dc5a1beed..4613610ce 100644
--- a/minix/include/minix/sef.h
+++ b/minix/include/minix/sef.h
@@ -73,6 +73,7 @@ int sef_cb_init_identity_state_transfer(int type, sef_init_info_t *info);
 int sef_cb_init_lu_identity_as_restart(int type, sef_init_info_t *info);
 int sef_cb_init_lu_generic(int type, sef_init_info_t *info);
 int sef_cb_init_response_rs_reply(message *m_ptr);
+int sef_cb_init_response_rs_asyn_once(message *m_ptr);
 
 /* Macros for predefined callback implementations. */
 #define SEF_CB_INIT_FRESH_NULL          sef_cb_init_null
diff --git a/minix/lib/libsys/sef_init.c b/minix/lib/libsys/sef_init.c
index 876e35f45..7b229717b 100644
--- a/minix/lib/libsys/sef_init.c
+++ b/minix/lib/libsys/sef_init.c
@@ -434,3 +434,19 @@ int sef_cb_init_response_rs_reply(message *m_ptr)
   return r;
 }
 
+/*===========================================================================*
+ *                       sef_cb_init_response_rs_asyn_once		     *
+ *===========================================================================*/
+int sef_cb_init_response_rs_asyn_once(message *m_ptr)
+{
+/* This response function is used by VM to avoid a boot-time deadlock. */
+  int r;
+
+  /* Inform RS that we completed initialization, asynchronously. */
+  r = asynsend3(RS_PROC_NR, m_ptr, AMF_NOREPLY);
+
+  /* Use a blocking reply call next time. */
+  sef_setcb_init_response(SEF_CB_INIT_RESPONSE_DEFAULT);
+
+  return r;
+}
diff --git a/minix/servers/rs/main.c b/minix/servers/rs/main.c
index ce3f320c2..6fd432bf7 100644
--- a/minix/servers/rs/main.c
+++ b/minix/servers/rs/main.c
@@ -807,9 +807,13 @@ endpoint_t endpoint;
       panic("unable to complete init for service: %d", m.m_source);
   }
 
-  /* Send a reply to unblock the service. */
-  m.m_type = OK;
-  reply(m.m_source, rp, &m);
+  /* Send a reply to unblock the service, except to VM, which sent the reply
+   * asynchronously.  Synchronous replies could lead to deadlocks there.
+   */
+  if (m.m_source != VM_PROC_NR) {
+      m.m_type = OK;
+      reply(m.m_source, rp, &m);
+  }
 
   /* Mark the slot as no longer initializing. */
   rp->r_flags &= ~RS_INITIALIZING;
diff --git a/minix/servers/vm/main.c b/minix/servers/vm/main.c
index 29ead3174..38800f498 100644
--- a/minix/servers/vm/main.c
+++ b/minix/servers/vm/main.c
@@ -200,6 +200,11 @@ static void sef_local_startup(void)
 	sef_setcb_init_fresh(sef_cb_init_fresh);
 	sef_setcb_init_lu(sef_cb_init_lu_restart);
 	sef_setcb_init_restart(sef_cb_init_lu_restart);
+	/* In order to avoid a deadlock at boot time, send the first RS_INIT
+	 * reply to RS asynchronously. After that, use sendrec as usual.
+	 */
+	if (__vm_init_fresh)
+		sef_setcb_init_response(sef_cb_init_response_rs_asyn_once);
 
 	/* Register signal callbacks. */
 	sef_setcb_signal_handler(sef_cb_signal_handler);