diff mbox series

Thread safe client destruction

Message ID 53af8ec6-4ece-09b2-9499-d46d0fdfaa15@cfa.harvard.edu (mailing list archive)
State New, archived
Headers show
Series Thread safe client destruction | expand

Commit Message

Attila Kovacs July 21, 2022, 6:41 p.m. UTC 
Hi again,


I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.

1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to TRUE, 
with no corresponding clearing when the operation (*xdr_res() call) is 
completed. This would leave other waiting operations blocked 
indefinitely, effectively deadlocked. For comparison, clnt_vd_freeres() 
in clnt_vc.c does not set the active state to TRUE. I believe the vc 
behavior is correct, while the dg behavior is a bug.

2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other 
blocked operations pending (such as clnt_*_call(), clnt_*_control(), or 
clnt_*_freeres()) but no active operation currently being executed, then 
the client gets destroyed. Then, as the other blocked operations get 
subsequently awoken, they will try operate on an invalid client handle, 
potentially causing unpredictable behavior and stack corruption.

The proposed fix is to introduce a simple mutexed counting variable into 
the client lock structure, which keeps track of the number of pending 
blocking operations on the client. This way, clnt_*_destroy() can check 
if there are any operations pending on a client, and keep waiting until 
all pending operations are completed, before the client is destroyed 
safely and its resources are freed.

Attached is a patch with the above fixes.

-- A.

Comments

Chuck Lever July 21, 2022, 6:45 p.m. UTC | #1 
> On Jul 21, 2022, at 2:41 PM, Attila Kovacs <attila.kovacs@cfa.harvard.edu> wrote:
> 
> Hi again,
> 
> 
> I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.

IIRC libtirpc is not MT-enabled because it was forked from the
Solaris libtirpc before MT-enablement was completed. I could
be remembering incorrectly.

Therefore I think we would need some unit tests along with the
improvements you propose. There's a test suite somewhere, but
someone else will need to provide details.

(and also, please inline your patch submissions rather than
attaching them, to make it easier for us to review. thanks!)


> 1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to TRUE, with no corresponding clearing when the operation (*xdr_res() call) is completed. This would leave other waiting operations blocked indefinitely, effectively deadlocked. For comparison, clnt_vd_freeres() in clnt_vc.c does not set the active state to TRUE. I believe the vc behavior is correct, while the dg behavior is a bug.
> 
> 2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other blocked operations pending (such as clnt_*_call(), clnt_*_control(), or clnt_*_freeres()) but no active operation currently being executed, then the client gets destroyed. Then, as the other blocked operations get subsequently awoken, they will try operate on an invalid client handle, potentially causing unpredictable behavior and stack corruption.
> 
> The proposed fix is to introduce a simple mutexed counting variable into the client lock structure, which keeps track of the number of pending blocking operations on the client. This way, clnt_*_destroy() can check if there are any operations pending on a client, and keep waiting until all pending operations are completed, before the client is destroyed safely and its resources are freed.
> 
> Attached is a patch with the above fixes.
> 
> -- A.<clnt_mt_safe_destroy.patch>

--
Chuck Lever
Attila Kovacs July 21, 2022, 7:19 p.m. UTC | #2 
Hi Chuck,


I'm not adding any new functionality here. I'm merely trying to fix 
what's already there but broken (in my opinion). :-)

-- A.


On 7/21/22 14:45, Chuck Lever III wrote:
> 
> 
>> On Jul 21, 2022, at 2:41 PM, Attila Kovacs <attila.kovacs@cfa.harvard.edu> wrote:
>>
>> Hi again,
>>
>>
>> I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.
> 
> IIRC libtirpc is not MT-enabled because it was forked from the
> Solaris libtirpc before MT-enablement was completed. I could
> be remembering incorrectly.
> 
> Therefore I think we would need some unit tests along with the
> improvements you propose. There's a test suite somewhere, but
> someone else will need to provide details.
> 
> (and also, please inline your patch submissions rather than
> attaching them, to make it easier for us to review. thanks!)
> 
> 
>> 1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to TRUE, with no corresponding clearing when the operation (*xdr_res() call) is completed. This would leave other waiting operations blocked indefinitely, effectively deadlocked. For comparison, clnt_vd_freeres() in clnt_vc.c does not set the active state to TRUE. I believe the vc behavior is correct, while the dg behavior is a bug.
>>
>> 2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other blocked operations pending (such as clnt_*_call(), clnt_*_control(), or clnt_*_freeres()) but no active operation currently being executed, then the client gets destroyed. Then, as the other blocked operations get subsequently awoken, they will try operate on an invalid client handle, potentially causing unpredictable behavior and stack corruption.
>>
>> The proposed fix is to introduce a simple mutexed counting variable into the client lock structure, which keeps track of the number of pending blocking operations on the client. This way, clnt_*_destroy() can check if there are any operations pending on a client, and keep waiting until all pending operations are completed, before the client is destroyed safely and its resources are freed.
>>
>> Attached is a patch with the above fixes.
>>
>> -- A.<clnt_mt_safe_destroy.patch>
> 
> --
> Chuck Lever
> 
> 
>
Steve Dickson July 21, 2022, 8:37 p.m. UTC | #3 
Hey!

On 7/21/22 3:19 PM, Attila Kovacs wrote:
> Hi Chuck,
> 
> 
> I'm not adding any new functionality here. I'm merely trying to fix 
> what's already there but broken (in my opinion). :-)
I agree with this.... And I will be more that willing
to reformat the patches make it easier to review

I don't want nits like that to get in the way this good work,
but it would be good if you could added the
     Signed-off-by: Attila Kovacs <attila.kovacs@cfa.harvard.edu>

line, which is described in [1]

steved.

[1] https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html
> 
> -- A.
> 
> 
> On 7/21/22 14:45, Chuck Lever III wrote:
>>
>>
>>> On Jul 21, 2022, at 2:41 PM, Attila Kovacs 
>>> <attila.kovacs@cfa.harvard.edu> wrote:
>>>
>>> Hi again,
>>>
>>>
>>> I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.
>>
>> IIRC libtirpc is not MT-enabled because it was forked from the
>> Solaris libtirpc before MT-enablement was completed. I could
>> be remembering incorrectly.
>>
>> Therefore I think we would need some unit tests along with the
>> improvements you propose. There's a test suite somewhere, but
>> someone else will need to provide details.
>>
>> (and also, please inline your patch submissions rather than
>> attaching them, to make it easier for us to review. thanks!)
>>
>>
>>> 1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to 
>>> TRUE, with no corresponding clearing when the operation (*xdr_res() 
>>> call) is completed. This would leave other waiting operations blocked 
>>> indefinitely, effectively deadlocked. For comparison, 
>>> clnt_vd_freeres() in clnt_vc.c does not set the active state to TRUE. 
>>> I believe the vc behavior is correct, while the dg behavior is a bug.
>>>
>>> 2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other 
>>> blocked operations pending (such as clnt_*_call(), clnt_*_control(), 
>>> or clnt_*_freeres()) but no active operation currently being 
>>> executed, then the client gets destroyed. Then, as the other blocked 
>>> operations get subsequently awoken, they will try operate on an 
>>> invalid client handle, potentially causing unpredictable behavior and 
>>> stack corruption.
>>>
>>> The proposed fix is to introduce a simple mutexed counting variable 
>>> into the client lock structure, which keeps track of the number of 
>>> pending blocking operations on the client. This way, clnt_*_destroy() 
>>> can check if there are any operations pending on a client, and keep 
>>> waiting until all pending operations are completed, before the client 
>>> is destroyed safely and its resources are freed.
>>>
>>> Attached is a patch with the above fixes.
>>>
>>> -- A.<clnt_mt_safe_destroy.patch>
>>
>> -- 
>> Chuck Lever
>>
>>
>>
>
Steve Dickson July 21, 2022, 9:38 p.m. UTC | #4 
Hello,

On 7/21/22 2:41 PM, Attila Kovacs wrote:
> Hi again,
> 
> 
> I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.
> 
> 1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to TRUE, 
> with no corresponding clearing when the operation (*xdr_res() call) is 
> completed. This would leave other waiting operations blocked 
> indefinitely, effectively deadlocked. For comparison, clnt_vd_freeres() 
> in clnt_vc.c does not set the active state to TRUE. I believe the vc 
> behavior is correct, while the dg behavior is a bug.
> 
> 2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other 
> blocked operations pending (such as clnt_*_call(), clnt_*_control(), or 
> clnt_*_freeres()) but no active operation currently being executed, then 
> the client gets destroyed. Then, as the other blocked operations get 
> subsequently awoken, they will try operate on an invalid client handle, 
> potentially causing unpredictable behavior and stack corruption.
> 
> The proposed fix is to introduce a simple mutexed counting variable into 
> the client lock structure, which keeps track of the number of pending 
> blocking operations on the client. This way, clnt_*_destroy() can check 
> if there are any operations pending on a client, and keep waiting until 
> all pending operations are completed, before the client is destroyed 
> safely and its resources are freed.
> 
> Attached is a patch with the above fixes.
There is a problem here... This patch does contain the first patch
you posted. So when I apply this patch it fails.

So Please apply your first patch, then apply the fixes from
your second patch... test... then when all is good...
Resent the second patch which will apply, cleanly,
with the first patch.

steved.

> 
> -- A.
Attila Kovacs July 21, 2022, 10 p.m. UTC | #5 
Hi Steve,

Yes, the two patches affect the same code area, so if they are both 
relative to master (which is how I submitted them originally) then they 
conflict.

Here is the second patch as a clean incremental on the first.

-- A.

-----------------------------------------------------------------------

diff --git a/src/clnt_dg.c b/src/clnt_dg.c
index 7c5d22e..24c825b 100644
--- a/src/clnt_dg.c
+++ b/src/clnt_dg.c
@@ -101,6 +101,7 @@ extern mutex_t clnt_fd_lock;
  #define	release_fd_lock(fd_lock, mask) {	\
  	mutex_lock(&clnt_fd_lock);	\
  	fd_lock->active = FALSE;	\
+	fd_lock->pending--;		\
  	thr_sigsetmask(SIG_SETMASK, &(mask), NULL); \
  	cond_signal(&fd_lock->cv);	\
  	mutex_unlock(&clnt_fd_lock);    \
@@ -311,6 +312,7 @@ clnt_dg_call(cl, proc, xargs, argsp, xresults, 
resultsp, utimeout)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
  	while (cu->cu_fd_lock->active)
  		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
  	cu->cu_fd_lock->active = TRUE;
@@ -571,11 +573,12 @@ clnt_dg_freeres(cl, xdr_res, res_ptr)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
  	while (cu->cu_fd_lock->active)
  		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
-	cu->cu_fd_lock->active = TRUE;
  	xdrs->x_op = XDR_FREE;
  	dummy = (*xdr_res)(xdrs, res_ptr);
+	cu->cu_fd_lock->pending--;
  	thr_sigsetmask(SIG_SETMASK, &mask, NULL);
  	cond_signal(&cu->cu_fd_lock->cv);
  	mutex_unlock(&clnt_fd_lock);
@@ -603,6 +606,7 @@ clnt_dg_control(cl, request, info)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
  	while (cu->cu_fd_lock->active)
  		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
  	cu->cu_fd_lock->active = TRUE;
@@ -743,7 +747,7 @@ clnt_dg_destroy(cl)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
-	while (cu_fd_lock->active)
+	while (cu_fd_lock->active || cu_fd_lock->pending > 0)
  		cond_wait(&cu_fd_lock->cv, &clnt_fd_lock);
  	if (cu->cu_closeit)
  		(void)close(cu_fd);
diff --git a/src/clnt_fd_locks.h b/src/clnt_fd_locks.h
index 359f995..6ba62cb 100644
--- a/src/clnt_fd_locks.h
+++ b/src/clnt_fd_locks.h
@@ -50,6 +50,7 @@ static unsigned int fd_locks_prealloc = 0;
  /* per-fd lock */
  struct fd_lock_t {
  	bool_t active;
+	int pending;        /* Number of pending operations on fd */
  	cond_t cv;
  };
  typedef struct fd_lock_t fd_lock_t;
@@ -180,6 +181,7 @@ fd_lock_t* fd_lock_create(int fd, fd_locks_t 
*fd_locks) {
  		item->fd = fd;
  		item->refs = 1;
  		item->fd_lock.active = FALSE;
+		item->fd_lock.pending = 0;
  		cond_init(&item->fd_lock.cv, 0, (void *) 0);
  		TAILQ_INSERT_HEAD(list, item, link);
  	} else {
diff --git a/src/clnt_vc.c b/src/clnt_vc.c
index 3c73e65..7610b4a 100644
--- a/src/clnt_vc.c
+++ b/src/clnt_vc.c
@@ -153,6 +153,7 @@ extern mutex_t  clnt_fd_lock;
  #define release_fd_lock(fd_lock, mask) {	\
  	mutex_lock(&clnt_fd_lock);	\
  	fd_lock->active = FALSE;	\
+	fd_lock->pending--;		\
  	thr_sigsetmask(SIG_SETMASK, &(mask), (sigset_t *) NULL);	\
  	cond_signal(&fd_lock->cv);	\
  	mutex_unlock(&clnt_fd_lock);    \
@@ -357,6 +358,7 @@ clnt_vc_call(cl, proc, xdr_args, args_ptr, 
xdr_results, results_ptr, timeout)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
  	while (ct->ct_fd_lock->active)
  		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
  	ct->ct_fd_lock->active = TRUE;
@@ -495,10 +497,12 @@ clnt_vc_freeres(cl, xdr_res, res_ptr)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
  	while (ct->ct_fd_lock->active)
  		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
  	xdrs->x_op = XDR_FREE;
  	dummy = (*xdr_res)(xdrs, res_ptr);
+	ct->ct_fd_lock->pending--;
  	thr_sigsetmask(SIG_SETMASK, &(mask), NULL);
  	cond_signal(&ct->ct_fd_lock->cv);
  	mutex_unlock(&clnt_fd_lock);
@@ -533,6 +537,7 @@ clnt_vc_control(cl, request, info)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
  	while (ct->ct_fd_lock->active)
  		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
  	ct->ct_fd_lock->active = TRUE;
@@ -655,7 +660,7 @@ clnt_vc_destroy(cl)
  	sigfillset(&newmask);
  	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
  	mutex_lock(&clnt_fd_lock);
-	while (ct_fd_lock->active)
+	while (ct_fd_lock->active || ct_fd_lock->pending > 0)
  		cond_wait(&ct_fd_lock->cv, &clnt_fd_lock);
  	if (ct->ct_closeit && ct->ct_fd != -1) {
  		(void)close(ct->ct_fd);

-------------------------------------------------------------



On 7/21/22 17:38, Steve Dickson wrote:
> Hello,
> 
> On 7/21/22 2:41 PM, Attila Kovacs wrote:
>> Hi again,
>>
>>
>> I found yet more potential MT flaws in clnt_dg.c and clnt_vg.c.
>>
>> 1. In clnt_dg.c in clnt_dg_freeres(), cu_fd_lock->active is set to 
>> TRUE, with no corresponding clearing when the operation (*xdr_res() 
>> call) is completed. This would leave other waiting operations blocked 
>> indefinitely, effectively deadlocked. For comparison, 
>> clnt_vd_freeres() in clnt_vc.c does not set the active state to TRUE. 
>> I believe the vc behavior is correct, while the dg behavior is a bug.
>>
>> 2. If clnt_dg_destroy() or clnt_vc_destroy() is awoken with other 
>> blocked operations pending (such as clnt_*_call(), clnt_*_control(), 
>> or clnt_*_freeres()) but no active operation currently being executed, 
>> then the client gets destroyed. Then, as the other blocked operations 
>> get subsequently awoken, they will try operate on an invalid client 
>> handle, potentially causing unpredictable behavior and stack corruption.
>>
>> The proposed fix is to introduce a simple mutexed counting variable 
>> into the client lock structure, which keeps track of the number of 
>> pending blocking operations on the client. This way, clnt_*_destroy() 
>> can check if there are any operations pending on a client, and keep 
>> waiting until all pending operations are completed, before the client 
>> is destroyed safely and its resources are freed.
>>
>> Attached is a patch with the above fixes.
> There is a problem here... This patch does contain the first patch
> you posted. So when I apply this patch it fails.
> 
> So Please apply your first patch, then apply the fixes from
> your second patch... test... then when all is good...
> Resent the second patch which will apply, cleanly,
> with the first patch.
> 
> steved.
> 
>>
>> -- A.
>
diff mbox series

Patch

diff --git a/src/clnt_dg.c b/src/clnt_dg.c
index b3d82e7..9f75e7c 100644
--- a/src/clnt_dg.c
+++ b/src/clnt_dg.c
@@ -101,6 +101,7 @@  extern mutex_t clnt_fd_lock;
 #define	release_fd_lock(fd_lock, mask) {	\
 	mutex_lock(&clnt_fd_lock);	\
 	fd_lock->active = FALSE;	\
+	fd_lock->pending--;		\
 	mutex_unlock(&clnt_fd_lock);	\
 	thr_sigsetmask(SIG_SETMASK, &(mask), NULL); \
 	cond_signal(&fd_lock->cv);	\
@@ -308,6 +309,7 @@  clnt_dg_call(cl, proc, xargs, argsp, xresults, resultsp, utimeout)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
 	while (cu->cu_fd_lock->active)
 		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
 	cu->cu_fd_lock->active = TRUE;
@@ -568,11 +570,12 @@  clnt_dg_freeres(cl, xdr_res, res_ptr)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
 	while (cu->cu_fd_lock->active)
 		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
-	cu->cu_fd_lock->active = TRUE;
 	xdrs->x_op = XDR_FREE;
 	dummy = (*xdr_res)(xdrs, res_ptr);
+	cu->cu_fd_lock->pending--;
 	mutex_unlock(&clnt_fd_lock);
 	thr_sigsetmask(SIG_SETMASK, &mask, NULL);
 	cond_signal(&cu->cu_fd_lock->cv);
@@ -600,6 +603,7 @@  clnt_dg_control(cl, request, info)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	cu->cu_fd_lock->pending++;
 	while (cu->cu_fd_lock->active)
 		cond_wait(&cu->cu_fd_lock->cv, &clnt_fd_lock);
 	cu->cu_fd_lock->active = TRUE;
@@ -740,7 +744,7 @@  clnt_dg_destroy(cl)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
-	while (cu_fd_lock->active)
+	while (cu_fd_lock->active || cu_fd_lock->pending > 0)
 		cond_wait(&cu_fd_lock->cv, &clnt_fd_lock);
 	if (cu->cu_closeit)
 		(void)close(cu_fd);
diff --git a/src/clnt_fd_locks.h b/src/clnt_fd_locks.h
index 359f995..6ba62cb 100644
--- a/src/clnt_fd_locks.h
+++ b/src/clnt_fd_locks.h
@@ -50,6 +50,7 @@  static unsigned int fd_locks_prealloc = 0;
 /* per-fd lock */
 struct fd_lock_t {
 	bool_t active;
+	int pending;        /* Number of pending operations on fd */
 	cond_t cv;
 };
 typedef struct fd_lock_t fd_lock_t;
@@ -180,6 +181,7 @@  fd_lock_t* fd_lock_create(int fd, fd_locks_t *fd_locks) {
 		item->fd = fd;
 		item->refs = 1;
 		item->fd_lock.active = FALSE;
+		item->fd_lock.pending = 0;
 		cond_init(&item->fd_lock.cv, 0, (void *) 0);
 		TAILQ_INSERT_HEAD(list, item, link);
 	} else {
diff --git a/src/clnt_vc.c b/src/clnt_vc.c
index a07e297..8f9ca74 100644
--- a/src/clnt_vc.c
+++ b/src/clnt_vc.c
@@ -153,6 +153,7 @@  extern mutex_t  clnt_fd_lock;
 #define release_fd_lock(fd_lock, mask) {	\
 	mutex_lock(&clnt_fd_lock);	\
 	fd_lock->active = FALSE;	\
+	fd_lock->pending--;		\
 	mutex_unlock(&clnt_fd_lock);	\
 	thr_sigsetmask(SIG_SETMASK, &(mask), (sigset_t *) NULL);	\
 	cond_signal(&fd_lock->cv);	\
@@ -353,6 +354,7 @@  clnt_vc_call(cl, proc, xdr_args, args_ptr, xdr_results, results_ptr, timeout)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
 	while (ct->ct_fd_lock->active)
 		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
 	ct->ct_fd_lock->active = TRUE;
@@ -491,10 +493,12 @@  clnt_vc_freeres(cl, xdr_res, res_ptr)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
 	while (ct->ct_fd_lock->active)
 		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
 	xdrs->x_op = XDR_FREE;
 	dummy = (*xdr_res)(xdrs, res_ptr);
+	ct->ct_fd_lock->pending--;
 	mutex_unlock(&clnt_fd_lock);
 	thr_sigsetmask(SIG_SETMASK, &(mask), NULL);
 	cond_signal(&ct->ct_fd_lock->cv);
@@ -529,6 +533,7 @@  clnt_vc_control(cl, request, info)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
+	ct->ct_fd_lock->pending++;
 	while (ct->ct_fd_lock->active)
 		cond_wait(&ct->ct_fd_lock->cv, &clnt_fd_lock);
 	ct->ct_fd_lock->active = TRUE;
@@ -651,7 +656,7 @@  clnt_vc_destroy(cl)
 	sigfillset(&newmask);
 	thr_sigsetmask(SIG_SETMASK, &newmask, &mask);
 	mutex_lock(&clnt_fd_lock);
-	while (ct_fd_lock->active)
+	while (ct_fd_lock->active || ct_fd_lock->pending > 0)
 		cond_wait(&ct_fd_lock->cv, &clnt_fd_lock);
 	if (ct->ct_closeit && ct->ct_fd != -1) {
 		(void)close(ct->ct_fd);