This has been a requested feature for a long time [1][2]. /proc/<pid> and /proc/<tgid>/task/<pid> will show POLLIN | POLLRDNORM when the <pid> is dead and POLLIN | POLLRDNORM | POLLERR when <pid> is dead and reaped.
The ability to tell whether the task exists given an fd isn't new -- readdir can do it. The ability to distinguish live and zombie tasks by fd may have minor security implications. It's conceivable, although unlikely, that some existing software expects directories in /proc to always have POLLIN set. The benefit of using POLLIN instead of something like POLLPRI is feature detection -- checking /proc/self for POLLIN will reliably detect this feature. [1] http://lwn.net/Articles/462177/ [2] http://0pointer.de/blog/projects/plumbers-wishlist-3.html Signed-off-by: Andy Lutomirski <l...@amacapital.net> --- fs/proc/base.c | 26 ++++++++++++++++++++++++++ include/linux/init_task.h | 2 ++ include/linux/sched.h | 3 +++ kernel/exit.c | 3 +++ kernel/fork.c | 4 ++++ 5 files changed, 38 insertions(+) diff --git a/fs/proc/base.c b/fs/proc/base.c index 3c231ad..ebab7ec 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2737,9 +2737,34 @@ static int proc_tgid_base_readdir(struct file * filp, tgid_base_stuff,ARRAY_SIZE(tgid_base_stuff)); } +static unsigned int proc_task_base_poll(struct file *filp, + struct poll_table_struct *wait) +{ + struct task_struct *task; + int retval; + + task = get_proc_task(filp->f_path.dentry->d_inode); + if (!task) + return POLLIN | POLLRDNORM | POLLERR; + + read_lock(&tasklist_lock); + poll_wait(filp, &task->detach_wqh, wait); + if (task_is_dead(task)) { + retval = POLLIN | POLLRDNORM; + } else { + retval = 0; + poll_wait(filp, &task->exit_wqh, wait); + } + read_unlock(&tasklist_lock); + + put_task_struct(task); + return retval; +} + static const struct file_operations proc_tgid_base_operations = { .read = generic_read_dir, .readdir = proc_tgid_base_readdir, + .poll = proc_task_base_poll, .llseek = default_llseek, }; @@ -3110,6 +3135,7 @@ static struct dentry *proc_tid_base_lookup(struct inode *dir, struct dentry *den static const struct file_operations proc_tid_base_operations = { .read = generic_read_dir, .readdir = proc_tid_base_readdir, + .poll = proc_task_base_poll, .llseek = default_llseek, }; diff --git a/include/linux/init_task.h b/include/linux/init_task.h index 6d087c5..093379e 100644 --- a/include/linux/init_task.h +++ b/include/linux/init_task.h @@ -202,6 +202,8 @@ extern struct task_group root_task_group; [PIDTYPE_SID] = INIT_PID_LINK(PIDTYPE_SID), \ }, \ .thread_group = LIST_HEAD_INIT(tsk.thread_group), \ + .exit_wqh = __WAIT_QUEUE_HEAD_INITIALIZER(tsk.exit_wqh), \ + .detach_wqh = __WAIT_QUEUE_HEAD_INITIALIZER(tsk.detach_wqh),\ INIT_IDS \ INIT_PERF_EVENTS(tsk) \ INIT_TRACE_IRQFLAGS \ diff --git a/include/linux/sched.h b/include/linux/sched.h index 0dd42a0..6034a37 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1548,6 +1548,9 @@ struct task_struct { #ifdef CONFIG_UPROBES struct uprobe_task *utask; #endif + + /* These are woken with tasklist_lock held. */ + wait_queue_head_t exit_wqh, detach_wqh; }; /* Future-safe accessor for struct task_struct's cpus_allowed. */ diff --git a/kernel/exit.c b/kernel/exit.c index 346616c..01c584b 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -212,6 +212,7 @@ repeat: leader->exit_state = EXIT_DEAD; } + wake_up_all(&p->detach_wqh); write_unlock_irq(&tasklist_lock); release_thread(p); call_rcu(&p->rcu, delayed_put_task_struct); @@ -775,6 +776,8 @@ static void exit_notify(struct task_struct *tsk, int group_dead) /* mt-exec, de_thread() is waiting for group leader */ if (unlikely(tsk->signal->notify_count < 0)) wake_up_process(tsk->signal->group_exit_task); + + wake_up_all(&tsk->exit_wqh); write_unlock_irq(&tasklist_lock); /* If the process is dead, release it - nobody will wait for it */ diff --git a/kernel/fork.c b/kernel/fork.c index 8b20ab7..356b32c 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -235,6 +235,8 @@ void __put_task_struct(struct task_struct *tsk) WARN_ON(!tsk->exit_state); WARN_ON(atomic_read(&tsk->usage)); WARN_ON(tsk == current); + WARN_ON(waitqueue_active(&tsk->exit_wqh)); + WARN_ON(waitqueue_active(&tsk->detach_wqh)); security_task_free(tsk); exit_creds(tsk); @@ -1285,6 +1287,8 @@ static struct task_struct *copy_process(unsigned long clone_flags, p->memcg_batch.do_batch = 0; p->memcg_batch.memcg = NULL; #endif + init_waitqueue_head(&p->exit_wqh); + init_waitqueue_head(&p->detach_wqh); /* Perform scheduler related setup. Assign this task to a CPU. */ sched_fork(p); -- 1.7.11.7 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
