[RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Paul E. McKenney]

Hello!

The grow_ary() code has a number of explicit memory barriers, as does
ipc_lock().  This patch gets rid of the need for some of these by
placing the array size in the same block of memory containing the
array itself, so that the array and the size cannot possibly get out
of sync.  Also uses rcu_assign_pointer() to get rid of the remaining
smp_wmb().

Untested, therefore probably broken.

Thoughts?

						Thanx, Paul

Signed-off-by: paulmck@us.ibm.com

 msg.c  |    3 +-
 sem.c  |    2 -
 util.c |   82 +++++++++++++++++++++++++++++------------------------------------
 util.h |    8 +++---
 4 files changed, 44 insertions(+), 51 deletions(-)


diff -urpN -X ../dontdiff linux-2.5-rcu_assign_pointer/ipc/msg.c linux-2.5-iloi/ipc/msg.c
--- linux-2.5-rcu_assign_pointer/ipc/msg.c	Mon Aug 30 15:38:19 2004
+++ linux-2.5-iloi/ipc/msg.c	Mon Sep  6 18:24:21 2004
@@ -380,7 +380,8 @@ asmlinkage long sys_msgctl (int msqid, i
 		int success_return;
 		if (!buf)
 			return -EFAULT;
-		if(cmd == MSG_STAT && msqid >= msg_ids.size)
+		if(cmd == MSG_STAT &&
+		   (msg_ids.entries == NULL || msqid >= msg_ids.entries->size))
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));
diff -urpN -X ../dontdiff linux-2.5-rcu_assign_pointer/ipc/sem.c linux-2.5-iloi/ipc/sem.c
--- linux-2.5-rcu_assign_pointer/ipc/sem.c	Mon Aug 30 15:38:19 2004
+++ linux-2.5-iloi/ipc/sem.c	Mon Sep  6 18:24:22 2004
@@ -523,7 +523,7 @@ static int semctl_nolock(int semid, int 
 		struct semid64_ds tbuf;
 		int id;
 
-		if(semid >= sem_ids.size)
+		if(sem_ids.entries == NULL || semid >= sem_ids.entries->size)
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));
diff -urpN -X ../dontdiff linux-2.5-rcu_assign_pointer/ipc/util.c linux-2.5-iloi/ipc/util.c
--- linux-2.5-rcu_assign_pointer/ipc/util.c	Mon Aug 30 15:38:19 2004
+++ linux-2.5-iloi/ipc/util.c	Mon Sep  6 18:26:06 2004
@@ -62,7 +62,6 @@ void __init ipc_init_ids(struct ipc_ids*
 
 	if(size > IPCMNI)
 		size = IPCMNI;
-	ids->size = size;
 	ids->in_use = 0;
 	ids->max_id = -1;
 	ids->seq = 0;
@@ -74,14 +73,17 @@ void __init ipc_init_ids(struct ipc_ids*
 		 	ids->seq_max = seq_limit;
 	}
 
-	ids->entries = ipc_rcu_alloc(sizeof(struct ipc_id)*size);
+	ids->entries = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*size +
+				     sizeof(struct ipc_id_ary));
 
-	if(ids->entries == NULL) {
+	if(ids->entries != NULL) {
+		ids->entries->size = size;
+	} else {
 		printk(KERN_ERR "ipc_init_ids() failed, ipc service disabled.\n");
-		ids->size = 0;
+		size = 0;
 	}
-	for(i=0;i<ids->size;i++)
-		ids->entries[i].p = NULL;
+	for(i=0;i<size;i++)
+		ids->entries->p[i] = NULL;
 }
 
 /**
@@ -104,7 +106,7 @@ int ipc_findkey(struct ipc_ids* ids, key
 	 * since ipc_ids.sem is held
 	 */
 	for (id = 0; id <= max_id; id++) {
-		p = ids->entries[id].p;
+		p = ids->entries->p[id];
 		if(p==NULL)
 			continue;
 		if (key == p->key)
@@ -118,36 +120,36 @@ int ipc_findkey(struct ipc_ids* ids, key
  */
 static int grow_ary(struct ipc_ids* ids, int newsize)
 {
-	struct ipc_id* new;
-	struct ipc_id* old;
+	struct ipc_id_ary* new;
+	struct ipc_id_ary* old;
 	int i;
+	int size = (ids->entries == NULL ? 0 : ids->entries->size);
 
 	if(newsize > IPCMNI)
 		newsize = IPCMNI;
-	if(newsize <= ids->size)
+	if(newsize <= size)
 		return newsize;
 
-	new = ipc_rcu_alloc(sizeof(struct ipc_id)*newsize);
+	new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
+			    sizeof(struct ipc_id_ary));
 	if(new == NULL)
-		return ids->size;
-	memcpy(new, ids->entries, sizeof(struct ipc_id)*ids->size);
-	for(i=ids->size;i<newsize;i++) {
-		new[i].p = NULL;
+		return size;
+	new->size = newsize;
+	memcpy(new->p, ids->entries->p, sizeof(struct kern_ipc_perm *)*size +
+					sizeof(struct ipc_id_ary));
+	for(i=size;i<newsize;i++) {
+		new->p[i] = NULL;
 	}
 	old = ids->entries;
 
 	/*
-	 * before setting the ids->entries to the new array, there must be a
-	 * smp_wmb() to make sure the memcpyed contents of the new array are
-	 * visible before the new array becomes visible.
+	 * Use rcu_assign_pointer() to make sure the memcpyed contents
+	 * of the new array are visible before the new array becomes visible.
 	 */
-	smp_wmb();	/* prevent seeing new array uninitialized. */
-	ids->entries = new;
-	smp_wmb();	/* prevent indexing into old array based on new size. */
-	ids->size = newsize;
+	rcu_assign_pointer(ids->entries, new);
 
 	ipc_rcu_putref(old);
-	return ids->size;
+	return newsize;
 }
 
 /**
@@ -175,7 +177,7 @@ int ipc_addid(struct ipc_ids* ids, struc
 	 * ipc_ids.sem is held
 	 */
 	for (id = 0; id < size; id++) {
-		if(ids->entries[id].p == NULL)
+		if(ids->entries->p[id] == NULL)
 			goto found;
 	}
 	return -1;
@@ -195,7 +197,7 @@ found:
 	new->deleted = 0;
 	rcu_read_lock();
 	spin_lock(&new->lock);
-	ids->entries[id].p = new;
+	ids->entries->p[id] = new;
 	return id;
 }
 
@@ -216,15 +218,15 @@ struct kern_ipc_perm* ipc_rmid(struct ip
 {
 	struct kern_ipc_perm* p;
 	int lid = id % SEQ_MULTIPLIER;
-	if(lid >= ids->size)
+	if(lid >= ids->entries->size)
 		BUG();
 
 	/* 
 	 * do not need a rcu_dereference()() here to force ordering
 	 * on Alpha, since the ipc_ids.sem is held.
 	 */	
-	p = ids->entries[lid].p;
-	ids->entries[lid].p = NULL;
+	p = ids->entries->p[lid];
+	ids->entries->p[lid] = NULL;
 	if(p==NULL)
 		BUG();
 	ids->in_use--;
@@ -234,7 +236,7 @@ struct kern_ipc_perm* ipc_rmid(struct ip
 			lid--;
 			if(lid == -1)
 				break;
-		} while (ids->entries[lid].p == NULL);
+		} while (ids->entries->p[lid] == NULL);
 		ids->max_id = lid;
 	}
 	p->deleted = 1;
@@ -493,9 +495,9 @@ struct kern_ipc_perm* ipc_get(struct ipc
 {
 	struct kern_ipc_perm* out;
 	int lid = id % SEQ_MULTIPLIER;
-	if(lid >= ids->size)
+	if((ids->entries == NULL) || (lid >= ids->entries->size))
 		return NULL;
-	out = ids->entries[lid].p;
+	out = ids->entries->p[lid];
 	return out;
 }
 
@@ -503,25 +505,15 @@ struct kern_ipc_perm* ipc_lock(struct ip
 {
 	struct kern_ipc_perm* out;
 	int lid = id % SEQ_MULTIPLIER;
-	struct ipc_id* entries;
+	struct ipc_id_ary* entries;
 
 	rcu_read_lock();
-	if(lid >= ids->size) {
+	entries = rcu_dereference(ids->entries);
+	if((entries == NULL) || (lid >= entries->size)) {
 		rcu_read_unlock();
 		return NULL;
 	}
-
-	/* 
-	 * Note: The following two read barriers are corresponding
-	 * to the two write barriers in grow_ary(). They guarantee 
-	 * the writes are seen in the same order on the read side. 
-	 * smp_rmb() has effect on all CPUs.  rcu_dereference()
-	 * is used if there are data dependency between two reads, and 
-	 * has effect only on Alpha.
-	 */
-	smp_rmb(); /* prevent indexing old array with new size */
-	entries = rcu_dereference(ids->entries);
-	out = entries[lid].p;
+	out = entries->p[lid];
 	if(out == NULL) {
 		rcu_read_unlock();
 		return NULL;
diff -urpN -X ../dontdiff linux-2.5-rcu_assign_pointer/ipc/util.h linux-2.5-iloi/ipc/util.h
--- linux-2.5-rcu_assign_pointer/ipc/util.h	Mon Aug 30 15:38:19 2004
+++ linux-2.5-iloi/ipc/util.h	Mon Sep  6 18:24:24 2004
@@ -16,17 +16,17 @@ void msg_init (void);
 void shm_init (void);
 
 struct ipc_ids {
-	int size;
 	int in_use;
 	int max_id;
 	unsigned short seq;
 	unsigned short seq_max;
 	struct semaphore sem;	
-	struct ipc_id* entries;
+	struct ipc_id_ary* entries;
 };
 
-struct ipc_id {
-	struct kern_ipc_perm* p;
+struct ipc_id_ary {
+	int size;
+	struct kern_ipc_perm *p[0];
 };
 
 void __init ipc_init_ids(struct ipc_ids* ids, int size);

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Hugh Dickins]

On Tue, 7 Sep 2004, Paul E. McKenney wrote:
> 
> The grow_ary() code has a number of explicit memory barriers, as does
> ipc_lock().  This patch gets rid of the need for some of these by
> placing the array size in the same block of memory containing the
> array itself, so that the array and the size cannot possibly get out
> of sync.  Also uses rcu_assign_pointer() to get rid of the remaining
> smp_wmb().

But Paul, if you keep removing all these examples of memory barriers,
how can I be expected to learn how to use them properly?

Seriously, good, yes, the fewer "mb"s the better.
I could always educate myself from the older source.

> Untested, therefore probably broken.

Agreed ;)

> Thoughts?

Wouldn't it be a little nicer to start ipc_ids off pointing to a
const ipc_id_ary of size 0, to avoid the various entries == NULL
tests you had to add?

Hugh

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Paul E. McKenney]

On Wed, Sep 08, 2004 at 04:39:43PM +0100, Hugh Dickins wrote:
> On Tue, 7 Sep 2004, Paul E. McKenney wrote:
> > 
> > The grow_ary() code has a number of explicit memory barriers, as does
> > ipc_lock().  This patch gets rid of the need for some of these by
> > placing the array size in the same block of memory containing the
> > array itself, so that the array and the size cannot possibly get out
> > of sync.  Also uses rcu_assign_pointer() to get rid of the remaining
> > smp_wmb().
> 
> But Paul, if you keep removing all these examples of memory barriers,
> how can I be expected to learn how to use them properly?

But Hugh, I left quite a few smp_wmb()s in there just for you!  ;-)

> Seriously, good, yes, the fewer "mb"s the better.
> I could always educate myself from the older source.

Agreed!

> > Untested, therefore probably broken.
> 
> Agreed ;)

Any specifics greatly appreciated, as always...

> > Thoughts?
> 
> Wouldn't it be a little nicer to start ipc_ids off pointing to a
> const ipc_id_ary of size 0, to avoid the various entries == NULL
> tests you had to add?

I like this one!!!  Will put a patch together.  Manfred's recent
patch applied a refcount, which negates the const part, but should
be no problem to put a size-zero structure in the struct ipc_ids.
(Having a separately allocated structure puts me back to checking
NULL pointers due to possibility of allocation failure.)

						Thanx, Paul

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Paul E. McKenney]

On Wed, Sep 08, 2004 at 03:07:53PM -0700, Paul E. McKenney wrote:
> On Wed, Sep 08, 2004 at 04:39:43PM +0100, Hugh Dickins wrote:
> > Wouldn't it be a little nicer to start ipc_ids off pointing to a
> > const ipc_id_ary of size 0, to avoid the various entries == NULL
> > tests you had to add?
> 
> I like this one!!!  Will put a patch together.  Manfred's recent
> patch applied a refcount, which negates the const part, but should
> be no problem to put a size-zero structure in the struct ipc_ids.
> (Having a separately allocated structure puts me back to checking
> NULL pointers due to possibility of allocation failure.)

And here, finally, is the updated patch.  Still untested.

Thoughts?

						Thanx, Paul

Signed-off-by: paulmck@us.ibm.com

diff -urpN -X ../dontdiff linux-2.5-rap/ipc/msg.c linux-2.5-iloi/ipc/msg.c
--- linux-2.5-rap/ipc/msg.c	Tue Sep  7 10:04:36 2004
+++ linux-2.5-iloi/ipc/msg.c	Fri Sep 10 16:43:10 2004
@@ -380,7 +380,7 @@ asmlinkage long sys_msgctl (int msqid, i
 		int success_return;
 		if (!buf)
 			return -EFAULT;
-		if(cmd == MSG_STAT && msqid >= msg_ids.size)
+		if(cmd == MSG_STAT && msqid >= msg_ids.entries->size)
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));
diff -urpN -X ../dontdiff linux-2.5-rap/ipc/sem.c linux-2.5-iloi/ipc/sem.c
--- linux-2.5-rap/ipc/sem.c	Tue Sep  7 10:04:36 2004
+++ linux-2.5-iloi/ipc/sem.c	Fri Sep 10 16:43:33 2004
@@ -523,7 +523,7 @@ static int semctl_nolock(int semid, int 
 		struct semid64_ds tbuf;
 		int id;
 
-		if(semid >= sem_ids.size)
+		if(semid >= sem_ids.entries->size)
 			return -EINVAL;
 
 		memset(&tbuf,0,sizeof(tbuf));
diff -urpN -X ../dontdiff linux-2.5-rap/ipc/util.c linux-2.5-iloi/ipc/util.c
--- linux-2.5-rap/ipc/util.c	Tue Sep  7 10:04:36 2004
+++ linux-2.5-iloi/ipc/util.c	Fri Sep 10 16:36:23 2004
@@ -62,7 +62,6 @@ void __init ipc_init_ids(struct ipc_ids*
 
 	if(size > IPCMNI)
 		size = IPCMNI;
-	ids->size = size;
 	ids->in_use = 0;
 	ids->max_id = -1;
 	ids->seq = 0;
@@ -74,14 +73,17 @@ void __init ipc_init_ids(struct ipc_ids*
 		 	ids->seq_max = seq_limit;
 	}
 
-	ids->entries = ipc_rcu_alloc(sizeof(struct ipc_id)*size);
+	ids->entries = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*size +
+				     sizeof(struct ipc_id_ary));
 
 	if(ids->entries == NULL) {
 		printk(KERN_ERR "ipc_init_ids() failed, ipc service disabled.\n");
-		ids->size = 0;
+		size = 0;
+		ids->entries = &ids->nullentry;
 	}
-	for(i=0;i<ids->size;i++)
-		ids->entries[i].p = NULL;
+	ids->entries->size = size;
+	for(i=0;i<size;i++)
+		ids->entries->p[i] = NULL;
 }
 
 /**
@@ -104,7 +106,7 @@ int ipc_findkey(struct ipc_ids* ids, key
 	 * since ipc_ids.sem is held
 	 */
 	for (id = 0; id <= max_id; id++) {
-		p = ids->entries[id].p;
+		p = ids->entries->p[id];
 		if(p==NULL)
 			continue;
 		if (key == p->key)
@@ -118,36 +120,36 @@ int ipc_findkey(struct ipc_ids* ids, key
  */
 static int grow_ary(struct ipc_ids* ids, int newsize)
 {
-	struct ipc_id* new;
-	struct ipc_id* old;
+	struct ipc_id_ary* new;
+	struct ipc_id_ary* old;
 	int i;
+	int size = ids->entries->size;
 
 	if(newsize > IPCMNI)
 		newsize = IPCMNI;
-	if(newsize <= ids->size)
+	if(newsize <= size)
 		return newsize;
 
-	new = ipc_rcu_alloc(sizeof(struct ipc_id)*newsize);
+	new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
+			    sizeof(struct ipc_id_ary));
 	if(new == NULL)
-		return ids->size;
-	memcpy(new, ids->entries, sizeof(struct ipc_id)*ids->size);
-	for(i=ids->size;i<newsize;i++) {
-		new[i].p = NULL;
+		return size;
+	new->size = newsize;
+	memcpy(new->p, ids->entries->p, sizeof(struct kern_ipc_perm *)*size +
+					sizeof(struct ipc_id_ary));
+	for(i=size;i<newsize;i++) {
+		new->p[i] = NULL;
 	}
 	old = ids->entries;
 
 	/*
-	 * before setting the ids->entries to the new array, there must be a
-	 * smp_wmb() to make sure the memcpyed contents of the new array are
-	 * visible before the new array becomes visible.
+	 * Use rcu_assign_pointer() to make sure the memcpyed contents
+	 * of the new array are visible before the new array becomes visible.
 	 */
-	smp_wmb();	/* prevent seeing new array uninitialized. */
-	ids->entries = new;
-	smp_wmb();	/* prevent indexing into old array based on new size. */
-	ids->size = newsize;
+	rcu_assign_pointer(ids->entries, new);
 
 	ipc_rcu_putref(old);
-	return ids->size;
+	return newsize;
 }
 
 /**
@@ -175,7 +177,7 @@ int ipc_addid(struct ipc_ids* ids, struc
 	 * ipc_ids.sem is held
 	 */
 	for (id = 0; id < size; id++) {
-		if(ids->entries[id].p == NULL)
+		if(ids->entries->p[id] == NULL)
 			goto found;
 	}
 	return -1;
@@ -195,7 +197,7 @@ found:
 	new->deleted = 0;
 	rcu_read_lock();
 	spin_lock(&new->lock);
-	ids->entries[id].p = new;
+	ids->entries->p[id] = new;
 	return id;
 }
 
@@ -216,15 +218,15 @@ struct kern_ipc_perm* ipc_rmid(struct ip
 {
 	struct kern_ipc_perm* p;
 	int lid = id % SEQ_MULTIPLIER;
-	if(lid >= ids->size)
+	if(lid >= ids->entries->size)
 		BUG();
 
 	/* 
 	 * do not need a rcu_dereference()() here to force ordering
 	 * on Alpha, since the ipc_ids.sem is held.
 	 */	
-	p = ids->entries[lid].p;
-	ids->entries[lid].p = NULL;
+	p = ids->entries->p[lid];
+	ids->entries->p[lid] = NULL;
 	if(p==NULL)
 		BUG();
 	ids->in_use--;
@@ -234,7 +236,7 @@ struct kern_ipc_perm* ipc_rmid(struct ip
 			lid--;
 			if(lid == -1)
 				break;
-		} while (ids->entries[lid].p == NULL);
+		} while (ids->entries->p[lid] == NULL);
 		ids->max_id = lid;
 	}
 	p->deleted = 1;
@@ -493,9 +495,9 @@ struct kern_ipc_perm* ipc_get(struct ipc
 {
 	struct kern_ipc_perm* out;
 	int lid = id % SEQ_MULTIPLIER;
-	if(lid >= ids->size)
+	if(lid >= ids->entries->size)
 		return NULL;
-	out = ids->entries[lid].p;
+	out = ids->entries->p[lid];
 	return out;
 }
 
@@ -503,25 +505,15 @@ struct kern_ipc_perm* ipc_lock(struct ip
 {
 	struct kern_ipc_perm* out;
 	int lid = id % SEQ_MULTIPLIER;
-	struct ipc_id* entries;
+	struct ipc_id_ary* entries;
 
 	rcu_read_lock();
-	if(lid >= ids->size) {
+	entries = rcu_dereference(ids->entries);
+	if(lid >= entries->size) {
 		rcu_read_unlock();
 		return NULL;
 	}
-
-	/* 
-	 * Note: The following two read barriers are corresponding
-	 * to the two write barriers in grow_ary(). They guarantee 
-	 * the writes are seen in the same order on the read side. 
-	 * smp_rmb() has effect on all CPUs.  rcu_dereference()
-	 * is used if there are data dependency between two reads, and 
-	 * has effect only on Alpha.
-	 */
-	smp_rmb(); /* prevent indexing old array with new size */
-	entries = rcu_dereference(ids->entries);
-	out = entries[lid].p;
+	out = entries->p[lid];
 	if(out == NULL) {
 		rcu_read_unlock();
 		return NULL;
diff -urpN -X ../dontdiff linux-2.5-rap/ipc/util.h linux-2.5-iloi/ipc/util.h
--- linux-2.5-rap/ipc/util.h	Tue Sep  7 10:04:36 2004
+++ linux-2.5-iloi/ipc/util.h	Fri Sep 10 17:52:17 2004
@@ -15,18 +15,19 @@ void sem_init (void);
 void msg_init (void);
 void shm_init (void);
 
-struct ipc_ids {
+struct ipc_id_ary {
 	int size;
+	struct kern_ipc_perm *p[0];
+};
+
+struct ipc_ids {
 	int in_use;
 	int max_id;
 	unsigned short seq;
 	unsigned short seq_max;
 	struct semaphore sem;	
-	struct ipc_id* entries;
-};
-
-struct ipc_id {
-	struct kern_ipc_perm* p;
+	struct ipc_id_ary nullentry;
+	struct ipc_id_ary* entries;
 };
 
 void __init ipc_init_ids(struct ipc_ids* ids, int size);

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Hugh Dickins]

On Fri, 10 Sep 2004, Paul E. McKenney wrote:
> On Wed, Sep 08, 2004 at 03:07:53PM -0700, Paul E. McKenney wrote:
> > On Wed, Sep 08, 2004 at 04:39:43PM +0100, Hugh Dickins wrote:
> > > Wouldn't it be a little nicer to start ipc_ids off pointing to a
> > > const ipc_id_ary of size 0, to avoid the various entries == NULL
> > > tests you had to add?
> > 
> > I like this one!!!  Will put a patch together.  Manfred's recent
> > patch applied a refcount, which negates the const part, but should
> > be no problem to put a size-zero structure in the struct ipc_ids.
> > (Having a separately allocated structure puts me back to checking
> > NULL pointers due to possibility of allocation failure.)
> 
> And here, finally, is the updated patch.  Still untested.

That looks good, Paul, thank you.  (But I too have not tested.)

Hugh

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Manfred Spraul]

Paul E. McKenney wrote:

>And here, finally, is the updated patch.  Still untested.
>
>Thoughts?
>
>  
>
Looks good.
I've even tried to test it, but doesn't compile with -rc1-bk11 due to 
missing rcu_assign_pointer.

--
    Manfred

Re: [RFC][PATCH] Put size in array to get rid of barriers in grow_ary()

[Paul E. McKenney]

On Mon, Sep 13, 2004 at 08:40:15PM +0200, Manfred Spraul wrote:
> Paul E. McKenney wrote:
> 
> >And here, finally, is the updated patch.  Still untested.
> >
> >Thoughts?
> >
> > 
> >
> Looks good.
> I've even tried to test it, but doesn't compile with -rc1-bk11 due to 
> missing rcu_assign_pointer.

My bad!  It needs the rcu_assign_pointer() patch as pre-req.

http://marc.theaimsgroup.com/?l=linux-kernel&m=109459678719365&w=2

Below is the part of that patch that is actually supplies
rcu_assign_pointer(), which should be all that is needed.

						Thanx, Paul

diff -urpN -X ../dontdiff linux-2.5/include/linux/rcupdate.h linux-2.5-rap/include/linux/rcupdate.h
--- linux-2.5/include/linux/rcupdate.h	Tue Sep  7 10:04:29 2004
+++ linux-2.5-rap/include/linux/rcupdate.h	Tue Sep  7 12:12:09 2004
@@ -238,6 +238,24 @@ static inline int rcu_pending(int cpu)
 				(_________p1); \
 				})
 
+/**
+ * rcu_assign_pointer - assign (publicize) a pointer to a newly
+ * initialized structure that will be dereferenced by RCU read-side
+ * critical sections.  Returns the value assigned.
+ *
+ * Inserts memory barriers on architectures that require them
+ * (pretty much all of them other than x86), and also prevents
+ * the compiler from reordering the code that initializes the
+ * structure after the pointer assignment.  More importantly, this
+ * call documents which pointers will be dereferenced by RCU read-side
+ * code.
+ */
+
+#define rcu_assign_pointer(p, v)	({ \
+						smp_wmb(); \
+						(p) = (v); \
+					})
+
 extern void rcu_init(void);
 extern void rcu_check_callbacks(int cpu, int user);
 extern void rcu_restart_cpu(int cpu);