freezer: implement and use kthread_freezable_should_stop()

Writeback and thinkpad_acpi have been using thaw_process() to prevent
deadlock between the freezer and kthread_stop(); unfortunately, this
is inherently racy - nothing prevents freezing from happening between
thaw_process() and kthread_stop().

This patch implements kthread_freezable_should_stop() which enters
refrigerator if necessary but is guaranteed to return if
kthread_stop() is invoked.  Both thaw_process() users are converted to
use the new function.

Note that this deadlock condition exists for many of freezable
kthreads.  They need to be converted to use the new should_stop or
freezable workqueue.

Tested with synthetic test case.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 71034f41a2ba..7ba8feae11b8 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -600,14 +600,10 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 
         /*
          * Finally, kill the kernel thread. We don't need to be RCU
-         * safe anymore, since the bdi is gone from visibility. Force
-         * unfreeze of the thread before calling kthread_stop(), otherwise
-         * it would never exet if it is currently stuck in the refrigerator.
+         * safe anymore, since the bdi is gone from visibility.
          */
-        if (bdi->wb.task) {
-                thaw_process(bdi->wb.task);
+        if (bdi->wb.task)
                 kthread_stop(bdi->wb.task);
-        }
 }
 
 /*