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, 6 insertions, 9 deletions

diff --git a/drivers/platform/x86/thinkpad_acpi.c b/drivers/platform/x86/thinkpad_acpi.c
index 7b828680b21..4b11fc91fa7 100644
--- a/drivers/platform/x86/thinkpad_acpi.c
+++ b/drivers/platform/x86/thinkpad_acpi.c
@@ -2456,8 +2456,9 @@ static int hotkey_kthread(void *data)
         u32 poll_mask, event_mask;
         unsigned int si, so;
         unsigned long t;
-        unsigned int change_detector, must_reset;
+        unsigned int change_detector;
         unsigned int poll_freq;
+        bool was_frozen;
 
         mutex_lock(&hotkey_thread_mutex);
 
@@ -2488,14 +2489,14 @@ static int hotkey_kthread(void *data)
                                 t = 100;        /* should never happen... */
                 }
                 t = msleep_interruptible(t);
-                if (unlikely(kthread_should_stop()))
+                if (unlikely(kthread_freezable_should_stop(&was_frozen)))
                         break;
-                must_reset = try_to_freeze();
-                if (t > 0 && !must_reset)
+
+                if (t > 0 && !was_frozen)
                         continue;
 
                 mutex_lock(&hotkey_thread_data_mutex);
-                if (must_reset || hotkey_config_change != change_detector) {
+                if (was_frozen || hotkey_config_change != change_detector) {
                         /* forget old state on thaw or config change */
                         si = so;
                         t = 0;
@@ -2528,10 +2529,6 @@ exit:
 static void hotkey_poll_stop_sync(void)
 {
         if (tpacpi_hotkey_task) {
-                if (frozen(tpacpi_hotkey_task) ||
-                    freezing(tpacpi_hotkey_task))
-                        thaw_process(tpacpi_hotkey_task);
-
                 kthread_stop(tpacpi_hotkey_task);
                 tpacpi_hotkey_task = NULL;
                 mutex_lock(&hotkey_thread_mutex);