cpu-timers: Avoid iterating over all threads in fastpath_timer_check()

Spread p->sighand->siglock locking scope to make sure that
fastpath_timer_check() never iterates over all threads. Without
locking there is small possibility that signal->cputimer will stop
running while we write values to signal->cputime_expires.

Calling thread_group_cputime() from fastpath_timer_check() is not only
bad because it is slow, also it is racy with __exit_signal() which can
lead to invalid signal->{s,u}time values.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 7d9d0fab1651310f8a4294a418a1d2df4551b7ba..564b3b0240ddbe2170d61f8a3d920b6a09fe3cd5 100644 (file)
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -550,7 +550,7 @@ 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, and interrupts disabled.
+ * for reading, interrupts disabled and p->sighand->siglock taken.
  */
 static void arm_timer(struct k_itimer *timer)
 {
@@ -569,9 +569,6 @@ static void arm_timer(struct k_itimer *timer)
        }
        head += CPUCLOCK_WHICH(timer->it_clock);
 
-       BUG_ON(!irqs_disabled());
-       spin_lock(&p->sighand->siglock);
-
        listpos = head;
        list_for_each_entry(next, head, entry) {
                if (cpu_time_before(timer->it_clock, nt->expires, next->expires))
@@ -606,8 +603,6 @@ static void arm_timer(struct k_itimer *timer)
                        break;
                }
        }
-
-       spin_unlock(&p->sighand->siglock);
 }
 
 /*
@@ -720,7 +715,6 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
                ret = TIMER_RETRY;
        } else
                list_del_init(&timer->it.cpu.entry);
-       spin_unlock(&p->sighand->siglock);
 
        /*
         * We need to sample the current value to convert the new
@@ -774,6 +768,7 @@ 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);
                goto out;
        }
@@ -793,6 +788,7 @@ int posix_cpu_timer_set(struct k_itimer *timer, int flags,
                arm_timer(timer);
        }
 
+       spin_unlock(&p->sighand->siglock);
        read_unlock(&tasklist_lock);
 
        /*
@@ -1206,6 +1202,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
                        goto out;
                }
                read_lock(&tasklist_lock); /* arm_timer needs it.  */
+               spin_lock(&p->sighand->siglock);
        } else {
                read_lock(&tasklist_lock);
                if (unlikely(p->signal == NULL)) {
@@ -1226,6 +1223,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
                        clear_dead_task(timer, now);
                        goto out_unlock;
                }
+               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.  */
@@ -1234,7 +1232,9 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
        /*
         * Now re-arm for the new expiry time.
         */
+       BUG_ON(!irqs_disabled());
        arm_timer(timer);
+       spin_unlock(&p->sighand->siglock);
 
 out_unlock:
        read_unlock(&tasklist_lock);