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Message-ID: <17002595.Gazy686Cvx@vostro.rjw.lan>
Date: Thu, 25 Jul 2013 22:06:46 +0200
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Colin Cross <ccross@...roid.com>
Cc: lkml <linux-kernel@...r.kernel.org>,
Michael Leun <lkml20130126@...ton.leun.net>,
Pavel Machek <pavel@....cz>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Linux PM list <linux-pm@...r.kernel.org>,
Darren Hart <dvhart@...ux.intel.com>
Subject: Re: [PATCH] power: set PF_SUSPEND_TASK flag on tasks that call freeze_processes
On Wednesday, July 24, 2013 05:41:33 PM Colin Cross wrote:
> Calling freeze_processes sets a global flag that will cause any
> process that calls try_to_freeze to enter the refrigerator. It
> skips sending a signal to the current task, but if the current
> task ever hits try_to_freeze all threads will be frozen and the
> system will deadlock.
>
> Set a new flag, PF_SUSPEND_TASK, on the task that calls
> freeze_processes. The flag notifies the freezer that the thread
> is involved in suspend and should not be frozen. Also add a
> WARN_ON in thaw_processes if the caller does not have the
> PF_SUSPEND_TASK flag set to catch if a different task calls
> thaw_processes than the one that called freeze_processes, leaving
> a task with PF_SUSPEND_TASK permanently set on it.
>
> Threads that spawn off a task with PF_SUSPEND_TASK set (which
> swsusp does) will also have PF_SUSPEND_TASK set, preventing them
> from freezing while they are helping with suspend, but they need
> to be dead by the time suspend is triggered, otherwise they may
> run when userspace is expected to be frozen. Add a WARN_ON in
> thaw_processes if more than one thread has the PF_SUSPEND_TASK
> flag set.
>
> Reported-by: Michael Leun <lkml20130126@...ton.leun.net>
> Tested-by: Michael Leun <lkml20130126@...ton.leun.net>
> Signed-off-by: Colin Cross <ccross@...roid.com>
> ---
>
> Resending not as an attachment for review.
I like this, but I wonder what other people think.
> If the extra process flag is considered too precious for this
> (there are only 2 left after this patch) I could get the
> same functionality by having freeze_processes() reject calls
> from a PF_KTHREAD|PF_NOFREEZE thread, and use PF_KTHREAD to
> determine if PF_NOFREEZE should be cleared in thaw_processes().
Can we spend an extra process flag on that?
Rafael
> include/linux/sched.h | 1 +
> kernel/freezer.c | 2 +-
> kernel/power/process.c | 11 +++++++++++
> 3 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 50d04b9..d722490 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1628,6 +1628,7 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
> #define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */
> #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */
> #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezable */
> +#define PF_SUSPEND_TASK 0x80000000 /* this thread called freeze_processes and should not be frozen */
>
> /*
> * Only the _current_ task can read/write to tsk->flags, but other
> diff --git a/kernel/freezer.c b/kernel/freezer.c
> index 8b2afc1..b462fa1 100644
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -33,7 +33,7 @@ static DEFINE_SPINLOCK(freezer_lock);
> */
> bool freezing_slow_path(struct task_struct *p)
> {
> - if (p->flags & PF_NOFREEZE)
> + if (p->flags & (PF_NOFREEZE | PF_SUSPEND_TASK))
> return false;
>
> if (pm_nosig_freezing || cgroup_freezing(p))
> diff --git a/kernel/power/process.c b/kernel/power/process.c
> index fc0df84..06ec886 100644
> --- a/kernel/power/process.c
> +++ b/kernel/power/process.c
> @@ -109,6 +109,8 @@ static int try_to_freeze_tasks(bool user_only)
>
> /**
> * freeze_processes - Signal user space processes to enter the refrigerator.
> + * The current thread will not be frozen. The same process that calls
> + * freeze_processes must later call thaw_processes.
> *
> * On success, returns 0. On failure, -errno and system is fully thawed.
> */
> @@ -120,6 +122,9 @@ int freeze_processes(void)
> if (error)
> return error;
>
> + /* Make sure this task doesn't get frozen */
> + current->flags |= PF_SUSPEND_TASK;
> +
> if (!pm_freezing)
> atomic_inc(&system_freezing_cnt);
>
> @@ -168,6 +173,7 @@ int freeze_kernel_threads(void)
> void thaw_processes(void)
> {
> struct task_struct *g, *p;
> + struct task_struct *curr = current;
>
> if (pm_freezing)
> atomic_dec(&system_freezing_cnt);
> @@ -182,10 +188,15 @@ void thaw_processes(void)
>
> read_lock(&tasklist_lock);
> do_each_thread(g, p) {
> + /* No other threads should have PF_SUSPEND_TASK set */
> + WARN_ON((p != curr) && (p->flags & PF_SUSPEND_TASK));
> __thaw_task(p);
> } while_each_thread(g, p);
> read_unlock(&tasklist_lock);
>
> + WARN_ON(!(curr->flags & PF_SUSPEND_TASK));
> + curr->flags &= ~PF_SUSPEND_TASK;
> +
> usermodehelper_enable();
>
> schedule();
>
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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