Messages in this thread Patch in this message | | | Date | Fri, 19 Mar 2010 19:40:17 +0100 | From | Oleg Nesterov <> | Subject | [PATCH 1/3] make task_struct->signal immutable/refcountable |
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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> ---
include/linux/sched.h | 2 +- kernel/exit.c | 3 --- kernel/fork.c | 23 ++++++++++++++++------- 3 files changed, 17 insertions(+), 11 deletions(-)
--- 34-rc1/include/linux/sched.h~8_IMMUTABLE_SIGNALL 2010-03-17 19:20:11.000000000 +0100 +++ 34-rc1/include/linux/sched.h 2010-03-19 17:32:44.000000000 +0100 @@ -518,6 +518,7 @@ struct thread_group_cputimer { * the locking of signal_struct. */ struct signal_struct { + atomic_t sigcnt; atomic_t count; atomic_t live; @@ -2100,7 +2101,6 @@ extern void flush_thread(void); extern void exit_thread(void); extern void exit_files(struct task_struct *); -extern void __cleanup_signal(struct signal_struct *); extern void __cleanup_sighand(struct sighand_struct *); extern void exit_itimers(struct signal_struct *); --- 34-rc1/kernel/exit.c~8_IMMUTABLE_SIGNALL 2010-03-18 22:46:41.000000000 +0100 +++ 34-rc1/kernel/exit.c 2010-03-19 17:25:36.000000000 +0100 @@ -135,8 +135,6 @@ static void __exit_signal(struct task_st * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals. */ flush_sigqueue(&tsk->pending); - - tsk->signal = NULL; tsk->sighand = NULL; spin_unlock(&sighand->siglock); @@ -151,7 +149,6 @@ static void __exit_signal(struct task_st */ task_rq_unlock_wait(tsk); tty_kref_put(sig->tty); - __cleanup_signal(sig); } } --- 34-rc1/kernel/fork.c~8_IMMUTABLE_SIGNALL 2010-03-18 22:45:14.000000000 +0100 +++ 34-rc1/kernel/fork.c 2010-03-19 17:51:51.000000000 +0100 @@ -158,6 +158,18 @@ void free_task(struct task_struct *tsk) } EXPORT_SYMBOL(free_task); +static inline void free_signal_struct(struct signal_struct *sig) +{ + thread_group_cputime_free(sig); + kmem_cache_free(signal_cachep, sig); +} + +static inline void put_signal_struct(struct signal_struct *sig) +{ + if (atomic_dec_and_test(&sig->sigcnt)) + free_signal_struct(sig); +} + void __put_task_struct(struct task_struct *tsk) { WARN_ON(!tsk->exit_state); @@ -166,6 +178,7 @@ void __put_task_struct(struct task_struc exit_creds(tsk); delayacct_tsk_free(tsk); + put_signal_struct(tsk->signal); if (!profile_handoff_task(tsk)) free_task(tsk); @@ -868,6 +881,7 @@ static int copy_signal(unsigned long clo if (!sig) return -ENOMEM; + atomic_set(&sig->sigcnt, 1); atomic_set(&sig->count, 1); atomic_set(&sig->live, 1); init_waitqueue_head(&sig->wait_chldexit); @@ -893,12 +907,6 @@ static int copy_signal(unsigned long clo return 0; } -void __cleanup_signal(struct signal_struct *sig) -{ - thread_group_cputime_free(sig); - kmem_cache_free(signal_cachep, sig); -} - static void copy_flags(unsigned long clone_flags, struct task_struct *p) { unsigned long new_flags = p->flags; @@ -1249,6 +1257,7 @@ static struct task_struct *copy_process( } if (clone_flags & CLONE_THREAD) { + atomic_inc(¤t->signal->sigcnt); atomic_inc(¤t->signal->count); atomic_inc(¤t->signal->live); p->group_leader = current->group_leader; @@ -1295,7 +1304,7 @@ bad_fork_cleanup_mm: mmput(p->mm); bad_fork_cleanup_signal: if (!(clone_flags & CLONE_THREAD)) - __cleanup_signal(p->signal); + free_signal_struct(p->signal); bad_fork_cleanup_sighand: __cleanup_sighand(p->sighand); bad_fork_cleanup_fs:
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