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SubjectRe: RCU : Abstracted RCU dereferencing [5/5]
Use abstracted RCU API to dereference RCU protected data. Hides
barrier details. Patch from Paul McKenney.

Thanks
Dipankar


This patch introduced an rcu_dereference() macro that replaces most
uses of smp_read_barrier_depends(). The new macro has the advantage
of explicitly documenting which pointers are protected by RCU -- in
contrast, it is sometimes difficult to figure out which pointer is
being protected by a given smp_read_barrier_depends() call.

Signed-off-by: Paul McKenney <paulmck@us.ibm.com>


arch/x86_64/kernel/mce.c | 8 +++-----
fs/dcache.c | 10 ++++------
include/linux/list.h | 27 ++++++++++++++-------------
include/linux/rcupdate.h | 16 ++++++++++++++++
ipc/util.c | 11 +++++------
net/bridge/br_input.c | 3 +--
net/core/dev.c | 3 +--
net/core/netfilter.c | 3 +--
net/decnet/dn_route.c | 17 ++++++++---------
net/ipv4/icmp.c | 3 +--
net/ipv4/ip_input.c | 3 +--
net/ipv4/route.c | 24 ++++++++++--------------
net/ipv6/icmp.c | 3 +--
net/ipv6/ip6_input.c | 3 +--
14 files changed, 67 insertions(+), 67 deletions(-)
diff -puN arch/x86_64/kernel/mce.c~rcu-use-deref-macro arch/x86_64/kernel/mce.c
--- linux-2.6.8-rc3-mm1/arch/x86_64/kernel/mce.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/arch/x86_64/kernel/mce.c 2004-08-07 15:30:07.000000000 +0530
@@ -49,8 +49,7 @@ static void mce_log(struct mce *mce)
mce->finished = 0;
smp_wmb();
for (;;) {
- entry = mcelog.next;
- read_barrier_depends();
+ entry = rcu_dereference(mcelog.next);
/* When the buffer fills up discard new entries. Assume
that the earlier errors are the more interesting. */
if (entry >= MCE_LOG_LEN) {
@@ -340,9 +339,8 @@ static ssize_t mce_read(struct file *fil
int i, err;

down(&mce_read_sem);
- next = mcelog.next;
- read_barrier_depends();
-
+ next = rcu_dereference(mcelog.next);
+
/* Only supports full reads right now */
if (*off != 0 || usize < MCE_LOG_LEN*sizeof(struct mce)) {
up(&mce_read_sem);
diff -puN fs/dcache.c~rcu-use-deref-macro fs/dcache.c
--- linux-2.6.8-rc3-mm1/fs/dcache.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/fs/dcache.c 2004-08-07 15:30:07.000000000 +0530
@@ -620,7 +620,7 @@ void shrink_dcache_parent(struct dentry
*
* Prune the dentries that are anonymous
*
- * parsing d_hash list does not read_barrier_depends() as it
+ * parsing d_hash list does not hlist_for_each_rcu() as it
* done under dcache_lock.
*
*/
@@ -977,11 +977,10 @@ struct dentry * __d_lookup(struct dentry

rcu_read_lock();

- hlist_for_each (node, head) {
+ hlist_for_each_rcu(node, head) {
struct dentry *dentry;
struct qstr *qstr;

- smp_read_barrier_depends();
dentry = hlist_entry(node, struct dentry, d_hash);

smp_rmb();
@@ -1008,8 +1007,7 @@ struct dentry * __d_lookup(struct dentry
if (dentry->d_parent != parent)
goto next;

- qstr = &dentry->d_name;
- smp_read_barrier_depends();
+ qstr = rcu_dereference(&dentry->d_name);
if (parent->d_op && parent->d_op->d_compare) {
if (parent->d_op->d_compare(parent, qstr, name))
goto next;
@@ -1062,7 +1060,7 @@ int d_validate(struct dentry *dentry, st
spin_lock(&dcache_lock);
base = d_hash(dparent, dentry->d_name.hash);
hlist_for_each(lhp,base) {
- /* read_barrier_depends() not required for d_hash list
+ /* hlist_for_each_rcu() not required for d_hash list
* as it is parsed under dcache_lock
*/
if (dentry == hlist_entry(lhp, struct dentry, d_hash)) {
diff -puN include/linux/list.h~rcu-use-deref-macro include/linux/list.h
--- linux-2.6.8-rc3-mm1/include/linux/list.h~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/include/linux/list.h 2004-08-07 16:36:41.000000000 +0530
@@ -423,11 +423,11 @@ static inline void list_splice_init(stru
*/
#define list_for_each_rcu(pos, head) \
for (pos = (head)->next, prefetch(pos->next); pos != (head); \
- pos = pos->next, ({ smp_read_barrier_depends(); 0;}), prefetch(pos->next))
+ pos = rcu_dereference(pos->next), prefetch(pos->next))

#define __list_for_each_rcu(pos, head) \
for (pos = (head)->next; pos != (head); \
- pos = pos->next, ({ smp_read_barrier_depends(); 0;}))
+ pos = rcu_dereference(pos->next))

/**
* list_for_each_safe_rcu - iterate over an rcu-protected list safe
@@ -442,7 +442,7 @@ static inline void list_splice_init(stru
*/
#define list_for_each_safe_rcu(pos, n, head) \
for (pos = (head)->next, n = pos->next; pos != (head); \
- pos = n, ({ smp_read_barrier_depends(); 0;}), n = pos->next)
+ pos = rcu_dereference(n), n = pos->next)

/**
* list_for_each_entry_rcu - iterate over rcu list of given type
@@ -458,8 +458,8 @@ static inline void list_splice_init(stru
for (pos = list_entry((head)->next, typeof(*pos), member), \
prefetch(pos->member.next); \
&pos->member != (head); \
- pos = list_entry(pos->member.next, typeof(*pos), member), \
- ({ smp_read_barrier_depends(); 0;}), \
+ pos = rcu_dereference(list_entry(pos->member.next, \
+ typeof(*pos), member)), \
prefetch(pos->member.next))


@@ -475,7 +475,7 @@ static inline void list_splice_init(stru
*/
#define list_for_each_continue_rcu(pos, head) \
for ((pos) = (pos)->next, prefetch((pos)->next); (pos) != (head); \
- (pos) = (pos)->next, ({ smp_read_barrier_depends(); 0;}), prefetch((pos)->next))
+ (pos) = rcu_dereference((pos)->next), prefetch((pos)->next))

/*
* Double linked lists with a single pointer list head.
@@ -581,12 +581,9 @@ static inline void hlist_add_head(struct
* or hlist_del_rcu(), running on this same list.
* However, it is perfectly legal to run concurrently with
* the _rcu list-traversal primitives, such as
- * hlist_for_each_entry(), but only if smp_read_barrier_depends()
- * is used to prevent memory-consistency problems on Alpha CPUs.
- * Regardless of the type of CPU, the list-traversal primitive
- * must be guarded by rcu_read_lock().
- *
- * OK, so why don't we have an hlist_for_each_entry_rcu()???
+ * hlist_for_each_rcu(), used to prevent memory-consistency
+ * problems on Alpha CPUs. Regardless of the type of CPU, the
+ * list-traversal primitive must be guarded by rcu_read_lock().
*/
static inline void hlist_add_head_rcu(struct hlist_node *n,
struct hlist_head *h)
@@ -631,6 +628,10 @@ static inline void hlist_add_after(struc
for (pos = (head)->first; pos && ({ n = pos->next; 1; }); \
pos = n)

+#define hlist_for_each_rcu(pos, head) \
+ for ((pos) = (head)->first; pos && ({ prefetch((pos)->next); 1; }); \
+ (pos) = rcu_dereference((pos)->next))
+
/**
* hlist_for_each_entry - iterate over list of given type
* @tpos: the type * to use as a loop counter.
@@ -696,7 +697,7 @@ static inline void hlist_add_after(struc
for (pos = (head)->first; \
pos && ({ prefetch(pos->next); 1;}) && \
({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
- pos = pos->next, ({ smp_read_barrier_depends(); 0; }) )
+ pos = rcu_dereference(pos->next))

#else
#warning "don't include kernel headers in userspace"
diff -puN include/linux/rcupdate.h~rcu-use-deref-macro include/linux/rcupdate.h
--- linux-2.6.8-rc3-mm1/include/linux/rcupdate.h~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/include/linux/rcupdate.h 2004-08-07 15:30:07.000000000 +0530
@@ -221,6 +221,22 @@ static inline int rcu_pending(int cpu)
* See rcu_read_lock_bh() for more information.
*/
#define rcu_read_unlock_bh() local_bh_enable()
+
+/**
+ * rcu_dereference - fetch an RCU-protected pointer in an
+ * RCU read-side critical section. This pointer may later
+ * be safely dereferenced.
+ *
+ * Inserts memory barriers on architectures that require them
+ * (currently only the Alpha), and, more importantly, documents
+ * exactly which pointers are protected by RCU.
+ */
+
+#define rcu_dereference(p) ({ \
+ typeof(p) _________p1 = p; \
+ smp_read_barrier_depends(); \
+ (_________p1); \
+ })

extern void rcu_init(void);
extern void rcu_check_callbacks(int cpu, int user);
diff -puN ipc/util.c~rcu-use-deref-macro ipc/util.c
--- linux-2.6.8-rc3-mm1/ipc/util.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/ipc/util.c 2004-08-07 15:30:07.000000000 +0530
@@ -100,7 +100,7 @@ int ipc_findkey(struct ipc_ids* ids, key
int max_id = ids->max_id;

/*
- * read_barrier_depends is not needed here
+ * rcu_dereference() is not needed here
* since ipc_ids.sem is held
*/
for (id = 0; id <= max_id; id++) {
@@ -171,7 +171,7 @@ int ipc_addid(struct ipc_ids* ids, struc
size = grow_ary(ids,size);

/*
- * read_barrier_depends() is not needed here since
+ * rcu_dereference()() is not needed here since
* ipc_ids.sem is held
*/
for (id = 0; id < size; id++) {
@@ -220,7 +220,7 @@ struct kern_ipc_perm* ipc_rmid(struct ip
BUG();

/*
- * do not need a read_barrier_depends() here to force ordering
+ * do not need a rcu_dereference()() here to force ordering
* on Alpha, since the ipc_ids.sem is held.
*/
p = ids->entries[lid].p;
@@ -515,13 +515,12 @@ struct kern_ipc_perm* ipc_lock(struct ip
* 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. read_barrier_depends()
+ * 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 = ids->entries;
- read_barrier_depends(); /*prevent seeing new array unitialized */
+ entries = rcu_dereference(ids->entries);
out = entries[lid].p;
if(out == NULL) {
rcu_read_unlock();
diff -puN net/bridge/br_input.c~rcu-use-deref-macro net/bridge/br_input.c
--- linux-2.6.8-rc3-mm1/net/bridge/br_input.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/bridge/br_input.c 2004-08-07 15:30:07.000000000 +0530
@@ -56,8 +56,7 @@ int br_handle_frame_finish(struct sk_buf
dest = skb->mac.ethernet->h_dest;

rcu_read_lock();
- p = skb->dev->br_port;
- smp_read_barrier_depends();
+ p = rcu_dereference(skb->dev->br_port);

if (p == NULL || p->state == BR_STATE_DISABLED) {
kfree_skb(skb);
diff -puN net/core/dev.c~rcu-use-deref-macro net/core/dev.c
--- linux-2.6.8-rc3-mm1/net/core/dev.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/core/dev.c 2004-08-07 15:30:07.000000000 +0530
@@ -1332,8 +1332,7 @@ int dev_queue_xmit(struct sk_buff *skb)
* also serializes access to the device queue.
*/

- q = dev->qdisc;
- smp_read_barrier_depends();
+ q = rcu_dereference(dev->qdisc);
#ifdef CONFIG_NET_CLS_ACT
skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
#endif
diff -puN net/core/netfilter.c~rcu-use-deref-macro net/core/netfilter.c
--- linux-2.6.8-rc3-mm1/net/core/netfilter.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/core/netfilter.c 2004-08-07 15:30:07.000000000 +0530
@@ -783,13 +783,12 @@ void nf_log_packet(int pf,
nf_logfn *logfn;

rcu_read_lock();
- logfn = nf_logging[pf];
+ logfn = rcu_dereference(nf_logging[pf]);
if (logfn) {
va_start(args, fmt);
vsnprintf(prefix, sizeof(prefix), fmt, args);
va_end(args);
/* We must read logging before nf_logfn[pf] */
- smp_read_barrier_depends();
logfn(hooknum, skb, in, out, prefix);
} else if (!reported) {
printk(KERN_WARNING "nf_log_packet: can\'t log yet, "
diff -puN net/decnet/dn_route.c~rcu-use-deref-macro net/decnet/dn_route.c
--- linux-2.6.8-rc3-mm1/net/decnet/dn_route.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/decnet/dn_route.c 2004-08-07 15:30:07.000000000 +0530
@@ -1,4 +1,3 @@
-
/*
* DECnet An implementation of the DECnet protocol suite for the LINUX
* operating system. DECnet is implemented using the BSD Socket
@@ -1175,8 +1174,8 @@ static int __dn_route_output_key(struct

if (!(flags & MSG_TRYHARD)) {
rcu_read_lock_bh();
- for(rt = dn_rt_hash_table[hash].chain; rt; rt = rt->u.rt_next) {
- smp_read_barrier_depends();
+ for(rt = rcu_dereference(dn_rt_hash_table[hash].chain); rt;
+ rt = rcu_dereference(rt->u.rt_next)) {
if ((flp->fld_dst == rt->fl.fld_dst) &&
(flp->fld_src == rt->fl.fld_src) &&
#ifdef CONFIG_DECNET_ROUTE_FWMARK
@@ -1454,8 +1453,8 @@ int dn_route_input(struct sk_buff *skb)
return 0;

rcu_read_lock();
- for(rt = dn_rt_hash_table[hash].chain; rt != NULL; rt = rt->u.rt_next) {
- read_barrier_depends();
+ for(rt = rcu_dereference(dn_rt_hash_table[hash].chain); rt != NULL;
+ rt = rcu_dereference(rt->u.rt_next)) {
if ((rt->fl.fld_src == cb->src) &&
(rt->fl.fld_dst == cb->dst) &&
(rt->fl.oif == 0) &&
@@ -1648,8 +1647,9 @@ int dn_cache_dump(struct sk_buff *skb, s
if (h > s_h)
s_idx = 0;
rcu_read_lock_bh();
- for(rt = dn_rt_hash_table[h].chain, idx = 0; rt; rt = rt->u.rt_next, idx++) {
- smp_read_barrier_depends();
+ for(rt = rcu_dereference(dn_rt_hash_table[h].chain), idx = 0;
+ rt;
+ rt = rcu_dereference(rt->u.rt_next), idx++) {
if (idx < s_idx)
continue;
skb->dst = dst_clone(&rt->u.dst);
@@ -1692,9 +1692,8 @@ static struct dn_route *dn_rt_cache_get_

static struct dn_route *dn_rt_cache_get_next(struct seq_file *seq, struct dn_route *rt)
{
- struct dn_rt_cache_iter_state *s = seq->private;
+ struct dn_rt_cache_iter_state *s = rcu_dereference(seq->private);

- smp_read_barrier_depends();
rt = rt->u.rt_next;
while(!rt) {
rcu_read_unlock_bh();
diff -puN net/ipv4/icmp.c~rcu-use-deref-macro net/ipv4/icmp.c
--- linux-2.6.8-rc3-mm1/net/ipv4/icmp.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/ipv4/icmp.c 2004-08-07 15:30:07.000000000 +0530
@@ -705,8 +705,7 @@ static void icmp_unreach(struct sk_buff
read_unlock(&raw_v4_lock);

rcu_read_lock();
- ipprot = inet_protos[hash];
- smp_read_barrier_depends();
+ ipprot = rcu_dereference(inet_protos[hash]);
if (ipprot && ipprot->err_handler)
ipprot->err_handler(skb, info);
rcu_read_unlock();
diff -puN net/ipv4/ip_input.c~rcu-use-deref-macro net/ipv4/ip_input.c
--- linux-2.6.8-rc3-mm1/net/ipv4/ip_input.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/ipv4/ip_input.c 2004-08-07 15:30:07.000000000 +0530
@@ -231,10 +231,9 @@ static inline int ip_local_deliver_finis
if (raw_sk)
raw_v4_input(skb, skb->nh.iph, hash);

- if ((ipprot = inet_protos[hash]) != NULL) {
+ if ((ipprot = rcu_dereference(inet_protos[hash])) != NULL) {
int ret;

- smp_read_barrier_depends();
if (!ipprot->no_policy &&
!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
kfree_skb(skb);
diff -puN net/ipv4/route.c~rcu-use-deref-macro net/ipv4/route.c
--- linux-2.6.8-rc3-mm1/net/ipv4/route.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/ipv4/route.c 2004-08-07 15:30:07.000000000 +0530
@@ -237,9 +237,8 @@ static struct rtable *rt_cache_get_first

static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
{
- struct rt_cache_iter_state *st = seq->private;
+ struct rt_cache_iter_state *st = rcu_dereference(seq->private);

- smp_read_barrier_depends();
r = r->u.rt_next;
while (!r) {
rcu_read_unlock_bh();
@@ -1004,10 +1003,9 @@ void ip_rt_redirect(u32 old_gw, u32 dadd
rthp=&rt_hash_table[hash].chain;

rcu_read_lock();
- while ((rth = *rthp) != NULL) {
+ while ((rth = rcu_dereference(*rthp)) != NULL) {
struct rtable *rt;

- smp_read_barrier_depends();
if (rth->fl.fl4_dst != daddr ||
rth->fl.fl4_src != skeys[i] ||
rth->fl.fl4_tos != tos ||
@@ -1259,9 +1257,8 @@ unsigned short ip_rt_frag_needed(struct
unsigned hash = rt_hash_code(daddr, skeys[i], tos);

rcu_read_lock();
- for (rth = rt_hash_table[hash].chain; rth;
- rth = rth->u.rt_next) {
- smp_read_barrier_depends();
+ for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
+ rth = rcu_dereference(rth->u.rt_next)) {
if (rth->fl.fl4_dst == daddr &&
rth->fl.fl4_src == skeys[i] &&
rth->rt_dst == daddr &&
@@ -1864,8 +1861,8 @@ int ip_route_input(struct sk_buff *skb,
hash = rt_hash_code(daddr, saddr ^ (iif << 5), tos);

rcu_read_lock();
- for (rth = rt_hash_table[hash].chain; rth; rth = rth->u.rt_next) {
- smp_read_barrier_depends();
+ for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
+ rth = rcu_dereference(rth->u.rt_next)) {
if (rth->fl.fl4_dst == daddr &&
rth->fl.fl4_src == saddr &&
rth->fl.iif == iif &&
@@ -2232,8 +2229,8 @@ int __ip_route_output_key(struct rtable
hash = rt_hash_code(flp->fl4_dst, flp->fl4_src ^ (flp->oif << 5), flp->fl4_tos);

rcu_read_lock_bh();
- for (rth = rt_hash_table[hash].chain; rth; rth = rth->u.rt_next) {
- smp_read_barrier_depends();
+ for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
+ rth = rcu_dereference(rth->u.rt_next)) {
if (rth->fl.fl4_dst == flp->fl4_dst &&
rth->fl.fl4_src == flp->fl4_src &&
rth->fl.iif == 0 &&
@@ -2464,9 +2461,8 @@ int ip_rt_dump(struct sk_buff *skb, str
if (h > s_h)
s_idx = 0;
rcu_read_lock_bh();
- for (rt = rt_hash_table[h].chain, idx = 0; rt;
- rt = rt->u.rt_next, idx++) {
- smp_read_barrier_depends();
+ for (rt = rcu_dereference(rt_hash_table[h].chain), idx = 0; rt;
+ rt = rcu_dereference(rt->u.rt_next), idx++) {
if (idx < s_idx)
continue;
skb->dst = dst_clone(&rt->u.dst);
diff -puN net/ipv6/icmp.c~rcu-use-deref-macro net/ipv6/icmp.c
--- linux-2.6.8-rc3-mm1/net/ipv6/icmp.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/ipv6/icmp.c 2004-08-07 15:30:07.000000000 +0530
@@ -530,8 +530,7 @@ static void icmpv6_notify(struct sk_buff
hash = nexthdr & (MAX_INET_PROTOS - 1);

rcu_read_lock();
- ipprot = inet6_protos[hash];
- smp_read_barrier_depends();
+ ipprot = rcu_dereference(inet6_protos[hash]);
if (ipprot && ipprot->err_handler)
ipprot->err_handler(skb, NULL, type, code, inner_offset, info);
rcu_read_unlock();
diff -puN net/ipv6/ip6_input.c~rcu-use-deref-macro net/ipv6/ip6_input.c
--- linux-2.6.8-rc3-mm1/net/ipv6/ip6_input.c~rcu-use-deref-macro 2004-08-07 15:30:07.000000000 +0530
+++ linux-2.6.8-rc3-mm1-dipankar/net/ipv6/ip6_input.c 2004-08-07 15:30:07.000000000 +0530
@@ -167,10 +167,9 @@ resubmit:
ipv6_raw_deliver(skb, nexthdr);

hash = nexthdr & (MAX_INET_PROTOS - 1);
- if ((ipprot = inet6_protos[hash]) != NULL) {
+ if ((ipprot = rcu_dereference(inet6_protos[hash])) != NULL) {
int ret;

- smp_read_barrier_depends();
if (ipprot->flags & INET6_PROTO_FINAL) {
struct ipv6hdr *hdr;

_
-
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