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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. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Cc: Alan Cox <alan@linux.intel.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <peterz@infradead.org> Acked-by: Roland McGrath <roland@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -527,6 +527,7 @@ struct thread_group_cputimer {
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* the locking of signal_struct.
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*/
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struct signal_struct {
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atomic_t sigcnt;
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atomic_t count;
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atomic_t live;
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@ -2101,7 +2102,6 @@ extern void flush_thread(void);
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extern void exit_thread(void);
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extern void exit_files(struct task_struct *);
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extern void __cleanup_signal(struct signal_struct *);
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extern void __cleanup_sighand(struct sighand_struct *);
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extern void exit_itimers(struct signal_struct *);
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@ -134,8 +134,6 @@ static void __exit_signal(struct task_struct *tsk)
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* doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
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*/
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flush_sigqueue(&tsk->pending);
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tsk->signal = NULL;
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tsk->sighand = NULL;
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spin_unlock(&sighand->siglock);
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@ -150,7 +148,6 @@ static void __exit_signal(struct task_struct *tsk)
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*/
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task_rq_unlock_wait(tsk);
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tty_kref_put(sig->tty);
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__cleanup_signal(sig);
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}
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}
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@ -165,6 +165,18 @@ void free_task(struct task_struct *tsk)
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}
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EXPORT_SYMBOL(free_task);
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static inline void free_signal_struct(struct signal_struct *sig)
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{
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thread_group_cputime_free(sig);
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kmem_cache_free(signal_cachep, sig);
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}
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static inline void put_signal_struct(struct signal_struct *sig)
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{
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if (atomic_dec_and_test(&sig->sigcnt))
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free_signal_struct(sig);
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}
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void __put_task_struct(struct task_struct *tsk)
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{
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WARN_ON(!tsk->exit_state);
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@ -173,6 +185,7 @@ void __put_task_struct(struct task_struct *tsk)
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exit_creds(tsk);
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delayacct_tsk_free(tsk);
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put_signal_struct(tsk->signal);
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if (!profile_handoff_task(tsk))
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free_task(tsk);
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@ -864,6 +877,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
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if (!sig)
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return -ENOMEM;
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atomic_set(&sig->sigcnt, 1);
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atomic_set(&sig->count, 1);
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atomic_set(&sig->live, 1);
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init_waitqueue_head(&sig->wait_chldexit);
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@ -889,12 +903,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
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return 0;
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}
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void __cleanup_signal(struct signal_struct *sig)
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{
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thread_group_cputime_free(sig);
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kmem_cache_free(signal_cachep, sig);
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}
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static void copy_flags(unsigned long clone_flags, struct task_struct *p)
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{
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unsigned long new_flags = p->flags;
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@ -1248,6 +1256,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
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}
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if (clone_flags & CLONE_THREAD) {
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atomic_inc(¤t->signal->sigcnt);
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atomic_inc(¤t->signal->count);
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atomic_inc(¤t->signal->live);
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p->group_leader = current->group_leader;
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@ -1294,7 +1303,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
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mmput(p->mm);
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bad_fork_cleanup_signal:
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if (!(clone_flags & CLONE_THREAD))
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__cleanup_signal(p->signal);
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free_signal_struct(p->signal);
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bad_fork_cleanup_sighand:
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__cleanup_sighand(p->sighand);
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bad_fork_cleanup_fs:
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