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[209.132.180.67]) by mx.google.com with ESMTP id 11-v6si3348051pfx.102.2018.10.29.12.23.48; Mon, 29 Oct 2018 12:24:04 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=bKF3jkV2; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726071AbeJ3ENX (ORCPT + 99 others); Tue, 30 Oct 2018 00:13:23 -0400 Received: from mail-qk1-f201.google.com ([209.85.222.201]:54364 "EHLO mail-qk1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725858AbeJ3ENX (ORCPT ); Tue, 30 Oct 2018 00:13:23 -0400 Received: by mail-qk1-f201.google.com with SMTP id u20-v6so10775274qka.21 for ; Mon, 29 Oct 2018 12:23:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=TDgDqU+PRPI+tahYx8UWJqG//Ge56nmot8k6iipA9H0=; b=bKF3jkV2bMLYAIhLUStwlYI1TDV6+Ihe5lXQCdme10cQw7cDVr+dK58PAG7rzxBJtz vvbKDE4dMF+mZIrxVdwXhQ3MbKpgiz1WBth/osj4pXOzW5TweXFuzGd8VyEgWwat1gf9 306Y+xvOy3ejUXfvdgtHllza2MUhZ02zCpQ1eZo8ib3I8l77v3lKi02ke4k8+QREUxZQ 9ywBdVfF2JWqFol4jFGxbgEEUaCEnB2//VBolU0tET/CGFsVK+VY3/ONTF4KMx1zO3a7 +gWgIFozshUhamW3n0wIPVX90ug6+4+siFx34YDScimrevDlKuHaY7y6blWgFtuQ8DON lsIg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=TDgDqU+PRPI+tahYx8UWJqG//Ge56nmot8k6iipA9H0=; b=GzqC2e4L5L6LmIwgAxPVIYGsB6sNNzzJY9FhR8Ig1AM/wAi9puGLXn59qChakFKStq BHTYVQuSyIy/vCvPsN4k2KQ45RZqoXRkxB6q7fBHubPo2ELa8Op1RDOGzroKFaxiLN3P fD3XZK2/2sM6XSGWZICHLeMHjgqucnibDx4ZXxrns/E+QdtLytEC/NEt9Z5t2GBPkAsT Y4YxJ9DfpcMtgjh8NhcEXIGyXme2PxBOtQaUUH4KrRnOTjkkVsH/ACUVVMi04faHdq9v F4e54b2liQuCbfGoycZ+uHrP+htoe2PWo9jU52VKd2v96s5iVazmmvsl3DHvLg2gtdZ8 WtZw== X-Gm-Message-State: AGRZ1gKjZZ1QXEeQITZ/b7BWcaVZHN1BadLiv/fzzzLoLVtjWnzo42zD 8v0YP8gd5Je/Qig/4H1Br4aFlooZP+CZXFkkv4FhCWAOjCcZSZ1gFcZ7RB5TZwYIT5qZjAFDU4T NB8NM+HUgovuLwC4H3Qkf0BHT97g/L+zaUNmYyohbn3Dh6hPSyHO2l7f37/IkW1JbiBaXOg== X-Received: by 2002:a37:ba44:: with SMTP id k65-v6mr12923911qkf.49.1540841000223; Mon, 29 Oct 2018 12:23:20 -0700 (PDT) Date: Mon, 29 Oct 2018 19:22:50 +0000 In-Reply-To: <20181029175322.189042-1-dancol@google.com> Message-Id: <20181029192250.130551-1-dancol@google.com> Mime-Version: 1.0 References: <20181029175322.189042-1-dancol@google.com> X-Mailer: git-send-email 2.19.1.568.g152ad8e336-goog Subject: [RFC PATCH v2] Minimal non-child process exit notification support From: Daniel Colascione To: linux-kernel@vger.kernel.org Cc: timmurray@google.com, joelaf@google.com, Daniel Colascione Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This patch adds a new file under /proc/pid, /proc/pid/exithand. Attempting to read from an exithand file will block until the corresponding process exits, at which point the read will successfully complete with EOF. The file descriptor supports both blocking operations and poll(2). It's intended to be a minimal interface for allowing a program to wait for the exit of a process that is not one of its children. Why might we want this interface? Android's lmkd kills processes in order to free memory in response to various memory pressure signals. It's desirable to wait until a killed process actually exits before moving on (if needed) to killing the next process. Since the processes that lmkd kills are not lmkd's children, lmkd currently lacks a way to wait for a process to actually die after being sent SIGKILL; today, lmkd resorts to polling the proc filesystem pid entry. This interface allow lmkd to give up polling and instead block and wait for process death. Signed-off-by: Daniel Colascione --- fs/proc/Makefile | 1 + fs/proc/base.c | 1 + fs/proc/exithand.c | 128 +++++++++++++++++++++++++++++++++++ fs/proc/internal.h | 4 ++ include/linux/sched/signal.h | 8 +++ kernel/exit.c | 2 + kernel/signal.c | 3 + 7 files changed, 147 insertions(+) create mode 100644 fs/proc/exithand.c diff --git a/fs/proc/Makefile b/fs/proc/Makefile index ead487e80510..21322280a2c1 100644 --- a/fs/proc/Makefile +++ b/fs/proc/Makefile @@ -27,6 +27,7 @@ proc-y += softirqs.o proc-y += namespaces.o proc-y += self.o proc-y += thread_self.o +proc-y += exithand.o proc-$(CONFIG_PROC_SYSCTL) += proc_sysctl.o proc-$(CONFIG_NET) += proc_net.o proc-$(CONFIG_PROC_KCORE) += kcore.o diff --git a/fs/proc/base.c b/fs/proc/base.c index 7e9f07bf260d..31bc6bbb6dc4 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -3006,6 +3006,7 @@ static const struct pid_entry tgid_base_stuff[] = { #ifdef CONFIG_LIVEPATCH ONE("patch_state", S_IRUSR, proc_pid_patch_state), #endif + REG("exithand", S_IRUGO, proc_tgid_exithand_operations), }; static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx) diff --git a/fs/proc/exithand.c b/fs/proc/exithand.c new file mode 100644 index 000000000000..c2ac31c52ff9 --- /dev/null +++ b/fs/proc/exithand.c @@ -0,0 +1,128 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Synchronous exit notification of non-child processes + * + * Simple file descriptor /proc/pid/exithand. Read blocks (and poll + * reports non-readable) until process either dies or becomes + * a zombie. + */ +#include +#include +#include +#include "internal.h" + +static int proc_tgid_exithand_open(struct inode *inode, struct file *file) +{ + /* If get_proc_task fails, it means the task is dead, which is + * fine, since a subsequent read will return immediately and + * indicate that, yes, the indicated process is dead. + */ + int res = 0; + struct task_struct *task = get_proc_task(inode); + + if (task) { + if (!thread_group_leader(task)) + res = -EINVAL; + put_task_struct(task); + } + return res; +} + +static ssize_t proc_tgid_exithand_read(struct file *file, + char __user *buf, + size_t count, loff_t *ppos) +{ + struct task_struct *task = NULL; + wait_queue_entry_t wait; + ssize_t res = 0; + bool locked = false; + + for (;;) { + /* Retrieve the task from the struct pid each time + * through the loop in case the exact struct task + * changes underneath us (e.g., if in exec.c, the + * execing process kills the group leader and starts + * using its PID). The struct signal should be the + * same though even in this case. + */ + task = get_proc_task(file_inode(file)); + res = 0; + if (!task) + goto out; /* No task? Must have died. */ + + BUG_ON(!thread_group_leader(task)); + + /* Synchronizes with exit.c machinery. */ + read_lock(&tasklist_lock); + locked = true; + + res = 0; + if (task->exit_state) + goto out; + + res = -EAGAIN; + if (file->f_flags & O_NONBLOCK) + goto out; + + /* Tell exit.c to go to the trouble of waking our + * runqueue when this process gets around to + * exiting. + */ + task->signal->exithand_is_interested = true; + + /* Even if the task identity changes, task->signal + * should be invariant across the wait, making it safe + * to go remove our wait record from the wait queue + * after we come back from schedule. + */ + + init_waitqueue_entry(&wait, current); + add_wait_queue(&wait_exithand, &wait); + + read_unlock(&tasklist_lock); + locked = false; + + put_task_struct(task); + task = NULL; + + set_current_state(TASK_INTERRUPTIBLE); + schedule(); + set_current_state(TASK_RUNNING); + remove_wait_queue(&wait_exithand, &wait); + + res = -ERESTARTSYS; + if (signal_pending(current)) + goto out; + } +out: + if (locked) + read_unlock(&tasklist_lock); + if (task) + put_task_struct(task); + return res; +} + +static __poll_t proc_tgid_exithand_poll(struct file *file, poll_table *wait) +{ + __poll_t mask = 0; + struct task_struct *task = get_proc_task(file_inode(file)); + + if (!task) { + mask |= POLLIN; + } else if (READ_ONCE(task->exit_state)) { + mask |= POLLIN; + } else { + read_lock(&tasklist_lock); + task->signal->exithand_is_interested = true; + read_unlock(&tasklist_lock); + poll_wait(file, &wait_exithand, wait); + } + if (task) + put_task_struct(task); + return mask; +} + +const struct file_operations proc_tgid_exithand_operations = { + .open = proc_tgid_exithand_open, + .read = proc_tgid_exithand_read, + .poll = proc_tgid_exithand_poll, +}; diff --git a/fs/proc/internal.h b/fs/proc/internal.h index 5185d7f6a51e..1009d20475bc 100644 --- a/fs/proc/internal.h +++ b/fs/proc/internal.h @@ -304,3 +304,7 @@ extern unsigned long task_statm(struct mm_struct *, unsigned long *, unsigned long *, unsigned long *, unsigned long *); extern void task_mem(struct seq_file *, struct mm_struct *); + +/* exithand.c */ + +extern const struct file_operations proc_tgid_exithand_operations; diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 13789d10a50e..f44397055429 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -74,6 +74,11 @@ struct multiprocess_signals { struct hlist_node node; }; +/* Need to stick the waitq for exithand outside process structures in + * case a process disappears across a poll. + */ +extern wait_queue_head_t wait_exithand; + /* * NOTE! "signal_struct" does not have its own * locking, because a shared signal_struct always @@ -87,6 +92,9 @@ struct signal_struct { int nr_threads; struct list_head thread_head; + /* Protected with tasklist_lock. */ + bool exithand_is_interested; + wait_queue_head_t wait_chldexit; /* for wait4() */ /* current thread group signal load-balancing target: */ diff --git a/kernel/exit.c b/kernel/exit.c index 0e21e6d21f35..44a4e3796f8b 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1485,6 +1485,8 @@ void __wake_up_parent(struct task_struct *p, struct task_struct *parent) { __wake_up_sync_key(&parent->signal->wait_chldexit, TASK_INTERRUPTIBLE, 1, p); + if (p->signal->exithand_is_interested) + __wake_up_sync(&wait_exithand, TASK_INTERRUPTIBLE, 0); } static long do_wait(struct wait_opts *wo) diff --git a/kernel/signal.c b/kernel/signal.c index 17565240b1c6..e156d48da70a 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -454,6 +454,9 @@ void flush_sigqueue(struct sigpending *queue) } } +wait_queue_head_t wait_exithand = + __WAIT_QUEUE_HEAD_INITIALIZER(wait_exithand); + /* * Flush all pending signals for this kthread. */ -- 2.19.1.568.g152ad8e336-goog