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Subject[PATCH] binder: take read mode of mmap_sem in binder_alloc_free_page()
Date
Restore the behavior of locking mmap_sem for reading in
binder_alloc_free_page(), as was first done in commit 3013bf62b67a
("binder: reduce mmap_sem write-side lock"). That change was
inadvertently reverted by commit 5cec2d2e5839 ("binder: fix race between
munmap() and direct reclaim").

In addition, change the name of the label for the error path to
accurately reflect that we're taking the lock for reading.

Backporting note: This fix is only needed when *both* of the commits
mentioned above are applied. That's an unlikely situation since they
both landed during the development of v5.1 but only one of them is
targeted for stable.

Fixes: 5cec2d2e5839 ("binder: fix race between munmap() and direct reclaim")
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
---

Hello - I was backporting 5cec2d2e5839 and didn't think it looked quite
right. The patch description doesn't specifically mention a need to acquire
mmap_sem for writing and the two commits (5cec2d2e5839 and 3013bf62b67a) landed
relatively close in time to each other so I have a feeling that a mistake could
have happened while resolving conflicts.

I also noticed that commit 3013bf62b67a was slightly incomplete because
it didn't rename the err_down_write_mmap_sem_failed label to
err_down_read_mmap_sem_failed.

I've tested this change by enabling CONFIG_ANDROID_BINDER_IPC_SELFTEST
and verified that the binderfs_test selftest passes.

Tyler

drivers/android/binder_alloc.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 195f120c4e8c..bb929eb87116 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -931,8 +931,8 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
mm = alloc->vma_vm_mm;
if (!mmget_not_zero(mm))
goto err_mmget;
- if (!down_write_trylock(&mm->mmap_sem))
- goto err_down_write_mmap_sem_failed;
+ if (!down_read_trylock(&mm->mmap_sem))
+ goto err_down_read_mmap_sem_failed;
vma = binder_alloc_get_vma(alloc);

list_lru_isolate(lru, item);
@@ -945,7 +945,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,

trace_binder_unmap_user_end(alloc, index);
}
- up_write(&mm->mmap_sem);
+ up_read(&mm->mmap_sem);
mmput(mm);

trace_binder_unmap_kernel_start(alloc, index);
@@ -959,7 +959,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
mutex_unlock(&alloc->mutex);
return LRU_REMOVED_RETRY;

-err_down_write_mmap_sem_failed:
+err_down_read_mmap_sem_failed:
mmput_async(mm);
err_mmget:
err_page_already_freed:
--
2.7.4
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