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Re: [RFC][PATCH] Make access to taks's nsproxy liter [message #19613] Wed, 08 August 2007 16:48 Go to next message
serue is currently offline  serue
Messages: 750
Registered: February 2006
Senior Member
Quoting Pavel Emelyanov (xemul@openvz.org):
> When someone wants to deal with some other taks's namespaces
> it has to lock the task and then to get the desired namespace
> if the one exists. This is slow on read-only paths and may be
> impossible in some cases.
> 
> E.g. Oleg recently noticed a race between unshare() and the
> (just sent for review) pid namespaces - when the task notifies
> the parent it has to know the parent's namespace, but taking
> the task_lock() is impossible there - the code is under write
> locked tasklist lock.
> 
> On the other hand switching the namespace on task (daemonize)
> and releasing the namespace (after the last task exit) is rather
> rare operation and we can sacrifice its speed to solve the
> issues above.
> 
> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> 
> ---
> 
> fs/proc/base.c          |   27 +++++++++++++++++----------
> include/linux/nsproxy.h |   19 +++++++------------
> kernel/exit.c           |    4 +---
> kernel/fork.c           |   11 +++++------
> kernel/nsproxy.c        |   38 ++++++++++++++++++++++++++++++--------
> 5 files changed, 60 insertions(+), 39 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ed2b224..614851a 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -371,18 +371,21 @@ struct proc_mounts {
> static int mounts_open(struct inode *inode, struct file *file)
> {
> 	struct task_struct *task = get_proc_task(inode);
> +	struct nsproxy *nsp;
> 	struct mnt_namespace *ns = NULL;
> 	struct proc_mounts *p;
> 	int ret = -EINVAL;
> 
> 	if (task) {
> -		task_lock(task);
> -		if (task->nsproxy) {
> -			ns = task->nsproxy->mnt_ns;
> +		rcu_read_lock();
> +		nsp = task_nsproxy(task);
> +		if (nsp) {
> +			ns = nsp->mnt_ns;
> 			if (ns)
> 				get_mnt_ns(ns);
> 		}
> -		task_unlock(task);
> +		rcu_read_unlock();
> +
> 		put_task_struct(task);
> 	}
> 
> @@ -445,16 +448,20 @@ static int mountstats_open(struct inode 
> 
> 	if (!ret) {
> 		struct seq_file *m = file->private_data;
> +		struct nsproxy *nsp;
> 		struct mnt_namespace *mnt_ns = NULL;
> 		struct task_struct *task = get_proc_task(inode);
> 
> 		if (task) {
> -			task_lock(task);
> -			if (task->nsproxy)
> -				mnt_ns = task->nsproxy->mnt_ns;
> -			if (mnt_ns)
> -				get_mnt_ns(mnt_ns);
> -			task_unlock(task);
> +			rcu_read_lock();
> +			nsp = task_nsproxy(task);
> +			if (nsp) {
> +				mnt_ns = nsp->mnt_ns;
> +				if (mnt_ns)
> +					get_mnt_ns(mnt_ns);
> +			}
> +			rcu_read_unlock();
> +
> 			put_task_struct(task);
> 		}
> 
> diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
> index 525d8fc..74f21fe 100644
> --- a/include/linux/nsproxy.h
> +++ b/include/linux/nsproxy.h
> @@ -32,8 +32,14 @@ struct nsproxy {
> };
> extern struct nsproxy init_nsproxy;
> 
> +static inline struct nsproxy *task_nsproxy(struct task_struct *tsk)
> +{
> +	return rcu_dereference(tsk->nsproxy);
> +}

Looks like a very nice cleanup as well.  But please add a comment
above task_nsproxy() that it must be called under rcu_read_lock()
or task_lock(task) (though I'll admit the rcu_dereference may make that
obvious)

> int copy_namespaces(unsigned long flags, struct task_struct *tsk);
> -void get_task_namespaces(struct task_struct *tsk);
> +void exit_task_namespaces(struct task_struct *tsk);
> +void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
> void free_nsproxy(struct nsproxy *ns);
> int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
> 	struct fs_struct *);
> @@ -45,17 +51,6 @@ static inline void put_nsproxy(struct ns
> 	}
> }
> 
> -static inline void exit_task_namespaces(struct task_struct *p)
> -{
> -	struct nsproxy *ns = p->nsproxy;
> -	if (ns) {
> -		task_lock(p);
> -		p->nsproxy = NULL;
> -		task_unlock(p);
> -		put_nsproxy(ns);
> -	}
> -}
> -
> #ifdef CONFIG_CONTAINER_NS
> int ns_container_clone(struct task_struct *tsk);
> #else
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 2f4f35e..a2aef1f 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -409,9 +409,7 @@ void daemonize(const char *name, ...)
> 	current->fs = fs;
> 	atomic_inc(&fs->count);
> 
> -	exit_task_namespaces(current);
> -	current->nsproxy = init_task.nsproxy;
> -	get_task_namespaces(current);
> +	switch_task_namespaces(current, init_task.nsproxy);
> 
> 	exit_files(current);
> 	current->files = init_task.files;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bc48e0f..0c6dd67 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1618,7 +1618,7 @@ asmlinkage long sys_unshare(unsigned lon
> 	struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
> 	struct files_struct *fd, *new_fd = NULL;
> 	struct sem_undo_list *new_ulist = NULL;
> -	struct nsproxy *new_nsproxy = NULL, *old_nsproxy = NULL;
> +	struct nsproxy *new_nsproxy = NULL;
> 
> 	check_unshare_flags(&unshare_flags);
> 
> @@ -1647,14 +1647,13 @@ asmlinkage long sys_unshare(unsigned lon
> 
> 	if (new_fs ||  new_mm || new_fd || new_ulist || new_nsproxy) {
> 
> -		task_lock(current);
> -
> 		if (new_nsproxy) {
> -			old_nsproxy = current->nsproxy;
> -			current->nsproxy = new_nsproxy;
> -			new_nsproxy = old_nsproxy;
> +			switch_task_namespaces(current, new_nsproxy);
> +			new_nsproxy = NULL;
> 		}
> 
> +		task_lock(current);
> +
> 		if (new_fs) {
> 			fs = current->fs;
> 			current->fs = new_fs;
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index 58843ae..91dca27 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -30,14 +30,6 @@ static inline void get_nsproxy(struct ns
> 	atomic_inc(&ns->count);
> }
> 
> -void get_task_namespaces(struct task_struct *tsk)
> -{
> -	struct nsproxy *ns = tsk->nsproxy;
> -	if (ns) {
> -		get_nsproxy(ns);
> -	}
> -}
> -
> /*
>  * creates a copy of "orig" with refcount 1.
>  */
> @@ -205,6 +197,36 @@ out:
> 	return err;
> }
> 
> +void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
> +{
> +	struct nsproxy *ns;
> +
> +	might_sleep();
> +
> +	ns = p->nsproxy;
> +	if (ns == new)
> +		return;
> +
> +	if (new)
> +		get_nsproxy(new);
> +	rcu_assign_pointer(p->nsproxy, new);
> +
> +	if (ns && atomic_dec_and_test(&ns->count)) {
> +		/*
> +		 * wait for others to get what they want from this
> +		 * nsproxy. cannot release this nsproxy via the
> +		 * call_rcu() since put_mnt_ns will want to sleep
> +		 */
> +		synchronize_rcu();
> +		free_nsproxy(ns);
> +	}
> +}

Also a comment above switch_task_namespaces() that it must be called
with task_lock held.

thanks,
-serge

> +
> +void exit_task_namespaces(struct task_struct *p)
> +{
> +	switch_task_namespaces(p, NULL);
> +}
> +
> static int __init nsproxy_cache_init(void)
> {
> 	nsproxy_cachep = kmem_cache_create("nsproxy", sizeof(struct nsproxy),
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Re: [RFC][PATCH] Make access to taks's nsproxy liter [message #19650 is a reply to message #19613] Wed, 08 August 2007 16:58 Go to previous messageGo to next message
Oleg Nesterov is currently offline  Oleg Nesterov
Messages: 143
Registered: August 2006
Senior Member
On 08/08, Serge E. Hallyn wrote:
>
> Quoting Pavel Emelyanov (xemul@openvz.org):
> > +void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
> > +{
> > +	struct nsproxy *ns;
> > +
> > +	might_sleep();
> > +
> > +	ns = p->nsproxy;
> > +	if (ns == new)
> > +		return;
> > +
> > +	if (new)
> > +		get_nsproxy(new);
> > +	rcu_assign_pointer(p->nsproxy, new);
> > +
> > +	if (ns && atomic_dec_and_test(&ns->count)) {
> > +		/*
> > +		 * wait for others to get what they want from this
> > +		 * nsproxy. cannot release this nsproxy via the
> > +		 * call_rcu() since put_mnt_ns will want to sleep
> > +		 */
> > +		synchronize_rcu();
> > +		free_nsproxy(ns);
> > +	}
> > +}
> 
> Also a comment above switch_task_namespaces() that it must be called
> with task_lock held.

Heh, indeed, sys_unshare() does this. might_sleep() won't be happy.

Oleg.

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Re: [RFC][PATCH] Make access to taks's nsproxy liter [message #19670 is a reply to message #19613] Thu, 09 August 2007 07:12 Go to previous message
Pavel Emelianov is currently offline  Pavel Emelianov
Messages: 1149
Registered: September 2006
Senior Member
[snip]

>> diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
>> index 525d8fc..74f21fe 100644
>> --- a/include/linux/nsproxy.h
>> +++ b/include/linux/nsproxy.h
>> @@ -32,8 +32,14 @@ struct nsproxy {
>> };
>> extern struct nsproxy init_nsproxy;
>>
>> +static inline struct nsproxy *task_nsproxy(struct task_struct *tsk)
>> +{
>> +	return rcu_dereference(tsk->nsproxy);
>> +}
> 
> Looks like a very nice cleanup as well.  But please add a comment
> above task_nsproxy() that it must be called under rcu_read_lock()
> or task_lock(task) (though I'll admit the rcu_dereference may make that
> obvious)

I will, but I think that rcu_dereference implies this. Anyway.

[snip]

>> +	if (ns == new)
>> +		return;
>> +
>> +	if (new)
>> +		get_nsproxy(new);
>> +	rcu_assign_pointer(p->nsproxy, new);
>> +
>> +	if (ns && atomic_dec_and_test(&ns->count)) {
>> +		/*
>> +		 * wait for others to get what they want from this
>> +		 * nsproxy. cannot release this nsproxy via the
>> +		 * call_rcu() since put_mnt_ns will want to sleep
>> +		 */
>> +		synchronize_rcu();
>> +		free_nsproxy(ns);
>> +	}
>> +}
> 
> Also a comment above switch_task_namespaces() that it must be called
> with task_lock held.

no! no locks here! free_nsproxy() may sleep when putting mnt_ns and
maybe some other. see - there's a hunk in sys_unshare that move the
task_lock() after switch_task_namespaces().

> thanks,
> -serge
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