Re: [PATCH v2 44/47] mm: shrinker: make global slab shrink lockless

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Hi Dave,

On 2023/7/27 07:09, Dave Chinner wrote:
On Wed, Jul 26, 2023 at 05:14:09PM +0800, Qi Zheng wrote:
On 2023/7/26 16:08, Dave Chinner wrote:
On Mon, Jul 24, 2023 at 05:43:51PM +0800, Qi Zheng wrote:
@@ -122,6 +126,13 @@ void shrinker_free_non_registered(struct shrinker *shrinker);
   void shrinker_register(struct shrinker *shrinker);
   void shrinker_unregister(struct shrinker *shrinker);
+static inline bool shrinker_try_get(struct shrinker *shrinker)
+	return READ_ONCE(shrinker->registered) &&
+	       refcount_inc_not_zero(&shrinker->refcount);

Why do we care about shrinker->registered here? If we don't set
the refcount to 1 until we have fully initialised everything, then
the shrinker code can key entirely off the reference count and
none of the lookup code needs to care about whether the shrinker is
registered or not.

The purpose of checking shrinker->registered here is to stop running
shrinker after calling shrinker_free(), which can prevent the following
situations from happening:

CPU 0                 CPU 1




I don't see any race here? What is wrong with having multiple active
users at once?

Maybe I'm overthinking. What I think is that if there are multiple users
at once, it may cause the above-mentioned livelock, which will cause
shrinker_free() to wait for a long time. But this probability should be
very low.

This should use a completion, then it is always safe under
rcu_read_lock().  This also gets rid of the shrinker_lock spin lock,
which only exists because we can't take a blocking lock under
rcu_read_lock(). i.e:

void shrinker_put(struct shrinker *shrinker)
	if (refcount_dec_and_test(&shrinker->refcount))

void shrinker_free()

I guess what you mean is shrinker_put(), because here may be the last

Yes, I did.

	 * lookups on the shrinker will now all fail as refcount has
	 * fallen to zero. We can now remove it from the lists and
	 * free it.
	call_rcu(shrinker->rcu_head, shrinker_free_rcu_cb);


@@ -686,11 +711,14 @@ EXPORT_SYMBOL(shrinker_free_non_registered);
   void shrinker_register(struct shrinker *shrinker)
-	down_write(&shrinker_rwsem);
-	list_add_tail(&shrinker->list, &shrinker_list);
-	shrinker->flags |= SHRINKER_REGISTERED;
+	refcount_set(&shrinker->refcount, 1);
+	spin_lock(&shrinker_lock);
+	list_add_tail_rcu(&shrinker->list, &shrinker_list);
+	spin_unlock(&shrinker_lock);
-	up_write(&shrinker_rwsem);
+	WRITE_ONCE(shrinker->registered, true);

This just looks wrong - you are trying to use WRITE_ONCE() as a
release barrier to indicate that the shrinker is now set up fully.
That's not necessary - the refcount is an atomic and along with the
rcu locks they should provides all the barriers we need. i.e.

The reason I used WRITE_ONCE() here is because the shrinker->registered
will be read and written concurrently (read in shrinker_try_get() and
written in shrinker_free()), which is why I added shrinker::registered
field instead of using SHRINKER_REGISTERED flag (this can reduce the
addition of WRITE_ONCE()/READ_ONCE()).

Using WRITE_ONCE/READ_ONCE doesn't provide memory barriers needed to
use the field like this. You need release/acquire memory ordering
here. i.e. smp_store_release()/smp_load_acquire().

As it is, the refcount_inc_not_zero() provides a control dependency,
as documented in include/linux/refcount.h, refcount_dec_and_test()
provides release memory ordering. The only thing I think we may need
is a write barrier before refcount_set(), such that if
refcount_inc_not_zero() sees a non-zero value, it is guaranteed to
see an initialised structure...

i.e. refcounts provide all the existence and initialisation
guarantees. Hence I don't see the need to use shrinker->registered
like this and it can remain a bit flag protected by the

Ah, I didn't consider the memory order with refcount when I added
WRITE_ONCE/READ_ONCE to shrinker->registered, just didn't want KCSAN
to complain (there are multiple visitors at the same time, one of which
is a writer).

And the livelock case mentioned above is indeed unlikely to happen, so
I will delete shrinker->registered in the next version.

void shrinker_register(struct shrinker *shrinker)
	list_add_tail_rcu(&shrinker->list, &shrinker_list);
	shrinker->flags |= SHRINKER_REGISTERED;

	 * now the shrinker is fully set up, take the first
	 * reference to it to indicate that lookup operations are
	 * now allowed to use it via shrinker_try_get().
	refcount_set(&shrinker->refcount, 1);

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index f1becfd45853..c5573066adbf 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -5,6 +5,7 @@
   #include <linux/seq_file.h>
   #include <linux/shrinker.h>
   #include <linux/memcontrol.h>
+#include <linux/rculist.h>
   /* defined in vmscan.c */
   extern struct rw_semaphore shrinker_rwsem;
@@ -161,17 +162,21 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
   	struct dentry *entry;
   	char buf[128];
-	int id;
-	lockdep_assert_held(&shrinker_rwsem);
+	int id, ret = 0;
   	/* debugfs isn't initialized yet, add debugfs entries later. */
   	if (!shrinker_debugfs_root)
   		return 0;
+	down_write(&shrinker_rwsem);
+	if (shrinker->debugfs_entry)
+		goto fail;
   	id = ida_alloc(&shrinker_debugfs_ida, GFP_KERNEL);
-	if (id < 0)
-		return id;
+	if (id < 0) {
+		ret = id;
+		goto fail;
+	}
   	shrinker->debugfs_id = id;
   	snprintf(buf, sizeof(buf), "%s-%d", shrinker->name, id);
@@ -180,7 +185,8 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
   	entry = debugfs_create_dir(buf, shrinker_debugfs_root);
   	if (IS_ERR(entry)) {
   		ida_free(&shrinker_debugfs_ida, id);
-		return PTR_ERR(entry);
+		ret = PTR_ERR(entry);
+		goto fail;
   	shrinker->debugfs_entry = entry;
@@ -188,7 +194,10 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
   	debugfs_create_file("scan", 0220, entry, shrinker,
-	return 0;
+	up_write(&shrinker_rwsem);
+	return ret;
   int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
@@ -243,6 +252,11 @@ struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
   	shrinker->name = NULL;
   	*debugfs_id = entry ? shrinker->debugfs_id : -1;
+	/*
+	 * Ensure that shrinker->registered has been set to false before
+	 * shrinker->debugfs_entry is set to NULL.
+	 */
+	smp_wmb();
   	shrinker->debugfs_entry = NULL;
   	return entry;
@@ -266,14 +280,26 @@ static int __init shrinker_debugfs_init(void)
   	shrinker_debugfs_root = dentry;
   	/* Create debugfs entries for shrinkers registered at boot */
-	down_write(&shrinker_rwsem);
-	list_for_each_entry(shrinker, &shrinker_list, list)
+	rcu_read_lock();
+	list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
+		if (!shrinker_try_get(shrinker))
+			continue;
+		rcu_read_unlock();
   		if (!shrinker->debugfs_entry) {
-			ret = shrinker_debugfs_add(shrinker);
-			if (ret)
-				break;
+			/* Paired with smp_wmb() in shrinker_debugfs_detach() */
+			smp_rmb();
+			if (READ_ONCE(shrinker->registered))
+				ret = shrinker_debugfs_add(shrinker);
-	up_write(&shrinker_rwsem);
+		rcu_read_lock();
+		shrinker_put(shrinker);
+		if (ret)
+			break;
+	}
+	rcu_read_unlock();
   	return ret;

And all this churn and complexity can go away because the
shrinker_rwsem is still used to protect shrinker_register()

My consideration is that during this process, there may be a
driver probe failure and then shrinker_free() is called (the
shrinker_debugfs_init() is called in late_initcall stage). In
this case, we need to use RCU+refcount to ensure that the shrinker
is not freed.

Yeah, you're trying to work around the lack of a
wait_for_completion() call in shrinker_free().

With that, this doesn't need RCU at all, and the iteration can be
done fully under the shrinker_rwsem() safely and so none of this
code needs to change.

Oh, indeed, here does not need to be changed.




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