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Date:   Mon, 12 Dec 2022 09:44:15 +0800
From:   Ian Kent <ikent@...hat.com>
To:     Brian Foster <bfoster@...hat.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     onestero@...hat.com, willy@...radead.org, ebiederm@...hat.com
Subject: Re: [PATCH v3 1/5] pid: replace pidmap_lock with xarray lock

On 3/12/22 01:16, Brian Foster wrote:
> As a first step to changing the struct pid tracking code from the
> idr over to the xarray, replace the custom pidmap_lock spinlock with
> the internal lock associated with the underlying xarray. This is
> effectively equivalent to using idr_lock() and friends, but since
> the goal is to disentangle from the idr, move directly to the
> underlying xarray api.
>
> Signed-off-by: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: Brian Foster <bfoster@...hat.com>

The xa array switch looks simpler than I expected, looks good.

Reviewed-by: Ian Kent <raven@...maw.net>

> ---
>   kernel/pid.c | 79 ++++++++++++++++++++++++++--------------------------
>   1 file changed, 40 insertions(+), 39 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 3fbc5e46b721..3622f8b13143 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -86,22 +86,6 @@ struct pid_namespace init_pid_ns = {
>   };
>   EXPORT_SYMBOL_GPL(init_pid_ns);
>   
> -/*
> - * Note: disable interrupts while the pidmap_lock is held as an
> - * interrupt might come in and do read_lock(&tasklist_lock).
> - *
> - * If we don't disable interrupts there is a nasty deadlock between
> - * detach_pid()->free_pid() and another cpu that does
> - * spin_lock(&pidmap_lock) followed by an interrupt routine that does
> - * read_lock(&tasklist_lock);
> - *
> - * After we clean up the tasklist_lock and know there are no
> - * irq handlers that take it we can leave the interrupts enabled.
> - * For now it is easier to be safe than to prove it can't happen.
> - */
> -
> -static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
> -
>   void put_pid(struct pid *pid)
>   {
>   	struct pid_namespace *ns;
> @@ -129,10 +113,11 @@ void free_pid(struct pid *pid)
>   	int i;
>   	unsigned long flags;
>   
> -	spin_lock_irqsave(&pidmap_lock, flags);
>   	for (i = 0; i <= pid->level; i++) {
>   		struct upid *upid = pid->numbers + i;
>   		struct pid_namespace *ns = upid->ns;
> +
> +		xa_lock_irqsave(&ns->idr.idr_rt, flags);
>   		switch (--ns->pid_allocated) {
>   		case 2:
>   		case 1:
> @@ -150,8 +135,8 @@ void free_pid(struct pid *pid)
>   		}
>   
>   		idr_remove(&ns->idr, upid->nr);
> +		xa_unlock_irqrestore(&ns->idr.idr_rt, flags);
>   	}
> -	spin_unlock_irqrestore(&pidmap_lock, flags);
>   
>   	call_rcu(&pid->rcu, delayed_put_pid);
>   }
> @@ -206,7 +191,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   		}
>   
>   		idr_preload(GFP_KERNEL);
> -		spin_lock_irq(&pidmap_lock);
> +		xa_lock_irq(&tmp->idr.idr_rt);
>   
>   		if (tid) {
>   			nr = idr_alloc(&tmp->idr, NULL, tid,
> @@ -233,7 +218,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
>   					      pid_max, GFP_ATOMIC);
>   		}
> -		spin_unlock_irq(&pidmap_lock);
> +		xa_unlock_irq(&tmp->idr.idr_rt);
>   		idr_preload_end();
>   
>   		if (nr < 0) {
> @@ -266,34 +251,38 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   	INIT_HLIST_HEAD(&pid->inodes);
>   
>   	upid = pid->numbers + ns->level;
> -	spin_lock_irq(&pidmap_lock);
> -	if (!(ns->pid_allocated & PIDNS_ADDING))
> -		goto out_unlock;
>   	for ( ; upid >= pid->numbers; --upid) {
> +		tmp = upid->ns;
> +
> +		xa_lock_irq(&tmp->idr.idr_rt);
> +		if (tmp == ns && !(tmp->pid_allocated & PIDNS_ADDING)) {
> +			xa_unlock_irq(&tmp->idr.idr_rt);
> +			put_pid_ns(ns);
> +			goto out_free;
> +		}
> +
>   		/* Make the PID visible to find_pid_ns. */
> -		idr_replace(&upid->ns->idr, pid, upid->nr);
> -		upid->ns->pid_allocated++;
> +		idr_replace(&tmp->idr, pid, upid->nr);
> +		tmp->pid_allocated++;
> +		xa_unlock_irq(&tmp->idr.idr_rt);
>   	}
> -	spin_unlock_irq(&pidmap_lock);
>   
>   	return pid;
>   
> -out_unlock:
> -	spin_unlock_irq(&pidmap_lock);
> -	put_pid_ns(ns);
> -
>   out_free:
> -	spin_lock_irq(&pidmap_lock);
>   	while (++i <= ns->level) {
>   		upid = pid->numbers + i;
> -		idr_remove(&upid->ns->idr, upid->nr);
> -	}
> +		tmp = upid->ns;
>   
> -	/* On failure to allocate the first pid, reset the state */
> -	if (ns->pid_allocated == PIDNS_ADDING)
> -		idr_set_cursor(&ns->idr, 0);
> +		xa_lock_irq(&tmp->idr.idr_rt);
>   
> -	spin_unlock_irq(&pidmap_lock);
> +		/* On failure to allocate the first pid, reset the state */
> +		if (tmp == ns && tmp->pid_allocated == PIDNS_ADDING)
> +			idr_set_cursor(&ns->idr, 0);
> +
> +		idr_remove(&tmp->idr, upid->nr);
> +		xa_unlock_irq(&tmp->idr.idr_rt);
> +	}
>   
>   	kmem_cache_free(ns->pid_cachep, pid);
>   	return ERR_PTR(retval);
> @@ -301,9 +290,9 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
>   
>   void disable_pid_allocation(struct pid_namespace *ns)
>   {
> -	spin_lock_irq(&pidmap_lock);
> +	xa_lock_irq(&ns->idr.idr_rt);
>   	ns->pid_allocated &= ~PIDNS_ADDING;
> -	spin_unlock_irq(&pidmap_lock);
> +	xa_unlock_irq(&ns->idr.idr_rt);
>   }
>   
>   struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
> @@ -647,6 +636,18 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
>   	return fd;
>   }
>   
> +/*
> + * Note: disable interrupts while the xarray lock is held as an interrupt might
> + * come in and do read_lock(&tasklist_lock).
> + *
> + * If we don't disable interrupts there is a nasty deadlock between
> + * detach_pid()->free_pid() and another cpu that does xa_lock() followed by an
> + * interrupt routine that does read_lock(&tasklist_lock);
> + *
> + * After we clean up the tasklist_lock and know there are no irq handlers that
> + * take it we can leave the interrupts enabled.  For now it is easier to be safe
> + * than to prove it can't happen.
> + */
>   void __init pid_idr_init(void)
>   {
>   	/* Verify no one has done anything silly: */

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