From a288c9a403bc7d8b9fb6d97183a691aac793337b Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 16 Oct 2012 15:03:14 -0700 Subject: [PATCH] freezer: add missing mb's to freezer_count() and freezer_should_skip() commit dd67d32dbc5de299d70cc9e10c6c1e29ffa56b92 upstream. A task is considered frozen enough between freezer_do_not_count() and freezer_count() and freezers use freezer_should_skip() to test this condition. This supposedly works because freezer_count() always calls try_to_freezer() after clearing %PF_FREEZER_SKIP. However, there currently is nothing which guarantees that freezer_count() sees %true freezing() after clearing %PF_FREEZER_SKIP when freezing is in progress, and vice-versa. A task can escape the freezing condition in effect by freezer_count() seeing !freezing() and freezer_should_skip() seeing %PF_FREEZER_SKIP. This patch adds smp_mb()'s to freezer_count() and freezer_should_skip() such that either %true freezing() is visible to freezer_count() or !PF_FREEZER_SKIP is visible to freezer_should_skip(). Signed-off-by: Tejun Heo Cc: Oleg Nesterov Cc: Rafael J. Wysocki Signed-off-by: Greg Kroah-Hartman --- include/linux/freezer.h | 50 ++++++++++++++++++++++++++++++++++------- 1 file changed, 42 insertions(+), 8 deletions(-) diff --git a/include/linux/freezer.h b/include/linux/freezer.h index d09af4b67cf1..ee899329e65a 100644 --- a/include/linux/freezer.h +++ b/include/linux/freezer.h @@ -75,28 +75,62 @@ static inline bool cgroup_freezing(struct task_struct *task) */ -/* Tell the freezer not to count the current task as freezable. */ +/** + * freezer_do_not_count - tell freezer to ignore %current + * + * Tell freezers to ignore the current task when determining whether the + * target frozen state is reached. IOW, the current task will be + * considered frozen enough by freezers. + * + * The caller shouldn't do anything which isn't allowed for a frozen task + * until freezer_cont() is called. Usually, freezer[_do_not]_count() pair + * wrap a scheduling operation and nothing much else. + */ static inline void freezer_do_not_count(void) { current->flags |= PF_FREEZER_SKIP; } -/* - * Tell the freezer to count the current task as freezable again and try to - * freeze it. +/** + * freezer_count - tell freezer to stop ignoring %current + * + * Undo freezer_do_not_count(). It tells freezers that %current should be + * considered again and tries to freeze if freezing condition is already in + * effect. */ static inline void freezer_count(void) { current->flags &= ~PF_FREEZER_SKIP; + /* + * If freezing is in progress, the following paired with smp_mb() + * in freezer_should_skip() ensures that either we see %true + * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP. + */ + smp_mb(); try_to_freeze(); } -/* - * Check if the task should be counted as freezable by the freezer +/** + * freezer_should_skip - whether to skip a task when determining frozen + * state is reached + * @p: task in quesion + * + * This function is used by freezers after establishing %true freezing() to + * test whether a task should be skipped when determining the target frozen + * state is reached. IOW, if this function returns %true, @p is considered + * frozen enough. */ -static inline int freezer_should_skip(struct task_struct *p) +static inline bool freezer_should_skip(struct task_struct *p) { - return !!(p->flags & PF_FREEZER_SKIP); + /* + * The following smp_mb() paired with the one in freezer_count() + * ensures that either freezer_count() sees %true freezing() or we + * see cleared %PF_FREEZER_SKIP and return %false. This makes it + * impossible for a task to slip frozen state testing after + * clearing %PF_FREEZER_SKIP. + */ + smp_mb(); + return p->flags & PF_FREEZER_SKIP; } /* -- 2.39.5