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>

7 files changed, 42 insertions, 23 deletions

diff --git a/drivers/platform/x86/thinkpad_acpi.c b/drivers/platform/x86/thinkpad_acpi.c
index 7b828680b21d..4b11fc91fa7d 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);
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 73c3992b2bb4..271fde50f0ee 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -947,7 +947,7 @@ int bdi_writeback_thread(void *data)
 
         trace_writeback_thread_start(bdi);
 
-        while (!kthread_should_stop()) {
+        while (!kthread_freezable_should_stop(NULL)) {
                 /*
                  * Remove own delayed wake-up timer, since we are already awake
                  * and we'll take care of the preriodic write-back.
@@ -977,8 +977,6 @@ int bdi_writeback_thread(void *data)
                          */
                         schedule();
                 }
-
-                try_to_freeze();
         }
 
         /* Flush any work that raced with us exiting */
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 7a9427e9fe47..d02b78448b0f 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -47,7 +47,7 @@ static inline bool should_send_signal(struct task_struct *p)
 /* Takes and releases task alloc lock using task_lock() */
 extern int thaw_process(struct task_struct *p);
 
-extern bool __refrigerator(void);
+extern bool __refrigerator(bool check_kthr_stop);
 extern int freeze_processes(void);
 extern int freeze_kernel_threads(void);
 extern void thaw_processes(void);
@@ -57,7 +57,7 @@ static inline bool try_to_freeze(void)
         might_sleep();
         if (likely(!freezing(current)))
                 return false;
-        return __refrigerator();
+        return __refrigerator(false);
 }
 
 extern bool freeze_task(struct task_struct *p, bool sig_only);
@@ -180,7 +180,7 @@ static inline void set_freeze_flag(struct task_struct *p) {}
 static inline void clear_freeze_flag(struct task_struct *p) {}
 static inline int thaw_process(struct task_struct *p) { return 1; }
 
-static inline bool __refrigerator(void) { return false; }
+static inline bool __refrigerator(bool check_kthr_stop) { return false; }
 static inline int freeze_processes(void) { return -ENOSYS; }
 static inline int freeze_kernel_threads(void) { return -ENOSYS; }
 static inline void thaw_processes(void) {}
diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 5cac19b3a266..0714b24c0e45 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -35,6 +35,7 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 void kthread_bind(struct task_struct *k, unsigned int cpu);
 int kthread_stop(struct task_struct *k);
 int kthread_should_stop(void);
+bool kthread_freezable_should_stop(bool *was_frozen);
 void *kthread_data(struct task_struct *k);
 
 int kthreadd(void *unused);
diff --git a/kernel/freezer.c b/kernel/freezer.c
index 732f14f5944f..b83c30e9483a 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -9,6 +9,7 @@
 #include <linux/export.h>
 #include <linux/syscalls.h>
 #include <linux/freezer.h>
+#include <linux/kthread.h>
 
 /*
  * freezing is complete, mark current process as frozen
@@ -23,7 +24,7 @@ static inline void frozen_process(void)
 }
 
 /* Refrigerator is place where frozen processes are stored :-). */
-bool __refrigerator(void)
+bool __refrigerator(bool check_kthr_stop)
 {
         /* Hmm, should we be allowed to suspend when there are realtime
            processes around? */
@@ -50,7 +51,8 @@ bool __refrigerator(void)
 
         for (;;) {
                 set_current_state(TASK_UNINTERRUPTIBLE);
-                if (!frozen(current))
+                if (!frozen(current) ||
+                    (check_kthr_stop && kthread_should_stop()))
                         break;
                 was_frozen = true;
                 schedule();
diff --git a/kernel/kthread.c b/kernel/kthread.c
index b6d216a92639..1c36deaae2f1 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -59,6 +59,31 @@ int kthread_should_stop(void)
 EXPORT_SYMBOL(kthread_should_stop);
 
 /**
+ * kthread_freezable_should_stop - should this freezable kthread return now?
+ * @was_frozen: optional out parameter, indicates whether %current was frozen
+ *
+ * kthread_should_stop() for freezable kthreads, which will enter
+ * refrigerator if necessary.  This function is safe from kthread_stop() /
+ * freezer deadlock and freezable kthreads should use this function instead
+ * of calling try_to_freeze() directly.
+ */
+bool kthread_freezable_should_stop(bool *was_frozen)
+{
+        bool frozen = false;
+
+        might_sleep();
+
+        if (unlikely(freezing(current)))
+                frozen = __refrigerator(true);
+
+        if (was_frozen)
+                *was_frozen = frozen;
+
+        return kthread_should_stop();
+}
+EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
+
+/**
  * kthread_data - return data value specified on kthread creation
  * @task: kthread task in question
  *
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);
-        }
 }
 
 /*