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freezer: add missing mb's to freezer_count() and freezer_should_skip()

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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 <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: stable@vger.kernel.org
This commit is contained in:
Tejun Heo 2012-10-16 15:03:14 -07:00
parent 5edee61ede
commit dd67d32dbc
1 changed files with 42 additions and 8 deletions
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50
include/linux/freezer.h
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@ -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;
}
/*