[PATCH] freezer: make freezing of user tasks and kernel threads clear

[Michal Hocko]

There are slightly different requirements for kernel threads and user
tasks freezing. User tasks might get killed while they are in the fridge
(e.g. by OOM killer) and we have to make sure that such a task will die
as soon as possible. This means that try_to_freeze cannot be called from
an arbitrary code path.

The obviously safe place is on the way out from the kernel. We have two
such places currently, get_signal_to_deliver which is the main point
for user tasks freezing called on the way to userspace and lguest which
expedites return to the userspace as well when signals are pending.

Any new caller should be added with extreme caution and proper
justification. This is quite hard right now because both kernel threads
and user tasks context are calling the same function. This patch
keeps the old try_to_freeze only for kernel threads and adds a new
try_to_freeze_user_task which is aimed at user tasks. Both check process
flag and warn about improper usage.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 drivers/lguest/core.c   |  2 +-
 include/linux/freezer.h | 28 +++++++++++++++++++++++++++-
 kernel/signal.c         |  2 +-
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 6590558d1d31..d8eb5ec8bcc7 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -239,7 +239,7 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		 * thing called the freezer.  If the Host is trying to suspend,
 		 * it stops us.
 		 */
-		try_to_freeze();
+		try_to_freeze_user_task();
 
 		/* Check for signals */
 		if (signal_pending(current))
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 7fd81b8c4897..2683ad5fdfe8 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -59,13 +59,39 @@ static inline bool try_to_freeze_unsafe(void)
 	return __refrigerator(false);
 }
 
-static inline bool try_to_freeze(void)
+/* A helper function. Do not use directly. */
+static inline bool __try_to_freeze(void)
 {
 	if (!(current->flags & PF_NOFREEZE))
 		debug_check_no_locks_held();
 	return try_to_freeze_unsafe();
 }
 
+/*
+ * Only kernel threads are allowed to call try_to_freeze.
+ */
+static inline bool try_to_freeze(void)
+{
+	WARN_ON(!(current->flags & PF_KTHREAD));
+
+	return __try_to_freeze();
+}
+
+/*
+ * User tasks might get frozen only on specific places
+ * where we know they will get to user space or die on
+ * signal very quickly.
+ *
+ * Every new user of this function has to be consulted with
+ * PM/freezer maintainers.
+ */
+static inline bool try_to_freeze_user_task(void)
+{
+	WARN_ON(current->flags & PF_KTHREAD);
+
+	return __try_to_freeze();
+}
+
 extern bool freeze_task(struct task_struct *p);
 extern bool set_freezable(void);
 
diff --git a/kernel/signal.c b/kernel/signal.c
index a4077e90f19f..c1860564a74d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2184,7 +2184,7 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka,
 	 * do_signal_stop() and ptrace_stop() do freezable_schedule() and
 	 * thus do not need another check after return.
 	 */
-	try_to_freeze();
+	try_to_freeze_user_task();
 
 relock:
 	spin_lock_irq(&sighand->siglock);
-- 
2.1.0

-- 
Michal Hocko
SUSE Labs
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