[PATCH] proc: convert do_task_stat() to use lock_task_sighand()

Drop tasklist_lock. ->siglock protects almost all interesting data
(including sub-threads traversal) except:

	->signal->tty
		protected by tty_mutex

	->real_parent
		the task can't be unhashed while we are holding
		->siglock, so ->real_parent can change from under us
		but we can safely dereference it under rcu_read_lock()

	->pgrp/->session
		we can get inconsistent numbers if the task does
		sys_setsid/daemonize at the same time. I hope this
		is acceptable.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 35 insertions, 28 deletions

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 279fbf542c88..35bd39dac247 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -321,7 +321,7 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
         sigset_t sigign, sigcatch;
         char state;
         int res;
-        pid_t ppid, pgid = -1, sid = -1;
+        pid_t ppid = 0, pgid = -1, sid = -1;
         int num_threads = 0;
         struct mm_struct *mm;
         unsigned long long start_time;
@@ -329,8 +329,8 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
         unsigned long  min_flt = 0,  maj_flt = 0;
         cputime_t cutime, cstime, utime, stime;
         unsigned long rsslim = 0;
-        struct task_struct *t;
         char tcomm[sizeof(task->comm)];
+        unsigned long flags;
 
         state = *get_task_state(task);
         vsize = eip = esp = 0;
@@ -348,15 +348,33 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
         cutime = cstime = utime = stime = cputime_zero;
 
         mutex_lock(&tty_mutex);
-        read_lock(&tasklist_lock);
-        if (task->sighand) {
-                spin_lock_irq(&task->sighand->siglock);
-                num_threads = atomic_read(&task->signal->count);
+        rcu_read_lock();
+        if (lock_task_sighand(task, &flags)) {
+                struct signal_struct *sig = task->signal;
+                struct tty_struct *tty = sig->tty;
+
+                if (tty) {
+                        /*
+                         * sig->tty is not stable, but tty_mutex
+                         * protects us from release_dev(tty)
+                         */
+                        barrier();
+                        tty_pgrp = tty->pgrp;
+                        tty_nr = new_encode_dev(tty_devnum(tty));
+                }
+
+                num_threads = atomic_read(&sig->count);
                 collect_sigign_sigcatch(task, &sigign, &sigcatch);
 
+                cmin_flt = sig->cmin_flt;
+                cmaj_flt = sig->cmaj_flt;
+                cutime = sig->cutime;
+                cstime = sig->cstime;
+                rsslim = sig->rlim[RLIMIT_RSS].rlim_cur;
+
                 /* add up live thread stats at the group level */
                 if (whole) {
-                        t = task;
+                        struct task_struct *t = task;
                         do {
                                 min_flt += t->min_flt;
                                 maj_flt += t->maj_flt;
@@ -364,31 +382,20 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
                                 stime = cputime_add(stime, t->stime);
                                 t = next_thread(t);
                         } while (t != task);
-                }
 
-                spin_unlock_irq(&task->sighand->siglock);
-        }
-        if (task->signal) {
-                if (task->signal->tty) {
-                        tty_pgrp = task->signal->tty->pgrp;
-                        tty_nr = new_encode_dev(tty_devnum(task->signal->tty));
+                        min_flt += sig->min_flt;
+                        maj_flt += sig->maj_flt;
+                        utime = cputime_add(utime, sig->utime);
+                        stime = cputime_add(stime, sig->stime);
                 }
+
+                sid = sig->session;
                 pgid = process_group(task);
-                sid = task->signal->session;
-                cmin_flt = task->signal->cmin_flt;
-                cmaj_flt = task->signal->cmaj_flt;
-                cutime = task->signal->cutime;
-                cstime = task->signal->cstime;
-                rsslim = task->signal->rlim[RLIMIT_RSS].rlim_cur;
-                if (whole) {
-                        min_flt += task->signal->min_flt;
-                        maj_flt += task->signal->maj_flt;
-                        utime = cputime_add(utime, task->signal->utime);
-                        stime = cputime_add(stime, task->signal->stime);
-                }
+                ppid = rcu_dereference(task->real_parent)->tgid;
+
+                unlock_task_sighand(task, &flags);
         }
-        ppid = pid_alive(task) ? task->group_leader->real_parent->tgid : 0;
-        read_unlock(&tasklist_lock);
+        rcu_read_unlock();
         mutex_unlock(&tty_mutex);
 
         if (!whole || num_threads<2)