[PATCH] run_posix_cpu_timers: remove a bogus BUG_ON()

do_exit() clears ->it_##clock##_expires, but nothing prevents
another cpu to attach the timer to exiting process after that.
arm_timer() tries to protect against this race, but the check
is racy.

After exit_notify() does 'write_unlock_irq(&tasklist_lock)' and
before do_exit() calls 'schedule() local timer interrupt can find
tsk->exit_state != 0. If that state was EXIT_DEAD (or another cpu
does sys_wait4) interrupted task has ->signal == NULL.

At this moment exiting task has no pending cpu timers, they were
cleanuped in __exit_signal()->posix_cpu_timers_exit{,_group}(),
so we can just return from irq.

John Stultz recently confirmed this bug, see

	http://marc.theaimsgroup.com/?l=linux-kernel&m=115015841413687

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 18 insertions, 26 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index e95b93282210..e06d0c10a24e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -881,14 +881,6 @@ fastcall NORET_TYPE void do_exit(long code)
 
         tsk->flags |= PF_EXITING;
 
-        /*
-         * Make sure we don't try to process any timer firings
-         * while we are already exiting.
-         */
-        tsk->it_virt_expires = cputime_zero;
-        tsk->it_prof_expires = cputime_zero;
-        tsk->it_sched_expires = 0;
-
         if (unlikely(in_atomic()))
                 printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
                                 current->comm, current->pid,
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9d9169aa2e24..4882bf1e094a 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1288,30 +1288,30 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 
 #undef  UNEXPIRED
 
-        BUG_ON(tsk->exit_state);
-
         /*
          * Double-check with locks held.
          */
         read_lock(&tasklist_lock);
-        spin_lock(&tsk->sighand->siglock);
+        if (likely(tsk->signal != NULL)) {
+                spin_lock(&tsk->sighand->siglock);
 
-        /*
-         * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
-         * all the timers that are firing, and put them on the firing list.
-         */
-        check_thread_timers(tsk, &firing);
-        check_process_timers(tsk, &firing);
+                /*
+                 * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
+                 * all the timers that are firing, and put them on the firing list.
+                 */
+                check_thread_timers(tsk, &firing);
+                check_process_timers(tsk, &firing);
 
-        /*
-         * We must release these locks before taking any timer's lock.
-         * There is a potential race with timer deletion here, as the
-         * siglock now protects our private firing list.  We have set
-         * the firing flag in each timer, so that a deletion attempt
-         * that gets the timer lock before we do will give it up and
-         * spin until we've taken care of that timer below.
-         */
-        spin_unlock(&tsk->sighand->siglock);
+                /*
+                 * We must release these locks before taking any timer's lock.
+                 * There is a potential race with timer deletion here, as the
+                 * siglock now protects our private firing list.  We have set
+                 * the firing flag in each timer, so that a deletion attempt
+                 * that gets the timer lock before we do will give it up and
+                 * spin until we've taken care of that timer below.
+                 */
+                spin_unlock(&tsk->sighand->siglock);
+        }
         read_unlock(&tasklist_lock);
 
         /*