linux-kernel - [PATCH 20/21] binder: reverse locking order in shrinker callback

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Message-ID: <20231102185934.773885-21-cmllamas@google.com>
Date:   Thu,  2 Nov 2023 18:59:21 +0000
From:   Carlos Llamas <cmllamas@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Arve Hjønnevåg" <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <brauner@...nel.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: [PATCH 20/21] binder: reverse locking order in shrinker callback

The locking order currently requires the alloc->mutex to be acquired
first followed by the mmap lock. However, the alloc->mutex is converted
into a spinlock in subsequent commits so the order needs to be reversed
to avoid nesting the sleeping mmap lock under the spinlock.

The shrinker's callback binder_alloc_free_page() is the only place that
needs to be reordered since other functions have been refactored and no
longer nest these locks.

Some minor cosmetic changes are also included in this patch.

Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
---
 drivers/android/binder_alloc.c | 44 ++++++++++++++++------------------
 1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index e739be7f2dd4..0ba9f524e0ff 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -1065,35 +1065,38 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 				       void *cb_arg)
 	__must_hold(lock)
 {
-	struct mm_struct *mm = NULL;
-	struct binder_lru_page *page = container_of(item,
-						    struct binder_lru_page,
-						    lru);
-	struct binder_alloc *alloc;
+	struct binder_lru_page *page = container_of(item, typeof(*page), lru);
+	struct binder_alloc *alloc = page->alloc;
+	struct mm_struct *mm = alloc->mm;
+	struct vm_area_struct *vma;
 	unsigned long page_addr;
 	size_t index;
-	struct vm_area_struct *vma;
 
-	alloc = page->alloc;
+	if (!mmget_not_zero(mm))
+		goto err_mmget;
+	if (!mmap_read_trylock(mm))
+		goto err_mmap_read_lock_failed;
 	if (!mutex_trylock(&alloc->mutex))
 		goto err_get_alloc_mutex_failed;
-
 	if (!page->page_ptr)
 		goto err_page_already_freed;
 
 	index = page - alloc->pages;
 	page_addr = alloc->buffer + index * PAGE_SIZE;
 
-	mm = alloc->mm;
-	if (!mmget_not_zero(mm))
-		goto err_mmget;
-	if (!mmap_read_trylock(mm))
-		goto err_mmap_read_lock_failed;
 	vma = vma_lookup(mm, page_addr);
 	if (vma && vma != binder_alloc_get_vma(alloc))
 		goto err_invalid_vma;
 
+	trace_binder_unmap_kernel_start(alloc, index);
+
+	__free_page(page->page_ptr);
+	page->page_ptr = NULL;
+
+	trace_binder_unmap_kernel_end(alloc, index);
+
 	list_lru_isolate(lru, item);
+	mutex_unlock(&alloc->mutex);
 	spin_unlock(lock);
 
 	if (vma) {
@@ -1103,28 +1106,21 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 
 		trace_binder_unmap_user_end(alloc, index);
 	}
+
 	mmap_read_unlock(mm);
 	mmput_async(mm);
 
-	trace_binder_unmap_kernel_start(alloc, index);
-
-	__free_page(page->page_ptr);
-	page->page_ptr = NULL;
-
-	trace_binder_unmap_kernel_end(alloc, index);
-
 	spin_lock(lock);
-	mutex_unlock(&alloc->mutex);
 	return LRU_REMOVED_RETRY;
 
 err_invalid_vma:
+err_page_already_freed:
+	mutex_unlock(&alloc->mutex);
+err_get_alloc_mutex_failed:
 	mmap_read_unlock(mm);
 err_mmap_read_lock_failed:
 	mmput_async(mm);
 err_mmget:
-err_page_already_freed:
-	mutex_unlock(&alloc->mutex);
-err_get_alloc_mutex_failed:
 	return LRU_SKIP;
 }
 
-- 
2.42.0.869.gea05f2083d-goog