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Both a crash reported on freebsd-current on Oct. 18 under the

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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
This commit is contained in:
Rick Macklem 2011-11-03 14:38:03 +00:00
parent 2b10b1f872
commit 2ba476324b
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=227059
1 changed files with 2 additions and 1 deletions
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3
sys/rpc/clnt_dg.c
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@ -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;
}