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>

2 files changed, 4 insertions, 3 deletions

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);