md: md_open returns -EBUSY when entering racing area
stable inclusion from linux-4.19.191 commit 5db6e4c5c1284cec6720afe27f202a3dca697def -------------------------------- commit 6a4db2a60306eb65bfb14ccc9fde035b74a4b4e7 upstream. commit d3374825 ("md: make devices disappear when they are no longer needed.") introduced protection between mddev creating & removing. The md_open shouldn't create mddev when all_mddevs list doesn't contain mddev. With currently code logic, there will be very easy to trigger soft lockup in non-preempt env. This patch changes md_open returning from -ERESTARTSYS to -EBUSY, which will break the infinitely retry when md_open enter racing area. This patch is partly fix soft lockup issue, full fix needs mddev_find is split into two functions: mddev_find & mddev_find_or_alloc. And md_open should call new mddev_find (it only does searching job). For more detail, please refer with Christoph's "split mddev_find" patch in later commits. Signed-off-by:Yang Yingliang <yangyingliang@huawei.com>
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