[PATCH] copy_process: cleanup bad_fork_cleanup_signal

__exit_signal() does important cleanups atomically under ->siglock.  It is
also called from copy_process's error path.  This is not good, for example we
can't move __unhash_process() under ->siglock for that reason.

We should not mix these 2 paths, just look at ugly 'if (p->sighand)' under
'bad_fork_cleanup_sighand:' label.  For copy_process() case it is sufficient
to just backout copy_signal(), nothing more.

Again, nobody can see this task yet.  For CLONE_THREAD case we just decrement
signal->count, otherwise nobody can see this ->signal and we can free it
lockless.

This patch assumes it is safe to do exit_thread_group_keys() without
tasklist_lock.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -1149,7 +1149,7 @@ extern void flush_thread(void);
 extern void exit_thread(void);
 
 extern void exit_files(struct task_struct *);
-extern void exit_signal(struct task_struct *);
+extern void __cleanup_signal(struct signal_struct *);
 extern void __exit_signal(struct task_struct *);
 extern void __exit_sighand(struct task_struct *);
 extern void exit_itimers(struct signal_struct *);

include/linux/slab.h

@@ -210,7 +210,6 @@ extern kmem_cache_t	*names_cachep;
 extern kmem_cache_t	*files_cachep;
 extern kmem_cache_t	*filp_cachep;
 extern kmem_cache_t	*fs_cachep;
-extern kmem_cache_t	*signal_cachep;
 extern kmem_cache_t	*sighand_cachep;
 extern kmem_cache_t	*bio_cachep;
 

kernel/fork.c

@@ -84,7 +84,7 @@ static kmem_cache_t *task_struct_cachep;
 #endif
 
 /* SLAB cache for signal_struct structures (tsk->signal) */
-kmem_cache_t *signal_cachep;
+static kmem_cache_t *signal_cachep;
 
 /* SLAB cache for sighand_struct structures (tsk->sighand) */
 kmem_cache_t *sighand_cachep;
@@ -872,6 +872,22 @@ static inline int copy_signal(unsigned long clone_flags, struct task_struct * ts
 	return 0;
 }
 
+void __cleanup_signal(struct signal_struct *sig)
+{
+	exit_thread_group_keys(sig);
+	kmem_cache_free(signal_cachep, sig);
+}
+
+static inline void cleanup_signal(struct task_struct *tsk)
+{
+	struct signal_struct *sig = tsk->signal;
+
+	atomic_dec(&sig->live);
+
+	if (atomic_dec_and_test(&sig->count))
+		__cleanup_signal(sig);
+}
+
 static inline void copy_flags(unsigned long clone_flags, struct task_struct *p)
 {
 	unsigned long new_flags = p->flags;
@@ -1206,10 +1222,9 @@ bad_fork_cleanup_mm:
 	if (p->mm)
 		mmput(p->mm);
 bad_fork_cleanup_signal:
-	exit_signal(p);
+	cleanup_signal(p);
 bad_fork_cleanup_sighand:
-	if (p->sighand)
-		__exit_sighand(p);
+	__exit_sighand(p);
 bad_fork_cleanup_fs:
 	exit_fs(p); /* blocking */
 bad_fork_cleanup_files:

kernel/signal.c

@@ -395,23 +395,10 @@ void __exit_signal(struct task_struct *tsk)
 	clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
 	flush_sigqueue(&tsk->pending);
 	if (sig) {
-		/*
-		 * We are cleaning up the signal_struct here.
-		 */
-		exit_thread_group_keys(sig);
-		kmem_cache_free(signal_cachep, sig);
+		__cleanup_signal(sig);
 	}
 }
 
-void exit_signal(struct task_struct *tsk)
-{
-	atomic_dec(&tsk->signal->live);
-
-	write_lock_irq(&tasklist_lock);
-	__exit_signal(tsk);
-	write_unlock_irq(&tasklist_lock);
-}
-
 /*
  * Flush all handlers for a task.
  */