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signals: 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>
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>
This commit is contained in:
Oleg Nesterov 2010-05-26 14:43:16 -07:00 • committed by Linus Torvalds
commit ea6d290ca3
3 changed files with 17 additions and 11 deletions
Download patch file Download diff file Expand all files Collapse all files

2
include/linux/sched.h
View file

@ -527,6 +527,7 @@ struct thread_group_cputimer {
* the locking of signal_struct.
*/
struct signal_struct {
atomic_t sigcnt;
atomic_t count;
atomic_t live;
@ -2101,7 +2102,6 @@ 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 *);

3
kernel/exit.c
View file

@ -134,8 +134,6 @@ static void __exit_signal(struct task_struct *tsk)
* doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
*/
flush_sigqueue(&tsk->pending);
tsk->signal = NULL;
tsk->sighand = NULL;
spin_unlock(&sighand->siglock);
@ -150,7 +148,6 @@ static void __exit_signal(struct task_struct *tsk)
*/
task_rq_unlock_wait(tsk);
tty_kref_put(sig->tty);
__cleanup_signal(sig);
}
}

23
kernel/fork.c
View file

@ -165,6 +165,18 @@ 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);
@ -173,6 +185,7 @@ void __put_task_struct(struct task_struct *tsk)
exit_creds(tsk);
delayacct_tsk_free(tsk);
put_signal_struct(tsk->signal);
if (!profile_handoff_task(tsk))
free_task(tsk);
@ -864,6 +877,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
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);
@ -889,12 +903,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
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;
@ -1248,6 +1256,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
}
if (clone_flags & CLONE_THREAD) {
atomic_inc(&current->signal->sigcnt);
atomic_inc(&current->signal->count);
atomic_inc(&current->signal->live);
p->group_leader = current->group_leader;
@ -1294,7 +1303,7 @@ 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: