Commit 3013bf62 authored Feb 18, 2019 by Minchan Kim Committed by Greg Kroah-Hartman Feb 19, 2019

binder: reduce mmap_sem write-side lock



binder has used write-side mmap_sem semaphore to release memory
mapped at address space of the process. However, right lock to
release pages is down_read, not down_write because page table lock
already protects the race for parallel freeing.

Please do not use mmap_sem write-side lock which is well known
contented lock.

Cc: Todd Kjos <tkjos@google.com>
Cc: Martijn Coenen <maco@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 7c11df42

drivers/android/binder_alloc.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -932,7 +932,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
		if (!mmget_not_zero(alloc->vma_vm_mm))
		goto err_mmget;
		mm = alloc->vma_vm_mm;
		if (!down_write_trylock(&mm->mmap_sem))
		if (!down_read_trylock(&mm->mmap_sem))
		goto err_down_write_mmap_sem_failed;
		}

		@@ -946,7 +946,7 @@ 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);
		}