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signals: make task_struct->signal immutable/refcountable

We have a lot of problems with accessing task_struct->signal, it can "disappear" at any moment. Even current can't use its ->signal safely after exit_notify(). ->siglock helps, but it is not convenient, not always possible, and sometimes it makes sense to use task->signal even after this task has already dead. This patch adds the reference counter, sigcnt, into signal_struct. This reference is owned by task_struct and it is dropped in __put_task_struct(). Perhaps it makes sense to export get/put_signal_struct() later, but currently I don't see the immediate reason. Rename __cleanup_signal() to free_signal_struct() and unexport it. With the previous changes it does nothing except kmem_cache_free(). Change __exit_signal() to not clear/free ->signal, it will be freed when the last reference to any thread in the thread group goes away. Note: - when the last thead exits signal->tty can point to nowhere, see the next patch. - with or without this patch signal_struct->count should go away, or at least it should be "int nr_threads" for fs/proc. This will be addressed later.

Oleg Nesterov committed 16 years ago

Commit ea6d2901 parent 4dec2a93 files · +17 −11Browse files

Unified · Split

3 files
  1. include/linux/sched.h +1 −1
  2. kernel/exit.c +0 −3
  3. kernel/fork.c +16 −7
include/linux/sched.h+1−1
@@ -527,6 +527,7 @@ struct thread_group_cputimer { · 526 unmodified lines
527 * the locking of signal_struct.527 * the locking of signal_struct.
528 */528 */
529struct signal_struct {529struct signal_struct {
530 atomic_t sigcnt;
530 atomic_t count;531 atomic_t count;
531 atomic_t live;532 atomic_t live;
532 533
@@ -2101,7 +2102,6 @@ extern void flush_thread(void); · 1568 unmodified lines
2101extern void exit_thread(void);2102extern void exit_thread(void);
2102 2103
2103extern void exit_files(struct task_struct *);2104extern void exit_files(struct task_struct *);
2104extern void __cleanup_signal(struct signal_struct *);
2105extern void __cleanup_sighand(struct sighand_struct *);2105extern void __cleanup_sighand(struct sighand_struct *);
2106 2106
2107extern void exit_itimers(struct signal_struct *);2107extern void exit_itimers(struct signal_struct *);
kernel/exit.c+0−3
@@ -134,8 +134,6 @@ static void __exit_signal(struct task_struct *tsk) · 133 unmodified lines
134 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.134 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
135 */135 */
136 flush_sigqueue(&tsk->pending);136 flush_sigqueue(&tsk->pending);
137
138 tsk->signal = NULL;
139 tsk->sighand = NULL;137 tsk->sighand = NULL;
140 spin_unlock(&sighand->siglock);138 spin_unlock(&sighand->siglock);
141 139
@@ -150,7 +148,6 @@ static void __exit_signal(struct task_struct *tsk) · 8 unmodified lines
150 */148 */
151 task_rq_unlock_wait(tsk);149 task_rq_unlock_wait(tsk);
152 tty_kref_put(sig->tty);150 tty_kref_put(sig->tty);
153 __cleanup_signal(sig);
154 }151 }
155}152}
156 153
kernel/fork.c+16−7
@@ -165,6 +165,18 @@ void free_task(struct task_struct *tsk) · 164 unmodified lines
165}165}
166EXPORT_SYMBOL(free_task);166EXPORT_SYMBOL(free_task);
167 167
168static inline void free_signal_struct(struct signal_struct *sig)
169{
170 thread_group_cputime_free(sig);
171 kmem_cache_free(signal_cachep, sig);
172}
173
174static inline void put_signal_struct(struct signal_struct *sig)
175{
176 if (atomic_dec_and_test(&sig->sigcnt))
177 free_signal_struct(sig);
178}
179
168void __put_task_struct(struct task_struct *tsk)180void __put_task_struct(struct task_struct *tsk)
169{181{
170 WARN_ON(!tsk->exit_state);182 WARN_ON(!tsk->exit_state);
@@ -173,6 +185,7 @@ void __put_task_struct(struct task_struct *tsk) · 2 unmodified lines
173 185
174 exit_creds(tsk);186 exit_creds(tsk);
175 delayacct_tsk_free(tsk);187 delayacct_tsk_free(tsk);
188 put_signal_struct(tsk->signal);
176 189
177 if (!profile_handoff_task(tsk))190 if (!profile_handoff_task(tsk))
178 free_task(tsk);191 free_task(tsk);
@@ -864,6 +877,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) · 685 unmodified lines
864 if (!sig)877 if (!sig)
865 return -ENOMEM;878 return -ENOMEM;
866 879
880 atomic_set(&sig->sigcnt, 1);
867 atomic_set(&sig->count, 1);881 atomic_set(&sig->count, 1);
868 atomic_set(&sig->live, 1);882 atomic_set(&sig->live, 1);
869 init_waitqueue_head(&sig->wait_chldexit);883 init_waitqueue_head(&sig->wait_chldexit);
@@ -889,12 +903,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) · 19 unmodified lines
889 return 0;903 return 0;
890}904}
891 905
892void __cleanup_signal(struct signal_struct *sig)
893{
894 thread_group_cputime_free(sig);
895 kmem_cache_free(signal_cachep, sig);
896}
897
898static void copy_flags(unsigned long clone_flags, struct task_struct *p)906static void copy_flags(unsigned long clone_flags, struct task_struct *p)
899{907{
900 unsigned long new_flags = p->flags;908 unsigned long new_flags = p->flags;
@@ -1248,6 +1256,7 @@ static struct task_struct *copy_process(unsigned long clone_flags, · 347 unmodified lines
1248 }1256 }
1249 1257
1250 if (clone_flags & CLONE_THREAD) {1258 if (clone_flags & CLONE_THREAD) {
1259 atomic_inc(&current->signal->sigcnt);
1251 atomic_inc(&current->signal->count);1260 atomic_inc(&current->signal->count);
1252 atomic_inc(&current->signal->live);1261 atomic_inc(&current->signal->live);
1253 p->group_leader = current->group_leader;1262 p->group_leader = current->group_leader;
@@ -1294,7 +1303,7 @@ bad_fork_cleanup_mm: · 40 unmodified lines
1294 mmput(p->mm);1303 mmput(p->mm);
1295bad_fork_cleanup_signal:1304bad_fork_cleanup_signal:
1296 if (!(clone_flags & CLONE_THREAD))1305 if (!(clone_flags & CLONE_THREAD))
1297 __cleanup_signal(p->signal);1306 free_signal_struct(p->signal);
1298bad_fork_cleanup_sighand:1307bad_fork_cleanup_sighand:
1299 __cleanup_sighand(p->sighand);1308 __cleanup_sighand(p->sighand);
1300bad_fork_cleanup_fs:1309bad_fork_cleanup_fs: