Merge branch 'timers/posix-timers-for-tip-v2' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks into timers/core

Pull posix cpu timer changes for v3.14 from Frederic Weisbecker:

 * Remove dying thread/process timers caching that was complicating the code
   for no significant win.

 * Remove early task reference release on dying timer sample read. Again it was
   not worth the code complication. The other timer's resources aren't released
   until timer_delete() is called anyway (or when the whole process dies).

 * Remove leftover arguments in reaped target cleanup

 * Consolidate some timer sampling code

 * Remove use of tasklist lock

 * Robustify sighand locking against exec and exit by using the safer
   lock_task_sighand() API instead of sighand raw locking.

 * Convert some unnecessary BUG_ON() to WARN_ON()

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 129 insertions, 183 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 79747b7d9420..3b8946416a5f 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -233,7 +233,8 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 
 /*
  * Sample a process (thread group) clock for the given group_leader task.
- * Must be called with tasklist_lock held for reading.
+ * Must be called with task sighand lock held for safe while_each_thread()
+ * traversal.
  */
 static int cpu_clock_sample_group(const clockid_t which_clock,
                                   struct task_struct *p,
@@ -260,30 +261,53 @@ static int cpu_clock_sample_group(const clockid_t which_clock,
         return 0;
 }
 
+static int posix_cpu_clock_get_task(struct task_struct *tsk,
+                                    const clockid_t which_clock,
+                                    struct timespec *tp)
+{
+        int err = -EINVAL;
+        unsigned long long rtn;
+
+        if (CPUCLOCK_PERTHREAD(which_clock)) {
+                if (same_thread_group(tsk, current))
+                        err = cpu_clock_sample(which_clock, tsk, &rtn);
+        } else {
+                unsigned long flags;
+                struct sighand_struct *sighand;
+
+                /*
+                 * while_each_thread() is not yet entirely RCU safe,
+                 * keep locking the group while sampling process
+                 * clock for now.
+                 */
+                sighand = lock_task_sighand(tsk, &flags);
+                if (!sighand)
+                        return err;
+
+                if (tsk == current || thread_group_leader(tsk))
+                        err = cpu_clock_sample_group(which_clock, tsk, &rtn);
+
+                unlock_task_sighand(tsk, &flags);
+        }
+
+        if (!err)
+                sample_to_timespec(which_clock, rtn, tp);
+
+        return err;
+}
+
 
 static int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 {
         const pid_t pid = CPUCLOCK_PID(which_clock);
-        int error = -EINVAL;
-        unsigned long long rtn;
+        int err = -EINVAL;
 
         if (pid == 0) {
                 /*
                  * Special case constant value for our own clocks.
                  * We don't have to do any lookup to find ourselves.
                  */
-                if (CPUCLOCK_PERTHREAD(which_clock)) {
-                        /*
-                         * Sampling just ourselves we can do with no locking.
-                         */
-                        error = cpu_clock_sample(which_clock,
-                                                 current, &rtn);
-                } else {
-                        read_lock(&tasklist_lock);
-                        error = cpu_clock_sample_group(which_clock,
-                                                       current, &rtn);
-                        read_unlock(&tasklist_lock);
-                }
+                err = posix_cpu_clock_get_task(current, which_clock, tp);
         } else {
                 /*
                  * Find the given PID, and validate that the caller
@@ -292,29 +316,12 @@ static int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
                 struct task_struct *p;
                 rcu_read_lock();
                 p = find_task_by_vpid(pid);
-                if (p) {
-                        if (CPUCLOCK_PERTHREAD(which_clock)) {
-                                if (same_thread_group(p, current)) {
-                                        error = cpu_clock_sample(which_clock,
-                                                                 p, &rtn);
-                                }
-                        } else {
-                                read_lock(&tasklist_lock);
-                                if (thread_group_leader(p) && p->sighand) {
-                                        error =
-                                            cpu_clock_sample_group(which_clock,
-                                                                   p, &rtn);
-                                }
-                                read_unlock(&tasklist_lock);
-                        }
-                }
+                if (p)
+                        err = posix_cpu_clock_get_task(p, which_clock, tp);
                 rcu_read_unlock();
         }
 
-        if (error)
-                return error;
-        sample_to_timespec(which_clock, rtn, tp);
-        return 0;
+        return err;
 }
 
 
@@ -371,36 +378,40 @@ static int posix_cpu_timer_create(struct k_itimer *new_timer)
  */
 static int posix_cpu_timer_del(struct k_itimer *timer)
 {
-        struct task_struct *p = timer->it.cpu.task;
         int ret = 0;
+        unsigned long flags;
+        struct sighand_struct *sighand;
+        struct task_struct *p = timer->it.cpu.task;
 
-        if (likely(p != NULL)) {
-                read_lock(&tasklist_lock);
-                if (unlikely(p->sighand == NULL)) {
-                        /*
-                         * We raced with the reaping of the task.
-                         * The deletion should have cleared us off the list.
-                         */
-                        BUG_ON(!list_empty(&timer->it.cpu.entry));
-                } else {
-                        spin_lock(&p->sighand->siglock);
-                        if (timer->it.cpu.firing)
-                                ret = TIMER_RETRY;
-                        else
-                                list_del(&timer->it.cpu.entry);
-                        spin_unlock(&p->sighand->siglock);
-                }
-                read_unlock(&tasklist_lock);
+        WARN_ON_ONCE(p == NULL);
 
-                if (!ret)
-                        put_task_struct(p);
+        /*
+         * Protect against sighand release/switch in exit/exec and process/
+         * thread timer list entry concurrent read/writes.
+         */
+        sighand = lock_task_sighand(p, &flags);
+        if (unlikely(sighand == NULL)) {
+                /*
+                 * We raced with the reaping of the task.
+                 * The deletion should have cleared us off the list.
+                 */
+                WARN_ON_ONCE(!list_empty(&timer->it.cpu.entry));
+        } else {
+                if (timer->it.cpu.firing)
+                        ret = TIMER_RETRY;
+                else
+                        list_del(&timer->it.cpu.entry);
+
+                unlock_task_sighand(p, &flags);
         }
 
+        if (!ret)
+                put_task_struct(p);
+
         return ret;
 }
 
-static void cleanup_timers_list(struct list_head *head,
-                                unsigned long long curr)
+static void cleanup_timers_list(struct list_head *head)
 {
         struct cpu_timer_list *timer, *next;
 
@@ -414,16 +425,11 @@ static void cleanup_timers_list(struct list_head *head,
  * time for later timer_gettime calls to return.
  * This must be called with the siglock held.
  */
-static void cleanup_timers(struct list_head *head,
-                           cputime_t utime, cputime_t stime,
-                           unsigned long long sum_exec_runtime)
+static void cleanup_timers(struct list_head *head)
 {
-
-        cputime_t ptime = utime + stime;
-
-        cleanup_timers_list(head, cputime_to_expires(ptime));
-        cleanup_timers_list(++head, cputime_to_expires(utime));
-        cleanup_timers_list(++head, sum_exec_runtime);
+        cleanup_timers_list(head);
+        cleanup_timers_list(++head);
+        cleanup_timers_list(++head);
 }
 
 /*
@@ -433,41 +439,14 @@ static void cleanup_timers(struct list_head *head,
  */
 void posix_cpu_timers_exit(struct task_struct *tsk)
 {
-        cputime_t utime, stime;
-
         add_device_randomness((const void*) &tsk->se.sum_exec_runtime,
                                                 sizeof(unsigned long long));
-        task_cputime(tsk, &utime, &stime);
-        cleanup_timers(tsk->cpu_timers,
-                       utime, stime, tsk->se.sum_exec_runtime);
+        cleanup_timers(tsk->cpu_timers);
 
 }
 void posix_cpu_timers_exit_group(struct task_struct *tsk)
 {
-        struct signal_struct *const sig = tsk->signal;
-        cputime_t utime, stime;
-
-        task_cputime(tsk, &utime, &stime);
-        cleanup_timers(tsk->signal->cpu_timers,
-                       utime + sig->utime, stime + sig->stime,
-                       tsk->se.sum_exec_runtime + sig->sum_sched_runtime);
-}
-
-static void clear_dead_task(struct k_itimer *itimer, unsigned long long now)
-{
-        struct cpu_timer_list *timer = &itimer->it.cpu;
-
-        /*
-         * That's all for this thread or process.
-         * We leave our residual in expires to be reported.
-         */
-        put_task_struct(timer->task);
-        timer->task = NULL;
-        if (timer->expires < now) {
-                timer->expires = 0;
-        } else {
-                timer->expires -= now;
-        }
+        cleanup_timers(tsk->signal->cpu_timers);
 }
 
 static inline int expires_gt(cputime_t expires, cputime_t new_exp)
@@ -477,8 +456,7 @@ static inline int expires_gt(cputime_t expires, cputime_t new_exp)
 
 /*
  * Insert the timer on the appropriate list before any timers that
- * expire later.  This must be called with the tasklist_lock held
- * for reading, interrupts disabled and p->sighand->siglock taken.
+ * expire later.  This must be called with the sighand lock held.
  */
 static void arm_timer(struct k_itimer *timer)
 {
@@ -569,7 +547,8 @@ static void cpu_timer_fire(struct k_itimer *timer)
 
 /*
  * Sample a process (thread group) timer for the given group_leader task.
- * Must be called with tasklist_lock held for reading.
+ * Must be called with task sighand lock held for safe while_each_thread()
+ * traversal.
  */
 static int cpu_timer_sample_group(const clockid_t which_clock,
                                   struct task_struct *p,
@@ -632,43 +611,39 @@ static inline void posix_cpu_timer_kick_nohz(void) { }
  * If we return TIMER_RETRY, it's necessary to release the timer's lock
  * and try again.  (This happens when the timer is in the middle of firing.)
  */
-static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
+static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
                                struct itimerspec *new, struct itimerspec *old)
 {
+        unsigned long flags;
+        struct sighand_struct *sighand;
         struct task_struct *p = timer->it.cpu.task;
         unsigned long long old_expires, new_expires, old_incr, val;
         int ret;
 
-        if (unlikely(p == NULL)) {
-                /*
-                 * Timer refers to a dead task's clock.
-                 */
-                return -ESRCH;
-        }
+        WARN_ON_ONCE(p == NULL);
 
         new_expires = timespec_to_sample(timer->it_clock, &new->it_value);
 
-        read_lock(&tasklist_lock);
         /*
-         * We need the tasklist_lock to protect against reaping that
-         * clears p->sighand.  If p has just been reaped, we can no
+         * Protect against sighand release/switch in exit/exec and p->cpu_timers
+         * and p->signal->cpu_timers read/write in arm_timer()
+         */
+        sighand = lock_task_sighand(p, &flags);
+        /*
+         * If p has just been reaped, we can no
          * longer get any information about it at all.
          */
-        if (unlikely(p->sighand == NULL)) {
-                read_unlock(&tasklist_lock);
-                put_task_struct(p);
-                timer->it.cpu.task = NULL;
+        if (unlikely(sighand == NULL)) {
                 return -ESRCH;
         }
 
         /*
          * Disarm any old timer after extracting its expiry time.
          */
-        BUG_ON(!irqs_disabled());
+        WARN_ON_ONCE(!irqs_disabled());
 
         ret = 0;
         old_incr = timer->it.cpu.incr;
-        spin_lock(&p->sighand->siglock);
         old_expires = timer->it.cpu.expires;
         if (unlikely(timer->it.cpu.firing)) {
                 timer->it.cpu.firing = -1;
@@ -725,12 +700,11 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
                  * disable this firing since we are already reporting
                  * it as an overrun (thanks to bump_cpu_timer above).
                  */
-                spin_unlock(&p->sighand->siglock);
-                read_unlock(&tasklist_lock);
+                unlock_task_sighand(p, &flags);
                 goto out;
         }
 
-        if (new_expires != 0 && !(flags & TIMER_ABSTIME)) {
+        if (new_expires != 0 && !(timer_flags & TIMER_ABSTIME)) {
                 new_expires += val;
         }
 
@@ -744,9 +718,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
                 arm_timer(timer);
         }
 
-        spin_unlock(&p->sighand->siglock);
-        read_unlock(&tasklist_lock);
-
+        unlock_task_sighand(p, &flags);
         /*
          * Install the new reload setting, and
          * set up the signal and overrun bookkeeping.
@@ -788,7 +760,8 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 {
         unsigned long long now;
         struct task_struct *p = timer->it.cpu.task;
-        int clear_dead;
+
+        WARN_ON_ONCE(p == NULL);
 
         /*
          * Easy part: convert the reload time.
@@ -801,52 +774,34 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
                 return;
         }
 
-        if (unlikely(p == NULL)) {
-                /*
-                 * This task already died and the timer will never fire.
-                 * In this case, expires is actually the dead value.
-                 */
-        dead:
-                sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
-                                   &itp->it_value);
-                return;
-        }
-
         /*
          * Sample the clock to take the difference with the expiry time.
          */
         if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                 cpu_clock_sample(timer->it_clock, p, &now);
-                clear_dead = p->exit_state;
         } else {
-                read_lock(&tasklist_lock);
-                if (unlikely(p->sighand == NULL)) {
+                struct sighand_struct *sighand;
+                unsigned long flags;
+
+                /*
+                 * Protect against sighand release/switch in exit/exec and
+                 * also make timer sampling safe if it ends up calling
+                 * thread_group_cputime().
+                 */
+                sighand = lock_task_sighand(p, &flags);
+                if (unlikely(sighand == NULL)) {
                         /*
                          * The process has been reaped.
                          * We can't even collect a sample any more.
                          * Call the timer disarmed, nothing else to do.
                          */
-                        put_task_struct(p);
-                        timer->it.cpu.task = NULL;
                         timer->it.cpu.expires = 0;
-                        read_unlock(&tasklist_lock);
-                        goto dead;
+                        sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
+                                           &itp->it_value);
                 } else {
                         cpu_timer_sample_group(timer->it_clock, p, &now);
-                        clear_dead = (unlikely(p->exit_state) &&
-                                      thread_group_empty(p));
+                        unlock_task_sighand(p, &flags);
                 }
-                read_unlock(&tasklist_lock);
-        }
-
-        if (unlikely(clear_dead)) {
-                /*
-                 * We've noticed that the thread is dead, but
-                 * not yet reaped.  Take this opportunity to
-                 * drop our task ref.
-                 */
-                clear_dead_task(timer, now);
-                goto dead;
         }
 
         if (now < timer->it.cpu.expires) {
@@ -1060,14 +1015,12 @@ static void check_process_timers(struct task_struct *tsk,
  */
 void posix_cpu_timer_schedule(struct k_itimer *timer)
 {
+        struct sighand_struct *sighand;
+        unsigned long flags;
         struct task_struct *p = timer->it.cpu.task;
         unsigned long long now;
 
-        if (unlikely(p == NULL))
-                /*
-                 * The task was cleaned up already, no future firings.
-                 */
-                goto out;
+        WARN_ON_ONCE(p == NULL);
 
         /*
          * Fetch the current sample and update the timer's expiry time.
@@ -1075,52 +1028,45 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
         if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
                 cpu_clock_sample(timer->it_clock, p, &now);
                 bump_cpu_timer(timer, now);
-                if (unlikely(p->exit_state)) {
-                        clear_dead_task(timer, now);
+                if (unlikely(p->exit_state))
+                        goto out;
+
+                /* Protect timer list r/w in arm_timer() */
+                sighand = lock_task_sighand(p, &flags);
+                if (!sighand)
                         goto out;
-                }
-                read_lock(&tasklist_lock); /* arm_timer needs it.  */
-                spin_lock(&p->sighand->siglock);
         } else {
-                read_lock(&tasklist_lock);
-                if (unlikely(p->sighand == NULL)) {
+                /*
+                 * Protect arm_timer() and timer sampling in case of call to
+                 * thread_group_cputime().
+                 */
+                sighand = lock_task_sighand(p, &flags);
+                if (unlikely(sighand == NULL)) {
                         /*
                          * The process has been reaped.
                          * We can't even collect a sample any more.
                          */
-                        put_task_struct(p);
-                        timer->it.cpu.task = p = NULL;
                         timer->it.cpu.expires = 0;
-                        read_unlock(&tasklist_lock);
                         goto out;
                 } else if (unlikely(p->exit_state) && thread_group_empty(p)) {
-                        /*
-                         * We've noticed that the thread is dead, but
-                         * not yet reaped.  Take this opportunity to
-                         * drop our task ref.
-                         */
-                        cpu_timer_sample_group(timer->it_clock, p, &now);
-                        clear_dead_task(timer, now);
-                        read_unlock(&tasklist_lock);
+                        unlock_task_sighand(p, &flags);
+                        /* Optimizations: if the process is dying, no need to rearm */
                         goto out;
                 }
-                spin_lock(&p->sighand->siglock);
                 cpu_timer_sample_group(timer->it_clock, p, &now);
                 bump_cpu_timer(timer, now);
-                /* Leave the tasklist_lock locked for the call below.  */
+                /* Leave the sighand locked for the call below.  */
         }
 
         /*
          * Now re-arm for the new expiry time.
          */
-        BUG_ON(!irqs_disabled());
+        WARN_ON_ONCE(!irqs_disabled());
         arm_timer(timer);
-        spin_unlock(&p->sighand->siglock);
-        read_unlock(&tasklist_lock);
+        unlock_task_sighand(p, &flags);
 
         /* Kick full dynticks CPUs in case they need to tick on the new timer */
         posix_cpu_timer_kick_nohz();
-
 out:
         timer->it_overrun_last = timer->it_overrun;
         timer->it_overrun = -1;
@@ -1204,7 +1150,7 @@ void run_posix_cpu_timers(struct task_struct *tsk)
         struct k_itimer *timer, *next;
         unsigned long flags;
 
-        BUG_ON(!irqs_disabled());
+        WARN_ON_ONCE(!irqs_disabled());
 
         /*
          * The fast path checks that there are no expired thread or thread
@@ -1271,7 +1217,7 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
 {
         unsigned long long now;
 
-        BUG_ON(clock_idx == CPUCLOCK_SCHED);
+        WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
         cpu_timer_sample_group(clock_idx, tsk, &now);
 
         if (oldval) {