pid: Implement PIDTYPE_TGID

Everywhere except in the pid array we distinguish between a tasks pid and
a tasks tgid (thread group id).  Even in the enumeration we want that
distinction sometimes so we have added __PIDTYPE_TGID.  With leader_pid
we almost have an implementation of PIDTYPE_TGID in struct signal_struct.

Add PIDTYPE_TGID as a first class member of the pid_type enumeration and
into the pids array.  Then remove the __PIDTYPE_TGID special case and the
leader_pid in signal_struct.

The net size increase is just an extra pointer added to struct pid and
an extra pair of pointers of an hlist_node added to task_struct.

The effect on code maintenance is the removal of a number of special
cases today and the potential to remove many more special cases as
PIDTYPE_TGID gets used to it's fullest.  The long term potential
is allowing zombie thread group leaders to exit, which will remove
a lot more special cases in the code.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

arch/ia64/kernel/asm-offsets.c

@@ -67,7 +67,7 @@ void foo(void)
 	DEFINE(IA64_SIGNAL_GROUP_STOP_COUNT_OFFSET,offsetof (struct signal_struct,
 							     group_stop_count));
 	DEFINE(IA64_SIGNAL_SHARED_PENDING_OFFSET,offsetof (struct signal_struct, shared_pending));
-	DEFINE(IA64_SIGNAL_LEADER_PID_OFFSET, offsetof (struct signal_struct, leader_pid));
+	DEFINE(IA64_SIGNAL_PIDS_TGID_OFFSET, offsetof (struct signal_struct, pids[PIDTYPE_TGID]));
 
 	BLANK();
 

arch/ia64/kernel/fsys.S

@@ -68,10 +68,10 @@ ENTRY(fsys_getpid)
 	add r9=TI_FLAGS+IA64_TASK_SIZE,r16
 	;;
 	ld4 r9=[r9]
-	add r17=IA64_SIGNAL_LEADER_PID_OFFSET,r17
+	add r17=IA64_SIGNAL_PIDS_TGID_OFFSET,r17
 	;;
 	and r9=TIF_ALLWORK_MASK,r9
-	ld8 r17=[r17]				// r17 = current->signal->leader_pid
+	ld8 r17=[r17]				// r17 = current->signal->pids[PIDTYPE_TGID]
 	;;
 	add r8=IA64_PID_LEVEL_OFFSET,r17
 	;;

arch/s390/kernel/perf_cpum_sf.c

@@ -665,7 +665,7 @@ static void cpumsf_output_event_pid(struct perf_event *event,
 		goto out;
 
 	/* Update the process ID (see also kernel/events/core.c) */
-	data->tid_entry.pid = cpumsf_pid_type(event, pid, __PIDTYPE_TGID);
+	data->tid_entry.pid = cpumsf_pid_type(event, pid, PIDTYPE_TGID);
 	data->tid_entry.tid = cpumsf_pid_type(event, pid, PIDTYPE_PID);
 
 	perf_output_sample(&handle, &header, data, event);

fs/exec.c

@@ -1146,6 +1146,7 @@ static int de_thread(struct task_struct *tsk)
 		 */
 		tsk->pid = leader->pid;
 		change_pid(tsk, PIDTYPE_PID, task_pid(leader));
+		transfer_pid(leader, tsk, PIDTYPE_TGID);
 		transfer_pid(leader, tsk, PIDTYPE_PGID);
 		transfer_pid(leader, tsk, PIDTYPE_SID);
 

include/linux/pid.h

@@ -7,11 +7,10 @@
 enum pid_type
 {
 	PIDTYPE_PID,
+	PIDTYPE_TGID,
 	PIDTYPE_PGID,
 	PIDTYPE_SID,
 	PIDTYPE_MAX,
-	/* only valid to __task_pid_nr_ns() */
-	__PIDTYPE_TGID
 };
 
 /*

include/linux/sched.h

@@ -1275,12 +1275,12 @@ static inline pid_t task_session_vnr(struct task_struct *tsk)
 
 static inline pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 {
-	return __task_pid_nr_ns(tsk, __PIDTYPE_TGID, ns);
+	return __task_pid_nr_ns(tsk, PIDTYPE_TGID, ns);
 }
 
 static inline pid_t task_tgid_vnr(struct task_struct *tsk)
 {
-	return __task_pid_nr_ns(tsk, __PIDTYPE_TGID, NULL);
+	return __task_pid_nr_ns(tsk, PIDTYPE_TGID, NULL);
 }
 
 static inline pid_t task_ppid_nr_ns(const struct task_struct *tsk, struct pid_namespace *ns)

include/linux/sched/signal.h

@@ -147,7 +147,6 @@ struct signal_struct {
 #endif
 
 	/* PID/PID hash table linkage. */
-	struct pid *leader_pid;
 	struct pid *pids[PIDTYPE_MAX];
 
 #ifdef CONFIG_NO_HZ_FULL
@@ -571,7 +570,7 @@ struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 
 static inline struct pid *task_tgid(struct task_struct *task)
 {
-	return task->signal->leader_pid;
+	return task->signal->pids[PIDTYPE_TGID];
 }
 
 /*
@@ -607,7 +606,7 @@ static inline bool thread_group_leader(struct task_struct *p)
  */
 static inline bool has_group_leader_pid(struct task_struct *p)
 {
-	return task_pid(p) == p->signal->leader_pid;
+	return task_pid(p) == task_tgid(p);
 }
 
 static inline

init/init_task.c

@@ -33,9 +33,9 @@ static struct signal_struct init_signals = {
 	},
 #endif
 	INIT_CPU_TIMERS(init_signals)
-	.leader_pid = &init_struct_pid,
 	.pids = {
 		[PIDTYPE_PID]	= &init_struct_pid,
+		[PIDTYPE_TGID]	= &init_struct_pid,
 		[PIDTYPE_PGID]	= &init_struct_pid,
 		[PIDTYPE_SID]	= &init_struct_pid,
 	},

kernel/events/core.c

@@ -1334,7 +1334,7 @@ static u32 perf_event_pid_type(struct perf_event *event, struct task_struct *p,
 
 static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
 {
-	return perf_event_pid_type(event, p, __PIDTYPE_TGID);
+	return perf_event_pid_type(event, p, PIDTYPE_TGID);
 }
 
 static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)

kernel/exit.c

@@ -73,6 +73,7 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
 	nr_threads--;
 	detach_pid(p, PIDTYPE_PID);
 	if (group_dead) {
+		detach_pid(p, PIDTYPE_TGID);
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
 

kernel/fork.c

@@ -1946,6 +1946,7 @@ static __latent_entropy struct task_struct *copy_process(
 
 		init_task_pid(p, PIDTYPE_PID, pid);
 		if (thread_group_leader(p)) {
+			init_task_pid(p, PIDTYPE_TGID, pid);
 			init_task_pid(p, PIDTYPE_PGID, task_pgrp(current));
 			init_task_pid(p, PIDTYPE_SID, task_session(current));
 
@@ -1954,7 +1955,6 @@ static __latent_entropy struct task_struct *copy_process(
 				p->signal->flags |= SIGNAL_UNKILLABLE;
 			}
 
-			p->signal->leader_pid = pid;
 			p->signal->tty = tty_kref_get(current->signal->tty);
 			/*
 			 * Inherit has_child_subreaper flag under the same
@@ -1965,6 +1965,7 @@ static __latent_entropy struct task_struct *copy_process(
 							 p->real_parent->signal->is_child_subreaper;
 			list_add_tail(&p->sibling, &p->real_parent->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
+			attach_pid(p, PIDTYPE_TGID);
 			attach_pid(p, PIDTYPE_PGID);
 			attach_pid(p, PIDTYPE_SID);
 			__this_cpu_inc(process_counts);

kernel/pid.c

@@ -269,8 +269,6 @@ static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
 {
 	return (type == PIDTYPE_PID) ?
 		&task->thread_pid :
-		(type == __PIDTYPE_TGID) ?
-		&task->signal->leader_pid :
 		&task->signal->pids[type];
 }
 

kernel/time/itimer.c

@@ -139,9 +139,10 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
 {
 	struct signal_struct *sig =
 		container_of(timer, struct signal_struct, real_timer);
+	struct pid *leader_pid = sig->pids[PIDTYPE_TGID];
 
-	trace_itimer_expire(ITIMER_REAL, sig->leader_pid, 0);
-	kill_pid_info(SIGALRM, SEND_SIG_PRIV, sig->leader_pid);
+	trace_itimer_expire(ITIMER_REAL, leader_pid, 0);
+	kill_pid_info(SIGALRM, SEND_SIG_PRIV, leader_pid);
 
 	return HRTIMER_NORESTART;
 }

kernel/time/posix-cpu-timers.c

@@ -895,7 +895,7 @@ static void check_cpu_itimer(struct task_struct *tsk, struct cpu_itimer *it,
 
 		trace_itimer_expire(signo == SIGPROF ?
 				    ITIMER_PROF : ITIMER_VIRTUAL,
-				    tsk->signal->leader_pid, cur_time);
+				    task_tgid(tsk), cur_time);
 		__group_send_sig_info(signo, SEND_SIG_PRIV, tsk);
 	}