[PATCH] RCU signal handling

RCU tasklist_lock and RCU signal handling: send signals RCU-read-locked
instead of tasklist_lock read-locked.  This is a scalability improvement on
SMP and a preemption-latency improvement under PREEMPT_RCU.

Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: William Irwin <wli@holomorphy.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

8 files changed, 143 insertions, 31 deletions

diff --git a/fs/exec.c b/fs/exec.c
index e75a9548da8e..e9650cd22a3b 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -760,7 +760,7 @@ no_thread_group:
                 spin_lock(&oldsighand->siglock);
                 spin_lock(&newsighand->siglock);
 
-                current->sighand = newsighand;
+                rcu_assign_pointer(current->sighand, newsighand);
                 recalc_sigpending();
 
                 spin_unlock(&newsighand->siglock);
@@ -768,7 +768,7 @@ no_thread_group:
                 write_unlock_irq(&tasklist_lock);
 
                 if (atomic_dec_and_test(&oldsighand->count))
-                        kmem_cache_free(sighand_cachep, oldsighand);
+                        sighand_free(oldsighand);
         }
 
         BUG_ON(!thread_group_leader(current));
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a74662077d60..a6af77e9b4cf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -34,6 +34,7 @@
 #include <linux/percpu.h>
 #include <linux/topology.h>
 #include <linux/seccomp.h>
+#include <linux/rcupdate.h>
 
 #include <linux/auxvec.h>       /* For AT_VECTOR_SIZE */
 
@@ -350,8 +351,16 @@ struct sighand_struct {
         atomic_t                count;
         struct k_sigaction      action[_NSIG];
         spinlock_t              siglock;
+        struct rcu_head         rcu;
 };
 
+extern void sighand_free_cb(struct rcu_head *rhp);
+
+static inline void sighand_free(struct sighand_struct *sp)
+{
+        call_rcu(&sp->rcu, sighand_free_cb);
+}
+
 /*
  * NOTE! "signal_struct" does not have it's own
  * locking, because a shared signal_struct always
@@ -844,6 +853,7 @@ struct task_struct {
         int cpuset_mems_generation;
 #endif
         atomic_t fs_excl;       /* holding fs exclusive resources */
+        struct rcu_head rcu;
 };
 
 static inline pid_t process_group(struct task_struct *tsk)
@@ -867,8 +877,26 @@ static inline int pid_alive(struct task_struct *p)
 extern void free_task(struct task_struct *tsk);
 extern void __put_task_struct(struct task_struct *tsk);
 #define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)
-#define put_task_struct(tsk) \
-do { if (atomic_dec_and_test(&(tsk)->usage)) __put_task_struct(tsk); } while(0)
+
+static inline int get_task_struct_rcu(struct task_struct *t)
+{
+        int oldusage;
+
+        do {
+                oldusage = atomic_read(&t->usage);
+                if (oldusage == 0)
+                        return 0;
+        } while (cmpxchg(&t->usage.counter, oldusage, oldusage+1) != oldusage);
+        return 1;
+}
+
+extern void __put_task_struct_cb(struct rcu_head *rhp);
+
+static inline void put_task_struct(struct task_struct *t)
+{
+        if (atomic_dec_and_test(&t->usage))
+                call_rcu(&t->rcu, __put_task_struct_cb);
+}
 
 /*
  * Per process flags
diff --git a/kernel/exit.c b/kernel/exit.c
index ee515683b92d..c73a7eb26de3 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -72,7 +72,6 @@ repeat:
                 __ptrace_unlink(p);
         BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
         __exit_signal(p);
-        __exit_sighand(p);
         /*
          * Note that the fastpath in sys_times depends on __exit_signal having
          * updated the counters before a task is removed from the tasklist of
diff --git a/kernel/fork.c b/kernel/fork.c
index fb8572a42297..7fe3adfa65cb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -743,6 +743,14 @@ int unshare_files(void)
 
 EXPORT_SYMBOL(unshare_files);
 
+void sighand_free_cb(struct rcu_head *rhp)
+{
+        struct sighand_struct *sp;
+
+        sp = container_of(rhp, struct sighand_struct, rcu);
+        kmem_cache_free(sighand_cachep, sp);
+}
+
 static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)
 {
         struct sighand_struct *sig;
@@ -752,7 +760,7 @@ static inline int copy_sighand(unsigned long clone_flags, struct task_struct * t
                 return 0;
         }
         sig = kmem_cache_alloc(sighand_cachep, GFP_KERNEL);
-        tsk->sighand = sig;
+        rcu_assign_pointer(tsk->sighand, sig);
         if (!sig)
                 return -ENOMEM;
         spin_lock_init(&sig->siglock);
diff --git a/kernel/pid.c b/kernel/pid.c
index edba31c681ac..1acc07246991 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -136,7 +136,7 @@ struct pid * fastcall find_pid(enum pid_type type, int nr)
         struct hlist_node *elem;
         struct pid *pid;
 
-        hlist_for_each_entry(pid, elem,
+        hlist_for_each_entry_rcu(pid, elem,
                         &pid_hash[type][pid_hashfn(nr)], pid_chain) {
                 if (pid->nr == nr)
                         return pid;
@@ -150,15 +150,15 @@ int fastcall attach_pid(task_t *task, enum pid_type type, int nr)
 
         task_pid = &task->pids[type];
         pid = find_pid(type, nr);
+        task_pid->nr = nr;
         if (pid == NULL) {
-                hlist_add_head(&task_pid->pid_chain,
-                                &pid_hash[type][pid_hashfn(nr)]);
                 INIT_LIST_HEAD(&task_pid->pid_list);
+                hlist_add_head_rcu(&task_pid->pid_chain,
+                                   &pid_hash[type][pid_hashfn(nr)]);
         } else {
                 INIT_HLIST_NODE(&task_pid->pid_chain);
-                list_add_tail(&task_pid->pid_list, &pid->pid_list);
+                list_add_tail_rcu(&task_pid->pid_list, &pid->pid_list);
         }
-        task_pid->nr = nr;
 
         return 0;
 }
@@ -170,20 +170,20 @@ static fastcall int __detach_pid(task_t *task, enum pid_type type)
 
         pid = &task->pids[type];
         if (!hlist_unhashed(&pid->pid_chain)) {
-                hlist_del(&pid->pid_chain);
 
-                if (list_empty(&pid->pid_list))
+                if (list_empty(&pid->pid_list)) {
                         nr = pid->nr;
-                else {
+                        hlist_del_rcu(&pid->pid_chain);
+                } else {
                         pid_next = list_entry(pid->pid_list.next,
                                                 struct pid, pid_list);
                         /* insert next pid from pid_list to hash */
-                        hlist_add_head(&pid_next->pid_chain,
-                                &pid_hash[type][pid_hashfn(pid_next->nr)]);
+                        hlist_replace_rcu(&pid->pid_chain,
+                                          &pid_next->pid_chain);
                 }
         }
 
-        list_del(&pid->pid_list);
+        list_del_rcu(&pid->pid_list);
         pid->nr = 0;
 
         return nr;
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index c9afc61240e4..0a669bd2f6d1 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -35,6 +35,7 @@
 #include <linux/init.h>
 #include <linux/spinlock.h>
 #include <linux/smp.h>
+#include <linux/rcupdate.h>
 #include <linux/interrupt.h>
 #include <linux/sched.h>
 #include <asm/atomic.h>
diff --git a/kernel/sched.c b/kernel/sched.c
index 6f46c94cc29e..92733091154c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -176,6 +176,13 @@ static unsigned int task_timeslice(task_t *p)
 #define task_hot(p, now, sd) ((long long) ((now) - (p)->last_ran)       \
                                 < (long long) (sd)->cache_hot_time)
 
+void __put_task_struct_cb(struct rcu_head *rhp)
+{
+        __put_task_struct(container_of(rhp, struct task_struct, rcu));
+}
+
+EXPORT_SYMBOL_GPL(__put_task_struct_cb);
+
 /*
  * These are the runqueue data structures:
  */
diff --git a/kernel/signal.c b/kernel/signal.c
index d7611f189ef7..64737c72dadd 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -329,13 +329,20 @@ void __exit_sighand(struct task_struct *tsk)
         /* Ok, we're done with the signal handlers */
         tsk->sighand = NULL;
         if (atomic_dec_and_test(&sighand->count))
-                kmem_cache_free(sighand_cachep, sighand);
+                sighand_free(sighand);
 }
 
 void exit_sighand(struct task_struct *tsk)
 {
         write_lock_irq(&tasklist_lock);
-        __exit_sighand(tsk);
+        rcu_read_lock();
+        if (tsk->sighand != NULL) {
+                struct sighand_struct *sighand = rcu_dereference(tsk->sighand);
+                spin_lock(&sighand->siglock);
+                __exit_sighand(tsk);
+                spin_unlock(&sighand->siglock);
+        }
+        rcu_read_unlock();
         write_unlock_irq(&tasklist_lock);
 }
 
@@ -345,12 +352,14 @@ void exit_sighand(struct task_struct *tsk)
 void __exit_signal(struct task_struct *tsk)
 {
         struct signal_struct * sig = tsk->signal;
-        struct sighand_struct * sighand = tsk->sighand;
+        struct sighand_struct * sighand;
 
         if (!sig)
                 BUG();
         if (!atomic_read(&sig->count))
                 BUG();
+        rcu_read_lock();
+        sighand = rcu_dereference(tsk->sighand);
         spin_lock(&sighand->siglock);
         posix_cpu_timers_exit(tsk);
         if (atomic_dec_and_test(&sig->count)) {
@@ -358,6 +367,7 @@ void __exit_signal(struct task_struct *tsk)
                 if (tsk == sig->curr_target)
                         sig->curr_target = next_thread(tsk);
                 tsk->signal = NULL;
+                __exit_sighand(tsk);
                 spin_unlock(&sighand->siglock);
                 flush_sigqueue(&sig->shared_pending);
         } else {
@@ -389,9 +399,11 @@ void __exit_signal(struct task_struct *tsk)
                 sig->nvcsw += tsk->nvcsw;
                 sig->nivcsw += tsk->nivcsw;
                 sig->sched_time += tsk->sched_time;
+                __exit_sighand(tsk);
                 spin_unlock(&sighand->siglock);
                 sig = NULL;     /* Marker for below.  */
         }
+        rcu_read_unlock();
         clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
         flush_sigqueue(&tsk->pending);
         if (sig) {
@@ -1080,18 +1092,28 @@ void zap_other_threads(struct task_struct *p)
 }
 
 /*
- * Must be called with the tasklist_lock held for reading!
+ * Must be called under rcu_read_lock() or with tasklist_lock read-held.
  */
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
         unsigned long flags;
+        struct sighand_struct *sp;
         int ret;
 
+retry:
         ret = check_kill_permission(sig, info, p);
-        if (!ret && sig && p->sighand) {
-                spin_lock_irqsave(&p->sighand->siglock, flags);
+        if (!ret && sig && (sp = p->sighand)) {
+                if (!get_task_struct_rcu(p))
+                        return -ESRCH;
+                spin_lock_irqsave(&sp->siglock, flags);
+                if (p->sighand != sp) {
+                        spin_unlock_irqrestore(&sp->siglock, flags);
+                        put_task_struct(p);
+                        goto retry;
+                }
                 ret = __group_send_sig_info(sig, info, p);
-                spin_unlock_irqrestore(&p->sighand->siglock, flags);
+                spin_unlock_irqrestore(&sp->siglock, flags);
+                put_task_struct(p);
         }
 
         return ret;
@@ -1136,14 +1158,21 @@ int
 kill_proc_info(int sig, struct siginfo *info, pid_t pid)
 {
         int error;
+        int acquired_tasklist_lock = 0;
         struct task_struct *p;
 
-        read_lock(&tasklist_lock);
+        rcu_read_lock();
+        if (unlikely(sig_kernel_stop(sig) || sig == SIGCONT)) {
+                read_lock(&tasklist_lock);
+                acquired_tasklist_lock = 1;
+        }
         p = find_task_by_pid(pid);
         error = -ESRCH;
         if (p)
                 error = group_send_sig_info(sig, info, p);
-        read_unlock(&tasklist_lock);
+        if (unlikely(acquired_tasklist_lock))
+                read_unlock(&tasklist_lock);
+        rcu_read_unlock();
         return error;
 }
 
@@ -1355,16 +1384,54 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 {
         unsigned long flags;
         int ret = 0;
+        struct sighand_struct *sh;
 
         BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
-        read_lock(&tasklist_lock);
+
+        /*
+         * The rcu based delayed sighand destroy makes it possible to
+         * run this without tasklist lock held. The task struct itself
+         * cannot go away as create_timer did get_task_struct().
+         *
+         * We return -1, when the task is marked exiting, so
+         * posix_timer_event can redirect it to the group leader
+         */
+        rcu_read_lock();
 
         if (unlikely(p->flags & PF_EXITING)) {
                 ret = -1;
                 goto out_err;
         }
 
-        spin_lock_irqsave(&p->sighand->siglock, flags);
+retry:
+        sh = rcu_dereference(p->sighand);
+
+        spin_lock_irqsave(&sh->siglock, flags);
+        if (p->sighand != sh) {
+                /* We raced with exec() in a multithreaded process... */
+                spin_unlock_irqrestore(&sh->siglock, flags);
+                goto retry;
+        }
+
+        /*
+         * We do the check here again to handle the following scenario:
+         *
+         * CPU 0                CPU 1
+         * send_sigqueue
+         * check PF_EXITING
+         * interrupt            exit code running
+         *                      __exit_signal
+         *                      lock sighand->siglock
+         *                      unlock sighand->siglock
+         * lock sh->siglock
+         * add(tsk->pending)    flush_sigqueue(tsk->pending)
+         *
+         */
+
+        if (unlikely(p->flags & PF_EXITING)) {
+                ret = -1;
+                goto out;
+        }
 
         if (unlikely(!list_empty(&q->list))) {
                 /*
@@ -1388,9 +1455,9 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
                 signal_wake_up(p, sig == SIGKILL);
 
 out:
-        spin_unlock_irqrestore(&p->sighand->siglock, flags);
+        spin_unlock_irqrestore(&sh->siglock, flags);
 out_err:
-        read_unlock(&tasklist_lock);
+        rcu_read_unlock();
 
         return ret;
 }
@@ -1402,7 +1469,9 @@ send_group_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
         int ret = 0;
 
         BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
+
         read_lock(&tasklist_lock);
+        /* Since it_lock is held, p->sighand cannot be NULL. */
         spin_lock_irqsave(&p->sighand->siglock, flags);
         handle_stop_signal(sig, p);
 
@@ -1436,7 +1505,7 @@ send_group_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 out:
         spin_unlock_irqrestore(&p->sighand->siglock, flags);
         read_unlock(&tasklist_lock);
-        return(ret);
+        return ret;
 }
 
 /*