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Message-Id: <20081217140741.0085e6a0.akpm@linux-foundation.org>
Date: Wed, 17 Dec 2008 14:07:41 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: menage@...gle.com
Cc: kamezawa.hiroyu@...fujitsu.com, lizf@...fujitsu.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] CGroups: Add css_tryget()
On Tue, 16 Dec 2008 03:30:58 -0800
menage@...gle.com wrote:
> This patch adds css_tryget(), that obtains a counted reference on a
> CSS. It is used in situations where the caller has a "weak" reference
> to the CSS, i.e. one that does not protect the cgroup from removal via
> a reference count, but would instead be cleaned up by a destroy()
> callback.
>
> css_tryget() will return true on success, or false if the cgroup is
> being removed.
>
> This is similar to Kamezawa Hiroyuki's patch from a week or two ago,
> but with the difference that in the event of css_tryget() racing with
> a cgroup_rmdir(), css_tryget() will only return false if the cgroup
> really does get removed.
>
> This implementation is done by biasing css->refcnt, so that a refcnt
> of 1 means "releasable" and 0 means "released or releasing". In the
> event of a race, css_tryget() distinguishes between "released" and
> "releasing" by checking for the CSS_REMOVED flag in css->flags.
>
> ...
>
> --- hierarchy_lock-mmotm-2008-12-09.orig/include/linux/cgroup.h
> +++ hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
> @@ -52,9 +52,9 @@ struct cgroup_subsys_state {
> * hierarchy structure */
> struct cgroup *cgroup;
>
> - /* State maintained by the cgroup system to allow
> - * subsystems to be "busy". Should be accessed via css_get()
> - * and css_put() */
> + /* State maintained by the cgroup system to allow subsystems
> + * to be "busy". Should be accessed via css_get(),
> + * css_tryget() and and css_put(). */
nanonit. This layout:
/*
* State maintained by the cgroup system to allow subsystems
* to be "busy". Should be accessed via css_get(),
* css_tryget() and and css_put().
*/
is conventional/preferred.
> atomic_t refcnt;
>
> @@ -64,11 +64,14 @@ struct cgroup_subsys_state {
> /* bits in struct cgroup_subsys_state flags field */
> enum {
> CSS_ROOT, /* This CSS is the root of the subsystem */
> + CSS_REMOVED, /* This CSS is dead */
> };
>
> /*
> - * Call css_get() to hold a reference on the cgroup;
> - *
> + * Call css_get() to hold a reference on the css; it can be used
> + * for a reference obtained via:
> + * - an existing ref-counted reference to the css
> + * - task->cgroups for a locked task
> */
>
> static inline void css_get(struct cgroup_subsys_state *css)
> @@ -77,9 +80,32 @@ static inline void css_get(struct cgroup
> if (!test_bit(CSS_ROOT, &css->flags))
> atomic_inc(&css->refcnt);
> }
> +
> +static inline bool css_is_removed(struct cgroup_subsys_state *css)
> +{
> + return test_bit(CSS_REMOVED, &css->flags);
> +}
> +
> +/*
> + * Call css_tryget() to take a reference on a css if your existing
> + * (known-valid) reference isn't already ref-counted. Returns false if
> + * the css has been destroyed.
> + */
> +
> +static inline bool css_tryget(struct cgroup_subsys_state *css)
> +{
> + if (test_bit(CSS_ROOT, &css->flags))
> + return true;
> + while (!atomic_inc_not_zero(&css->refcnt)) {
> + if (test_bit(CSS_REMOVED, &css->flags))
> + return false;
> + }
> + return true;
> +}
This looks too large to inline.
We should have a cpu_relax() in the loop?
And possibly a cond_resched().
It would be better if these polling loops didn't exist at all, of
course. But I guess if you could work out a way of doing that, this
patch wouldn't exist.
>
> ...
>
> +/*
> + * Atomically mark all (or else none) of the cgroup's CSS objects as
> + * CSS_REMOVED. Return true on success, or false if the cgroup has
> + * busy subsystems. Call with cgroup_mutex held
> + */
> +
> +static int cgroup_clear_css_refs(struct cgroup *cgrp)
> +{
> + struct cgroup_subsys *ss;
> + unsigned long flags;
> + bool failed = false;
> + local_irq_save(flags);
please put a blank line between end-of-locals and start-of-code.
> + for_each_subsys(cgrp->root, ss) {
> + struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> + int refcnt;
> + do {
> + /* We can only remove a CSS with a refcnt==1 */
> + refcnt = atomic_read(&css->refcnt);
> + if (refcnt > 1) {
> + failed = true;
> + goto done;
> + }
> + BUG_ON(!refcnt);
> + /*
> + * Drop the refcnt to 0 while we check other
> + * subsystems. This will cause any racing
> + * css_tryget() to spin until we set the
> + * CSS_REMOVED bits or abort
> + */
> + } while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);
This loop also should have a cpu_relax(), I think?
> + }
> + done:
> + for_each_subsys(cgrp->root, ss) {
> + struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> + if (failed) {
> + /*
> + * Restore old refcnt if we previously managed
> + * to clear it from 1 to 0
> + */
> + if (!atomic_read(&css->refcnt))
> + atomic_set(&css->refcnt, 1);
> + } else {
> + /* Commit the fact that the CSS is removed */
> + set_bit(CSS_REMOVED, &css->flags);
> + }
> + }
> + local_irq_restore(flags);
> + return !failed;
> +}
--
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