[PATCH v2 2/7] radix-tree: Use local_lock for protection

[PATCH v2 2/7] radix-tree: Use local_lock for protection

[Sebastian Andrzej Siewior]

The radix-tree and idr preload mechanisms use preempt_disable() to protect
the complete operation between xxx_preload() and xxx_preload_end().

As the code inside the preempt disabled section acquires regular spinlocks,
which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
eventually calls into a memory allocator, this conflicts with the RT
semantics.

Convert it to a local_lock which allows RT kernels to substitute them with
a real per CPU lock. On non RT kernels this maps to preempt_disable() as
before, but provides also lockdep coverage of the critical region.
No functional change.

Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/idr.h        |  5 +----
 include/linux/radix-tree.h |  6 +-----
 lib/radix-tree.c           | 29 ++++++++++++++++++++++-------
 3 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/include/linux/idr.h b/include/linux/idr.h
index ac6e946b6767b..839da8f2f6f13 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -169,10 +169,7 @@ static inline bool idr_is_empty(const struct idr *idr)
  * Each idr_preload() should be matched with an invocation of this
  * function.  See idr_preload() for details.
  */
-static inline void idr_preload_end(void)
-{
-	preempt_enable();
-}
+void idr_preload_end(void);
 
 /**
  * idr_for_each_entry() - Iterate over an IDR's elements of a given type.
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 63e62372443a5..040b1fd0ab940 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -226,6 +226,7 @@ unsigned int radix_tree_gang_lookup(const struct radix_tree_root *,
 			unsigned int max_items);
 int radix_tree_preload(gfp_t gfp_mask);
 int radix_tree_maybe_preload(gfp_t gfp_mask);
+void radix_tree_preload_end(void);
 void radix_tree_init(void);
 void *radix_tree_tag_set(struct radix_tree_root *,
 			unsigned long index, unsigned int tag);
@@ -243,11 +244,6 @@ unsigned int radix_tree_gang_lookup_tag_slot(const struct radix_tree_root *,
 		unsigned int max_items, unsigned int tag);
 int radix_tree_tagged(const struct radix_tree_root *, unsigned int tag);
 
-static inline void radix_tree_preload_end(void)
-{
-	preempt_enable();
-}
-
 void __rcu **idr_get_free(struct radix_tree_root *root,
 			      struct radix_tree_iter *iter, gfp_t gfp,
 			      unsigned long max);
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 2ee6ae3b0ade0..609aeb900b550 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -20,6 +20,7 @@
 #include <linux/kernel.h>
 #include <linux/kmemleak.h>
 #include <linux/percpu.h>
+#include <linux/locallock.h>
 #include <linux/preempt.h>		/* in_interrupt() */
 #include <linux/radix-tree.h>
 #include <linux/rcupdate.h>
@@ -27,7 +28,6 @@
 #include <linux/string.h>
 #include <linux/xarray.h>
 
-
 /*
  * Radix tree node cache.
  */
@@ -59,11 +59,14 @@ struct kmem_cache *radix_tree_node_cachep;
  * Per-cpu pool of preloaded nodes
  */
 struct radix_tree_preload {
+	struct local_lock lock;
 	unsigned nr;
 	/* nodes->parent points to next preallocated node */
 	struct radix_tree_node *nodes;
 };
-static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
+static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) =
+	{ .lock = INIT_LOCAL_LOCK(lock),
+	  .nr = 0, };
 
 static inline struct radix_tree_node *entry_to_node(void *ptr)
 {
@@ -332,14 +335,14 @@ static __must_check int __radix_tree_preload(gfp_t gfp_mask, unsigned nr)
 	 */
 	gfp_mask &= ~__GFP_ACCOUNT;
 
-	preempt_disable();
+	local_lock(&radix_tree_preloads.lock);
 	rtp = this_cpu_ptr(&radix_tree_preloads);
 	while (rtp->nr < nr) {
-		preempt_enable();
+		local_unlock(&radix_tree_preloads.lock);
 		node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
 		if (node == NULL)
 			goto out;
-		preempt_disable();
+		local_lock(&radix_tree_preloads.lock);
 		rtp = this_cpu_ptr(&radix_tree_preloads);
 		if (rtp->nr < nr) {
 			node->parent = rtp->nodes;
@@ -381,11 +384,17 @@ int radix_tree_maybe_preload(gfp_t gfp_mask)
 	if (gfpflags_allow_blocking(gfp_mask))
 		return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
 	/* Preloading doesn't help anything with this gfp mask, skip it */
-	preempt_disable();
+	local_lock(&radix_tree_preloads.lock);
 	return 0;
 }
 EXPORT_SYMBOL(radix_tree_maybe_preload);
 
+void radix_tree_preload_end(void)
+{
+	local_unlock(&radix_tree_preloads.lock);
+}
+EXPORT_SYMBOL(radix_tree_preload_end);
+
 static unsigned radix_tree_load_root(const struct radix_tree_root *root,
 		struct radix_tree_node **nodep, unsigned long *maxindex)
 {
@@ -1470,10 +1479,16 @@ EXPORT_SYMBOL(radix_tree_tagged);
 void idr_preload(gfp_t gfp_mask)
 {
 	if (__radix_tree_preload(gfp_mask, IDR_PRELOAD_SIZE))
-		preempt_disable();
+		local_lock(&radix_tree_preloads.lock);
 }
 EXPORT_SYMBOL(idr_preload);
 
+void idr_preload_end(void)
+{
+	local_unlock(&radix_tree_preloads.lock);
+}
+EXPORT_SYMBOL(idr_preload_end);
+
 void __rcu **idr_get_free(struct radix_tree_root *root,
 			      struct radix_tree_iter *iter, gfp_t gfp,
 			      unsigned long max)
-- 
2.27.0.rc0

Re: [PATCH v2 2/7] radix-tree: Use local_lock for protection

[Ingo Molnar]

* Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> The radix-tree and idr preload mechanisms use preempt_disable() to protect
> the complete operation between xxx_preload() and xxx_preload_end().
> 
> As the code inside the preempt disabled section acquires regular spinlocks,
> which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
> eventually calls into a memory allocator, this conflicts with the RT
> semantics.
> 
> Convert it to a local_lock which allows RT kernels to substitute them with
> a real per CPU lock. On non RT kernels this maps to preempt_disable() as
> before, but provides also lockdep coverage of the critical region.
> No functional change.
> 
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/idr.h        |  5 +----
>  include/linux/radix-tree.h |  6 +-----
>  lib/radix-tree.c           | 29 ++++++++++++++++++++++-------
>  3 files changed, 24 insertions(+), 16 deletions(-)

> -static inline void idr_preload_end(void)
> -{
> -	preempt_enable();
> -}
> +void idr_preload_end(void);

> +void idr_preload_end(void)
> +{
> +	local_unlock(&radix_tree_preloads.lock);
> +}
> +EXPORT_SYMBOL(idr_preload_end);

> +void radix_tree_preload_end(void);

> -static inline void radix_tree_preload_end(void)
> -{
> -	preempt_enable();
> -}

> +void radix_tree_preload_end(void)
> +{
> +	local_unlock(&radix_tree_preloads.lock);
> +}
> +EXPORT_SYMBOL(radix_tree_preload_end);

Since upstream we are still mapping the local_lock primitives to
preempt_disable()/preempt_enable(), I believe these uninlining changes should not be done
in this patch, i.e. idr_preload_end() and radix_tree_preload_end() should stay inline.

Thanks,

	Ingo

Re: [PATCH v2 2/7] radix-tree: Use local_lock for protection

[Matthew Wilcox]

On Mon, May 25, 2020 at 08:29:54AM +0200, Ingo Molnar wrote:
> > +void radix_tree_preload_end(void)
> > +{
> > +	local_unlock(&radix_tree_preloads.lock);
> > +}
> > +EXPORT_SYMBOL(radix_tree_preload_end);
> 
> Since upstream we are still mapping the local_lock primitives to
> preempt_disable()/preempt_enable(), I believe these uninlining changes should not be done
> in this patch, i.e. idr_preload_end() and radix_tree_preload_end() should stay inline.

But radix_tree_preloads is static, and I wouldn't be terribly happy to
see that exported to modules.

Re: [PATCH v2 2/7] radix-tree: Use local_lock for protection

[Ingo Molnar]

* Matthew Wilcox <willy@infradead.org> wrote:

> On Mon, May 25, 2020 at 08:29:54AM +0200, Ingo Molnar wrote:
> > > +void radix_tree_preload_end(void)
> > > +{
> > > +	local_unlock(&radix_tree_preloads.lock);
> > > +}
> > > +EXPORT_SYMBOL(radix_tree_preload_end);
> > 
> > Since upstream we are still mapping the local_lock primitives to
> > preempt_disable()/preempt_enable(), I believe these uninlining changes should not be done
> > in this patch, i.e. idr_preload_end() and radix_tree_preload_end() should stay inline.
> 
> But radix_tree_preloads is static, and I wouldn't be terribly happy to
> see that exported to modules.

Well, it seems a bit silly to make radix_tree_preload_end() a 
standalone function, on most distro kernels that don't have 
CONFIG_PREEMPT=y, preempt_enable() is a NOP:

 0000000000002bf0 <radix_tree_preload_end>:
     2bf0:       c3                      retq   

I.e. we'd be introducing a separate function call for no good reason.

Thanks,

	Ingo

Re: [PATCH v2 2/7] radix-tree: Use local_lock for protection

[Sebastian Andrzej Siewior]

On 2020-05-25 08:29:54 [+0200], Ingo Molnar wrote:
> Since upstream we are still mapping the local_lock primitives to
> preempt_disable()/preempt_enable(), I believe these uninlining changes should not be done
> in this patch, i.e. idr_preload_end() and radix_tree_preload_end() should stay inline.

That means we need to export the per-CPU struct radix_tree_preload in
order to access the ::lock from an inline function.

Something like this then:

diff --git a/include/linux/idr.h b/include/linux/idr.h
index ac6e946b6767b..3ade03e5c7af3 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -171,7 +171,7 @@ static inline bool idr_is_empty(const struct idr *idr)
  */
 static inline void idr_preload_end(void)
 {
-	preempt_enable();
+	local_unlock(&radix_tree_preloads.lock);
 }
 
 /**
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 63e62372443a5..1dcc43ac75aed 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -16,11 +16,20 @@
 #include <linux/spinlock.h>
 #include <linux/types.h>
 #include <linux/xarray.h>
+#include <linux/locallock.h>
 
 /* Keep unconverted code working */
 #define radix_tree_root		xarray
 #define radix_tree_node		xa_node
 
+struct radix_tree_preload {
+	struct local_lock lock;
+	unsigned nr;
+	/* nodes->parent points to next preallocated node */
+	struct radix_tree_node *nodes;
+};
+DECLARE_PER_CPU(struct radix_tree_preload, radix_tree_preloads);
+
 /*
  * The bottom two bits of the slot determine how the remaining bits in the
  * slot are interpreted:
@@ -245,7 +254,7 @@ int radix_tree_tagged(const struct radix_tree_root *, unsigned int tag);
 
 static inline void radix_tree_preload_end(void)
 {
-	preempt_enable();
+	local_unlock(&radix_tree_preloads.lock);
 }
 
 void __rcu **idr_get_free(struct radix_tree_root *root,
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 2ee6ae3b0ade0..1c46840b4f1d3 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -20,6 +20,7 @@
 #include <linux/kernel.h>
 #include <linux/kmemleak.h>
 #include <linux/percpu.h>
+#include <linux/locallock.h>
 #include <linux/preempt.h>		/* in_interrupt() */
 #include <linux/radix-tree.h>
 #include <linux/rcupdate.h>
@@ -27,7 +28,6 @@
 #include <linux/string.h>
 #include <linux/xarray.h>
 
-
 /*
  * Radix tree node cache.
  */
@@ -58,12 +58,10 @@ struct kmem_cache *radix_tree_node_cachep;
 /*
  * Per-cpu pool of preloaded nodes
  */
-struct radix_tree_preload {
-	unsigned nr;
-	/* nodes->parent points to next preallocated node */
-	struct radix_tree_node *nodes;
+DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = {
+	.lock = INIT_LOCAL_LOCK(lock),
 };
-static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
+EXPORT_PER_CPU_SYMBOL_GPL(radix_tree_preloads);
 
 static inline struct radix_tree_node *entry_to_node(void *ptr)
 {
@@ -332,14 +330,14 @@ static __must_check int __radix_tree_preload(gfp_t gfp_mask, unsigned nr)
 	 */
 	gfp_mask &= ~__GFP_ACCOUNT;
 
-	preempt_disable();
+	local_lock(&radix_tree_preloads.lock);
 	rtp = this_cpu_ptr(&radix_tree_preloads);
 	while (rtp->nr < nr) {
-		preempt_enable();
+		local_unlock(&radix_tree_preloads.lock);
 		node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
 		if (node == NULL)
 			goto out;
-		preempt_disable();
+		local_lock(&radix_tree_preloads.lock);
 		rtp = this_cpu_ptr(&radix_tree_preloads);
 		if (rtp->nr < nr) {
 			node->parent = rtp->nodes;
@@ -381,7 +379,7 @@ int radix_tree_maybe_preload(gfp_t gfp_mask)
 	if (gfpflags_allow_blocking(gfp_mask))
 		return __radix_tree_preload(gfp_mask, RADIX_TREE_PRELOAD_SIZE);
 	/* Preloading doesn't help anything with this gfp mask, skip it */
-	preempt_disable();
+	local_lock(&radix_tree_preloads.lock);
 	return 0;
 }
 EXPORT_SYMBOL(radix_tree_maybe_preload);
@@ -1470,7 +1468,7 @@ EXPORT_SYMBOL(radix_tree_tagged);
 void idr_preload(gfp_t gfp_mask)
 {
 	if (__radix_tree_preload(gfp_mask, IDR_PRELOAD_SIZE))
-		preempt_disable();
+		local_lock(&radix_tree_preloads.lock);
 }
 EXPORT_SYMBOL(idr_preload);
 
-- 
2.27.0.rc0


> Thanks,
> 
> 	Ingo

Sebastian