[RFC][PATCH] Improve readability by hiding read_barrier_depends() calls

From: Paul E. McKenney (paulmck_at_us.ibm.com)
Date: 08/04/04

  • Next message: Zwane Mwaikambo: "Re: CPU hotplug broken in 2.6.8-rc2 ?" 
    Date:	Wed, 4 Aug 2004 07:28:45 -0700
To: dipankar@in.ibm.com, ak@suse.de, maneesh@in.ibm.com, shemminger@osdl.org

    Hello!

    Updated based on feedback, and merged forward to 2.6.8-rc3.

    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.

    Thoughts?

                                                    Thanx, Paul

    diff -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/include/linux/rcupdate.h linux-2.5-rcu_dereference/include/linux/rcupdate.h
    --- linux-2.5-rcu_read_lock_comments/include/linux/rcupdate.h Wed Aug 4 06:44:57 2004
    +++ linux-2.5-rcu_dereference/include/linux/rcupdate.h Wed Aug 4 07:04:22 2004
    @@ -181,6 +181,22 @@ static inline int rcu_pending(int cpu)
      * others' way, as long as they do so.
      */
     
    +/**
    + * 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);
     extern void rcu_restart_cpu(int cpu);
    diff -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/arch/x86_64/kernel/mce.c linux-2.5-rcu_dereference/arch/x86_64/kernel/mce.c
    --- linux-2.5-rcu_read_lock_comments/arch/x86_64/kernel/mce.c Mon Aug 2 11:31:01 2004
    +++ linux-2.5-rcu_dereference/arch/x86_64/kernel/mce.c Wed Aug 4 06:46:43 2004
    @@ -48,8 +48,7 @@ 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) {
    @@ -333,9 +332,8 @@ 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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/fs/dcache.c linux-2.5-rcu_dereference/fs/dcache.c
    --- linux-2.5-rcu_read_lock_comments/fs/dcache.c Mon Aug 2 11:31:45 2004
    +++ linux-2.5-rcu_dereference/fs/dcache.c Wed Aug 4 06:46:45 2004
    @@ -613,7 +613,7 @@ 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.
      *
      */
    @@ -970,11 +970,10 @@ 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();
    @@ -1001,8 +1000,7 @@ 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;
    @@ -1055,7 +1053,7 @@ 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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/include/linux/list.h linux-2.5-rcu_dereference/include/linux/list.h
    --- linux-2.5-rcu_read_lock_comments/include/linux/list.h Mon Aug 2 11:32:10 2004
    +++ linux-2.5-rcu_dereference/include/linux/list.h Wed Aug 4 06:54:12 2004
    @@ -420,11 +420,11 @@ 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
    @@ -439,7 +439,7 @@ 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
    @@ -455,8 +455,8 @@ 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))
     
     
    @@ -472,7 +472,7 @@ 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.
    @@ -578,12 +578,9 @@ 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)
    @@ -624,6 +621,10 @@ static inline void hlist_add_after(struc
             for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \
                  pos = pos->next)
     
    +#define hlist_for_each_rcu(pos, head) \
    + for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \
    + pos = rcu_dereference(pos->next))
    +
     #define hlist_for_each_safe(pos, n, head) \
             for (pos = (head)->first; pos && ({ n = pos->next; 1; }); \
                  pos = n)
    @@ -693,7 +694,7 @@ 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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/ipc/util.c linux-2.5-rcu_dereference/ipc/util.c
    --- linux-2.5-rcu_read_lock_comments/ipc/util.c Mon Aug 2 11:32:13 2004
    +++ linux-2.5-rcu_dereference/ipc/util.c Wed Aug 4 06:46:52 2004
    @@ -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++) {
    @@ -172,7 +172,7 @@ 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++) {
    @@ -221,7 +221,7 @@ 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;
    @@ -481,13 +481,12 @@ 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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/bridge/br_input.c linux-2.5-rcu_dereference/net/bridge/br_input.c
    --- linux-2.5-rcu_read_lock_comments/net/bridge/br_input.c Mon Aug 2 11:32:14 2004
    +++ linux-2.5-rcu_dereference/net/bridge/br_input.c Wed Aug 4 06:46:54 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/core/dev.c linux-2.5-rcu_dereference/net/core/dev.c
    --- linux-2.5-rcu_read_lock_comments/net/core/dev.c Mon Aug 2 11:32:15 2004
    +++ linux-2.5-rcu_dereference/net/core/dev.c Wed Aug 4 06:58:22 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/core/netfilter.c linux-2.5-rcu_dereference/net/core/netfilter.c
    --- linux-2.5-rcu_read_lock_comments/net/core/netfilter.c Mon Aug 2 11:32:18 2004
    +++ linux-2.5-rcu_dereference/net/core/netfilter.c Wed Aug 4 06:46:54 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/decnet/dn_route.c linux-2.5-rcu_dereference/net/decnet/dn_route.c
    --- linux-2.5-rcu_read_lock_comments/net/decnet/dn_route.c Mon Aug 2 11:32:18 2004
    +++ linux-2.5-rcu_dereference/net/decnet/dn_route.c Wed Aug 4 06:46:55 2004
    @@ -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();
    - for(rt = dn_rt_hash_table[hash].chain; rt; rt = rt->u.rt_next) {
    - 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 +1647,8 @@ int dn_cache_dump(struct sk_buff *skb, s
                     if (h > s_h)
                             s_idx = 0;
                     rcu_read_lock();
    - for(rt = dn_rt_hash_table[h].chain, idx = 0; rt; rt = rt->u.rt_next, idx++) {
    - 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 +1691,8 @@ 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();
    diff -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/ipv4/icmp.c linux-2.5-rcu_dereference/net/ipv4/icmp.c
    --- linux-2.5-rcu_read_lock_comments/net/ipv4/icmp.c Mon Aug 2 11:32:18 2004
    +++ linux-2.5-rcu_dereference/net/ipv4/icmp.c Wed Aug 4 06:46:56 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/ipv4/ip_input.c linux-2.5-rcu_dereference/net/ipv4/ip_input.c
    --- linux-2.5-rcu_read_lock_comments/net/ipv4/ip_input.c Mon Aug 2 11:32:18 2004
    +++ linux-2.5-rcu_dereference/net/ipv4/ip_input.c Wed Aug 4 06:46:57 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/ipv4/route.c linux-2.5-rcu_dereference/net/ipv4/route.c
    --- linux-2.5-rcu_read_lock_comments/net/ipv4/route.c Mon Aug 2 11:32:18 2004
    +++ linux-2.5-rcu_dereference/net/ipv4/route.c Wed Aug 4 06:46:59 2004
    @@ -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();
    @@ -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();
    - 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();
    - 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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/ipv6/icmp.c linux-2.5-rcu_dereference/net/ipv6/icmp.c
    --- linux-2.5-rcu_read_lock_comments/net/ipv6/icmp.c Mon Aug 2 11:32:19 2004
    +++ linux-2.5-rcu_dereference/net/ipv6/icmp.c Wed Aug 4 07:00:13 2004
    @@ -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 -urpN -X ../dontdiff linux-2.5-rcu_read_lock_comments/net/ipv6/ip6_input.c linux-2.5-rcu_dereference/net/ipv6/ip6_input.c
    --- linux-2.5-rcu_read_lock_comments/net/ipv6/ip6_input.c Mon Aug 2 11:32:19 2004
    +++ linux-2.5-rcu_dereference/net/ipv6/ip6_input.c Wed Aug 4 06:47:00 2004
    @@ -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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