[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>

1 files changed, 83 insertions, 14 deletions

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;
 }
 
 /*