Date: Mon, 25 Dec 2023 16:26:09 +0800
From: "Aiqun Yu (Maria)" <quic_aiquny@...cinc.com>
To: <ebiederm@...ssion.com>
CC: <kernel@...cinc.com>, <quic_pkondeti@...cinc.com>, <keescook@...omium.or>,
        <viro@...iv.linux.org.uk>, <brauner@...nel.org>, <oleg@...hat.com>,
        <dhowells@...hat.com>, <jarkko@...nel.org>, <paul@...l-moore.com>,
        <jmorris@...ei.org>, <serge@...lyn.com>, <linux-mm@...ck.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <keyrings@...r.kernel.org>, <linux-security-module@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock



On 12/25/2023 4:19 PM, Maria Yu wrote:
> As a rwlock for tasklist_lock, there are multiple scenarios to acquire
> read lock which write lock needed to be waiting for.
> In freeze_process/thaw_processes it can take about 200+ms for holding read
> lock of tasklist_lock by walking and freezing/thawing tasks in commercial
> devices. And write_lock_irq will have preempt disabled and local irq
> disabled to spin until the tasklist_lock can be acquired. This leading to
> a bad responsive performance of current system.
> Take an example:
> 1. cpu0 is holding read lock of tasklist_lock to thaw_processes.
> 2. cpu1 is waiting write lock of tasklist_lock to exec a new thread with
>     preempt_disabled and local irq disabled.
> 3. cpu2 is waiting write lock of tasklist_lock to do_exit with
>     preempt_disabled and local irq disabled.
> 4. cpu3 is waiting write lock of tasklist_lock to do_exit with
>     preempt_disabled and local irq disabled.
> So introduce a write lock/unlock wrapper for tasklist_lock specificly.
> The current taskslist_lock writers all have write_lock_irq to hold
> tasklist_lock, and write_unlock_irq to release tasklist_lock, that means
> the writers are not suitable or workable to wait on tasklist_lock in irq
> disabled scenarios. So the write lock/unlock wrapper here only follow the
> current design of directly use local_irq_disable and local_irq_enable,
> and not take already irq disabled writer callers into account.
> Use write_trylock in the loop and enabled irq for cpu to repsond if lock
> cannot be taken.
Pls ignore this patch.
Change is not ready for review.
Re-send by mistake.
> 
> Signed-off-by: Maria Yu <quic_aiquny@...cinc.com>
> ---
>   fs/exec.c                  | 10 +++++-----
>   include/linux/sched/task.h | 29 +++++++++++++++++++++++++++++
>   kernel/exit.c              | 16 ++++++++--------
>   kernel/fork.c              |  6 +++---
>   kernel/ptrace.c            | 12 ++++++------
>   kernel/sys.c               |  8 ++++----
>   security/keys/keyctl.c     |  4 ++--
>   7 files changed, 57 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 4aa19b24f281..030eef6852eb 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1086,7 +1086,7 @@ static int de_thread(struct task_struct *tsk)
>   
>   		for (;;) {
>   			cgroup_threadgroup_change_begin(tsk);
> -			write_lock_irq(&tasklist_lock);
> +			write_lock_tasklist_lock();
>   			/*
>   			 * Do this under tasklist_lock to ensure that
>   			 * exit_notify() can't miss ->group_exec_task
> @@ -1095,7 +1095,7 @@ static int de_thread(struct task_struct *tsk)
>   			if (likely(leader->exit_state))
>   				break;
>   			__set_current_state(TASK_KILLABLE);
> -			write_unlock_irq(&tasklist_lock);
> +			write_unlock_tasklist_lock();
>   			cgroup_threadgroup_change_end(tsk);
>   			schedule();
>   			if (__fatal_signal_pending(tsk))
> @@ -1150,7 +1150,7 @@ static int de_thread(struct task_struct *tsk)
>   		 */
>   		if (unlikely(leader->ptrace))
>   			__wake_up_parent(leader, leader->parent);
> -		write_unlock_irq(&tasklist_lock);
> +		write_unlock_tasklist_lock();
>   		cgroup_threadgroup_change_end(tsk);
>   
>   		release_task(leader);
> @@ -1198,13 +1198,13 @@ static int unshare_sighand(struct task_struct *me)
>   
>   		refcount_set(&newsighand->count, 1);
>   
> -		write_lock_irq(&tasklist_lock);
> +		write_lock_tasklist_lock();
>   		spin_lock(&oldsighand->siglock);
>   		memcpy(newsighand->action, oldsighand->action,
>   		       sizeof(newsighand->action));
>   		rcu_assign_pointer(me->sighand, newsighand);
>   		spin_unlock(&oldsighand->siglock);
> -		write_unlock_irq(&tasklist_lock);
> +		write_unlock_tasklist_lock();
>   
>   		__cleanup_sighand(oldsighand);
>   	}
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index a23af225c898..6f69d9a3c868 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -50,6 +50,35 @@ struct kernel_clone_args {
>    * a separate lock).
>    */
>   extern rwlock_t tasklist_lock;
> +
> +/*
> + * Tasklist_lock is a special lock, it takes a good amount of time of
> + * taskslist_lock readers to finish, and the pure write_irq_lock api
> + * will do local_irq_disable at the very first, and put the current cpu
> + * waiting for the lock while is non-responsive for interrupts.
> + *
> + * The current taskslist_lock writers all have write_lock_irq to hold
> + * tasklist_lock, and write_unlock_irq to release tasklist_lock, that
> + * means the writers are not suitable or workable to wait on
> + * tasklist_lock in irq disabled scenarios. So the write lock/unlock
> + * wrapper here only follow the current design of directly use
> + * local_irq_disable and local_irq_enable.
> + */
> +static inline void write_lock_tasklist_lock(void)
> +{
> +	while (1) {
> +		local_irq_disable();
> +		if (write_trylock(&tasklist_lock))
> +			break;
> +		local_irq_enable();
> +		cpu_relax();
> +	}
> +}
> +static inline void write_unlock_tasklist_lock(void)
> +{
> +	write_unlock_irq(&tasklist_lock);
> +}
> +
>   extern spinlock_t mmlist_lock;
>   
>   extern union thread_union init_thread_union;
> diff --git a/kernel/exit.c b/kernel/exit.c
> index ee9f43bed49a..18b00f477079 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -251,7 +251,7 @@ void release_task(struct task_struct *p)
>   
>   	cgroup_release(p);
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	ptrace_release_task(p);
>   	thread_pid = get_pid(p->thread_pid);
>   	__exit_signal(p);
> @@ -275,7 +275,7 @@ void release_task(struct task_struct *p)
>   			leader->exit_state = EXIT_DEAD;
>   	}
>   
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   	seccomp_filter_release(p);
>   	proc_flush_pid(thread_pid);
>   	put_pid(thread_pid);
> @@ -598,7 +598,7 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
>   		return reaper;
>   	}
>   
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   
>   	list_for_each_entry_safe(p, n, dead, ptrace_entry) {
>   		list_del_init(&p->ptrace_entry);
> @@ -606,7 +606,7 @@ static struct task_struct *find_child_reaper(struct task_struct *father,
>   	}
>   
>   	zap_pid_ns_processes(pid_ns);
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   
>   	return father;
>   }
> @@ -730,7 +730,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>   	struct task_struct *p, *n;
>   	LIST_HEAD(dead);
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	forget_original_parent(tsk, &dead);
>   
>   	if (group_dead)
> @@ -758,7 +758,7 @@ 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_exec_task);
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   
>   	list_for_each_entry_safe(p, n, &dead, ptrace_entry) {
>   		list_del_init(&p->ptrace_entry);
> @@ -1172,7 +1172,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
>   	wo->wo_stat = status;
>   
>   	if (state == EXIT_TRACE) {
> -		write_lock_irq(&tasklist_lock);
> +		write_lock_tasklist_lock();
>   		/* We dropped tasklist, ptracer could die and untrace */
>   		ptrace_unlink(p);
>   
> @@ -1181,7 +1181,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
>   		if (do_notify_parent(p, p->exit_signal))
>   			state = EXIT_DEAD;
>   		p->exit_state = state;
> -		write_unlock_irq(&tasklist_lock);
> +		write_unlock_tasklist_lock();
>   	}
>   	if (state == EXIT_DEAD)
>   		release_task(p);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..06c4b4ab9102 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2623,7 +2623,7 @@ __latent_entropy struct task_struct *copy_process(
>   	 * Make it visible to the rest of the system, but dont wake it up yet.
>   	 * Need tasklist lock for parent etc handling!
>   	 */
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   
>   	/* CLONE_PARENT re-uses the old parent */
>   	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
> @@ -2714,7 +2714,7 @@ __latent_entropy struct task_struct *copy_process(
>   	hlist_del_init(&delayed.node);
>   	spin_unlock(&current->sighand->siglock);
>   	syscall_tracepoint_update(p);
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   
>   	if (pidfile)
>   		fd_install(pidfd, pidfile);
> @@ -2735,7 +2735,7 @@ __latent_entropy struct task_struct *copy_process(
>   bad_fork_cancel_cgroup:
>   	sched_core_free(p);
>   	spin_unlock(&current->sighand->siglock);
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   	cgroup_cancel_fork(p, args);
>   bad_fork_put_pidfd:
>   	if (clone_flags & CLONE_PIDFD) {
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index d8b5e13a2229..a8d7e2d06f3e 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -435,7 +435,7 @@ static int ptrace_attach(struct task_struct *task, long request,
>   	if (retval)
>   		goto unlock_creds;
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	retval = -EPERM;
>   	if (unlikely(task->exit_state))
>   		goto unlock_tasklist;
> @@ -479,7 +479,7 @@ static int ptrace_attach(struct task_struct *task, long request,
>   
>   	retval = 0;
>   unlock_tasklist:
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   unlock_creds:
>   	mutex_unlock(&task->signal->cred_guard_mutex);
>   out:
> @@ -508,7 +508,7 @@ static int ptrace_traceme(void)
>   {
>   	int ret = -EPERM;
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	/* Are we already being traced? */
>   	if (!current->ptrace) {
>   		ret = security_ptrace_traceme(current->parent);
> @@ -522,7 +522,7 @@ static int ptrace_traceme(void)
>   			ptrace_link(current, current->real_parent);
>   		}
>   	}
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   
>   	return ret;
>   }
> @@ -588,7 +588,7 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
>   	/* Architecture-specific hardware disable .. */
>   	ptrace_disable(child);
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	/*
>   	 * We rely on ptrace_freeze_traced(). It can't be killed and
>   	 * untraced by another thread, it can't be a zombie.
> @@ -600,7 +600,7 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
>   	 */
>   	child->exit_code = data;
>   	__ptrace_detach(current, child);
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   
>   	proc_ptrace_connector(child, PTRACE_DETACH);
>   
> diff --git a/kernel/sys.c b/kernel/sys.c
> index e219fcfa112d..0b1647d3ed32 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1088,7 +1088,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
>   	/* From this point forward we keep holding onto the tasklist lock
>   	 * so that our parent does not change from under us. -DaveM
>   	 */
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   
>   	err = -ESRCH;
>   	p = find_task_by_vpid(pid);
> @@ -1136,7 +1136,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
>   	err = 0;
>   out:
>   	/* All paths lead to here, thus we are safe. -DaveM */
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   	rcu_read_unlock();
>   	return err;
>   }
> @@ -1229,7 +1229,7 @@ int ksys_setsid(void)
>   	pid_t session = pid_vnr(sid);
>   	int err = -EPERM;
>   
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   	/* Fail if I am already a session leader */
>   	if (group_leader->signal->leader)
>   		goto out;
> @@ -1247,7 +1247,7 @@ int ksys_setsid(void)
>   
>   	err = session;
>   out:
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   	if (err > 0) {
>   		proc_sid_connector(group_leader);
>   		sched_autogroup_create_attach(group_leader);
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 19be69fa4d05..dd8aed20486a 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -1652,7 +1652,7 @@ long keyctl_session_to_parent(void)
>   
>   	me = current;
>   	rcu_read_lock();
> -	write_lock_irq(&tasklist_lock);
> +	write_lock_tasklist_lock();
>   
>   	ret = -EPERM;
>   	oldwork = NULL;
> @@ -1702,7 +1702,7 @@ long keyctl_session_to_parent(void)
>   	if (!ret)
>   		newwork = NULL;
>   unlock:
> -	write_unlock_irq(&tasklist_lock);
> +	write_unlock_tasklist_lock();
>   	rcu_read_unlock();
>   	if (oldwork)
>   		put_cred(container_of(oldwork, struct cred, rcu));
> 
> base-commit: 88035e5694a86a7167d490bb95e9df97a9bb162b

-- 
Thx and BRs,
Aiqun(Maria) Yu

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