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Subject[PATCH 2/2] android: binder: Don't get mm from task
Date
Use binder_alloc struct's mm_struct rather than getting
a reference to the mm struct through get_task_mm to
avoid a potential deadlock between lru lock, task lock and
dentry lock, since a thread can be holding the task lock
and the dentry lock while trying to acquire the lru lock.

Acked-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Sherry Yang <sherryy@android.com>
---
drivers/android/binder_alloc.c | 22 +++++++++-------------
drivers/android/binder_alloc.h | 1 -
2 files changed, 9 insertions(+), 14 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index b87ecf77f9d1..e283670695db 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -215,17 +215,12 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
}
}

- if (need_mm)
- mm = get_task_mm(alloc->tsk);
+ if (need_mm && mmget_not_zero(alloc->vma_vm_mm))
+ mm = alloc->vma_vm_mm;

if (mm) {
down_write(&mm->mmap_sem);
vma = alloc->vma;
- if (vma && mm != alloc->vma_vm_mm) {
- pr_err("%d: vma mm and task mm mismatch\n",
- alloc->pid);
- vma = NULL;
- }
}

if (!vma && need_mm) {
@@ -718,6 +713,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
barrier();
alloc->vma = vma;
alloc->vma_vm_mm = vma->vm_mm;
+ mmgrab(alloc->vma_vm_mm);

return 0;

@@ -793,6 +789,8 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
vfree(alloc->buffer);
}
mutex_unlock(&alloc->mutex);
+ if (alloc->vma_vm_mm)
+ mmdrop(alloc->vma_vm_mm);

binder_alloc_debug(BINDER_DEBUG_OPEN_CLOSE,
"%s: %d buffers %d, pages %d\n",
@@ -887,7 +885,6 @@ int binder_alloc_get_allocated_count(struct binder_alloc *alloc)
void binder_alloc_vma_close(struct binder_alloc *alloc)
{
WRITE_ONCE(alloc->vma, NULL);
- WRITE_ONCE(alloc->vma_vm_mm, NULL);
}

/**
@@ -924,9 +921,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
page_addr = (uintptr_t)alloc->buffer + index * PAGE_SIZE;
vma = alloc->vma;
if (vma) {
- mm = get_task_mm(alloc->tsk);
- if (!mm)
- goto err_get_task_mm_failed;
+ if (!mmget_not_zero(alloc->vma_vm_mm))
+ goto err_mmget;
+ mm = alloc->vma_vm_mm;
if (!down_write_trylock(&mm->mmap_sem))
goto err_down_write_mmap_sem_failed;
}
@@ -961,7 +958,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,

err_down_write_mmap_sem_failed:
mmput_async(mm);
-err_get_task_mm_failed:
+err_mmget:
err_page_already_freed:
mutex_unlock(&alloc->mutex);
err_get_alloc_mutex_failed:
@@ -1000,7 +997,6 @@ struct shrinker binder_shrinker = {
*/
void binder_alloc_init(struct binder_alloc *alloc)
{
- alloc->tsk = current->group_leader;
alloc->pid = current->group_leader->pid;
mutex_init(&alloc->mutex);
INIT_LIST_HEAD(&alloc->buffers);
diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h
index a3a3602c689c..2dd33b6df104 100644
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -100,7 +100,6 @@ struct binder_lru_page {
*/
struct binder_alloc {
struct mutex mutex;
- struct task_struct *tsk;
struct vm_area_struct *vma;
struct mm_struct *vma_vm_mm;
void *buffer;
--
2.11.0 (Apple Git-81)
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