linux-kernel - Re: per cpun+ spin locks coexistence?

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Date:	Fri, 14 Mar 2008 10:54:22 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Peter Teoh <htmldeveloper@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <htejun@...il.com>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: per cpun+ spin locks coexistence?

Peter Teoh wrote:
> Help me out this one - in fs/file.c, there is a function free_fdtable_rcu():
>
> void free_fdtable_rcu(struct rcu_head *rcu)
> {
>        struct fdtable *fdt = container_of(rcu, struct fdtable, rcu);
>        struct fdtable_defer *fddef;
>
>        BUG_ON(!fdt);
>
>        if (fdt->max_fds <= NR_OPEN_DEFAULT) {
>                /*
>                 * This fdtable is embedded in the files structure and that
>                 * structure itself is getting destroyed.
>                 */
>                kmem_cache_free(files_cachep,
>                                container_of(fdt, struct files_struct,
> fdtab));
>                return;
>        }
>        if (fdt->max_fds <= (PAGE_SIZE / sizeof(struct file *))) {
>                kfree(fdt->fd);
>                kfree(fdt->open_fds);
>                kfree(fdt);
>        } else {
>                fddef = &get_cpu_var(fdtable_defer_list);
>                spin_lock(&fddef->lock);
>                fdt->next = fddef->next;
>                fddef->next = fdt;
>                /* vmallocs are handled from the workqueue context */
>                schedule_work(&fddef->wq);
>                spin_unlock(&fddef->lock);
>                put_cpu_var(fdtable_defer_list);
>        }
> }
>
> Notice above that get_cpu_var() is followed by spin_lock().   Does this
> make sense?   get_cpu_var() will return a variable that is only
> accessible by the current CPU - guaranteed it will not be touch (read or
> write) by another CPU, right? 

No, not true.  percpu is for stuff which is generally only touched by 
one CPU, but there's nothing stopping other processors from accessing it 
with per_cpu(var, cpu).

Besides, the lock isn't locking the percpu list head, but the thing on 
the head of the list, presumably to prevent races with the workqueue.  
(Though the list structure is nonstandard, so its not completely clear.)

    J
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