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From: Rick Macklem <rmacklem@FreeBSD.org>
Date: Thu, 3 Nov 2011 14:38:03 +0000 (UTC)
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Subject: svn commit: r227059 - head/sys/rpc
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Author: rmacklem
Date: Thu Nov  3 14:38:03 2011
New Revision: 227059
URL: http://svn.freebsd.org/changeset/base/227059

Log:
  Both a crash reported on freebsd-current on Oct. 18 under the
  subject heading "mtx_lock() of destroyed mutex on NFS" and
  PR# 156168 appear to be caused by clnt_dg_destroy() closing
  down the socket prematurely. When to close down the socket
  is controlled by a reference count (cs_refs), but clnt_dg_create()
  checks for sb_upcall being non-NULL to decide if a new socket
  is needed. I believe the crashes were caused by the following race:
    clnt_dg_destroy() finds cs_refs == 0 and decides to delete socket
    clnt_dg_destroy() then loses race with clnt_dg_create() for
      acquisition of the SOCKBUF_LOCK()
    clnt_dg_create() finds sb_upcall != NULL and increments cs_refs to 1
    clnt_dg_destroy() then acquires SOCKBUF_LOCK(), sets sb_upcall to
      NULL and destroys socket
  
  This patch fixes the above race by changing clnt_dg_destroy() so
  that it acquires SOCKBUF_LOCK() before testing cs_refs.
  
  Tested by:	bz
  PR:		156168
  Reviewed by:	dfr
  MFC after:	2 weeks

Modified:
  head/sys/rpc/clnt_dg.c

Modified: head/sys/rpc/clnt_dg.c
==============================================================================
--- head/sys/rpc/clnt_dg.c	Thu Nov  3 14:36:56 2011	(r227058)
+++ head/sys/rpc/clnt_dg.c	Thu Nov  3 14:38:03 2011	(r227059)
@@ -1001,12 +1001,12 @@ clnt_dg_destroy(CLIENT *cl)
 	cs = cu->cu_socket->so_rcv.sb_upcallarg;
 	clnt_dg_close(cl);
 
+	SOCKBUF_LOCK(&cu->cu_socket->so_rcv);
 	mtx_lock(&cs->cs_lock);
 
 	cs->cs_refs--;
 	if (cs->cs_refs == 0) {
 		mtx_unlock(&cs->cs_lock);
-		SOCKBUF_LOCK(&cu->cu_socket->so_rcv);
 		soupcall_clear(cu->cu_socket, SO_RCV);
 		clnt_dg_upcallsdone(cu->cu_socket, cs);
 		SOCKBUF_UNLOCK(&cu->cu_socket->so_rcv);
@@ -1015,6 +1015,7 @@ clnt_dg_destroy(CLIENT *cl)
 		lastsocketref = TRUE;
 	} else {
 		mtx_unlock(&cs->cs_lock);
+		SOCKBUF_UNLOCK(&cu->cu_socket->so_rcv);
 		lastsocketref = FALSE;
 	}