[PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock()

[Hugh Dickins]

Since commit 2a9127fcf229 ("mm: rewrite wait_on_page_bit_common() logic")
improved unlock_page(), it has become more noticeable how cow_user_page()
in a kernel with CONFIG_DMA_API_DEBUG=y can create and suffer from heavy
contention on DMA debug's radix_lock in debug_dma_assert_idle().

It is only doing a lookup: use rcu_read_lock() and rcu_read_unlock()
instead; though that does require the static ents[] to be moved onstack...

...but, hold on, isn't that radix_tree_gang_lookup() and loop doing quite
the wrong thing: searching CACHELINES_PER_PAGE entries for an exact match
with the first cacheline of the page in question?  radix_tree_gang_lookup()
is the right tool for the job, but we need nothing more than to check the
first entry it can find, reporting if that falls anywhere within the page.

(Is RCU safe here?  As safe as using the spinlock was.  The entries are
never freed, so don't need to be freed by RCU.  They may be reused, and
there is a faint chance of a race, with an offending entry reused while
printing its error info; but the spinlock did not prevent that either,
and I agree that it's not worth worrying about.)

Fixes: 3b7a6418c749 ("dma debug: account for cachelines and read-only mappings in overlap tracking")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 kernel/dma/debug.c |   27 +++++++++------------------
 1 file changed, 9 insertions(+), 18 deletions(-)

--- v5.9-rc/kernel/dma/debug.c	2020-08-05 18:17:57.544203766 -0700
+++ linux/kernel/dma/debug.c	2020-08-12 19:53:33.159070245 -0700
@@ -565,11 +565,8 @@ static void active_cacheline_remove(stru
  */
 void debug_dma_assert_idle(struct page *page)
 {
-	static struct dma_debug_entry *ents[CACHELINES_PER_PAGE];
-	struct dma_debug_entry *entry = NULL;
-	void **results = (void **) &ents;
-	unsigned int nents, i;
-	unsigned long flags;
+	struct dma_debug_entry *entry;
+	unsigned long pfn;
 	phys_addr_t cln;
 
 	if (dma_debug_disabled())
@@ -578,20 +575,14 @@ void debug_dma_assert_idle(struct page *
 	if (!page)
 		return;
 
-	cln = (phys_addr_t) page_to_pfn(page) << CACHELINE_PER_PAGE_SHIFT;
-	spin_lock_irqsave(&radix_lock, flags);
-	nents = radix_tree_gang_lookup(&dma_active_cacheline, results, cln,
-				       CACHELINES_PER_PAGE);
-	for (i = 0; i < nents; i++) {
-		phys_addr_t ent_cln = to_cacheline_number(ents[i]);
+	pfn = page_to_pfn(page);
+	cln = (phys_addr_t) pfn << CACHELINE_PER_PAGE_SHIFT;
 
-		if (ent_cln == cln) {
-			entry = ents[i];
-			break;
-		} else if (ent_cln >= cln + CACHELINES_PER_PAGE)
-			break;
-	}
-	spin_unlock_irqrestore(&radix_lock, flags);
+	rcu_read_lock();
+	if (!radix_tree_gang_lookup(&dma_active_cacheline, (void **) &entry,
+				    cln, 1) || entry->pfn != pfn)
+		entry = NULL;
+	rcu_read_unlock();
 
 	if (!entry)
 		return;