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SubjectRe: [linux-pm] [GIT PULL] One more power management fix for 2.6.37
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On Thursday, November 04, 2010, Dominik Brodowski wrote:
> On Thu, Nov 04, 2010 at 06:04:05AM +0100, Rafael J. Wysocki wrote:
> > On Wednesday, November 03, 2010, Dominik Brodowski wrote:
> > > > There's apparently an ordering problem with dpm_list_mtx and
> > > > socket->skt_mutex. Lockdep details appended.
> > > >
> > > > Dominik, Rafael? What's the proper locking order here, and
> > > > how do we fix this?
> > >
> > > Thanks for noting this; let's see:
> > >
> > > - We add a PCMCIA device holding skt_mutex, therefore we have the ordering
> > > (1) skt_mutex -> (2) dpm_list_mtx
> > >
> > > - If we're suspending, dpm_list_mtx is held, but we need to acquire
> > > skt_mutex as we modify some data being protected by skt_mutex
> > > (1) dpm_list_mtx -> (2) skt_mutex
> > >
> > > Rafael, any idea on how to solve this? How do other subsystems handle such
> > > an issue? Do they call device_add() with no locks held at all?
> >
> > They usually do from what I can tell.
> >
> > Also only a few of them implement the ->suspend_noirq() callback, which is the
> > one executed under dpm_list_mtx.
> >
> > What exactly is protected by skt_mutex ?
>
> e.g.
> struct pcmcia_socket {
> ...
> u_int suspended_state;
> int resume_status;
> ...
> }
>
> Furthermore, one has to acquire skt_mutex first before obtaining ops_mutex,
> which protects many more fields (and asserts exclusion for some code paths),
> see Documentation/pcmcia/locking.txt for details.

OK, so I think we can relax the locking in dpm_[suspend/resume]_noirq() to
avoid executing callbacks under dpm_list_mtx, like in the (untested) patch
below.

Alan, do you see any immediate problem with that?

Rafael

---
drivers/base/power/main.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
Index: linux-2.6/drivers/base/power/main.c
===================================================================
--- linux-2.6.orig/drivers/base/power/main.c
+++ linux-2.6/drivers/base/power/main.c
@@ -480,15 +480,23 @@ void dpm_resume_noirq(pm_message_t state

mutex_lock(&dpm_list_mtx);
transition_started = false;
- list_for_each_entry(dev, &dpm_list, power.entry)
+ list_for_each_entry(dev, &dpm_list, power.entry) {
+ get_device(dev);
if (dev->power.status > DPM_OFF) {
int error;

dev->power.status = DPM_OFF;
+
+ mutex_unlock(&dpm_list_mtx);
+
error = device_resume_noirq(dev, state);
+
+ mutex_lock(&dpm_list_mtx);
if (error)
pm_dev_err(dev, state, " early", error);
}
+ put_device(dev);
+ }
mutex_unlock(&dpm_list_mtx);
dpm_show_time(starttime, state, "early");
resume_device_irqs();
@@ -796,12 +804,19 @@ int dpm_suspend_noirq(pm_message_t state
suspend_device_irqs();
mutex_lock(&dpm_list_mtx);
list_for_each_entry_reverse(dev, &dpm_list, power.entry) {
+ get_device(dev);
+ mutex_unlock(&dpm_list_mtx);
+
error = device_suspend_noirq(dev, state);
+
+ mutex_lock(&dpm_list_mtx);
if (error) {
pm_dev_err(dev, state, " late", error);
+ put_device(dev);
break;
}
dev->power.status = DPM_OFF_IRQ;
+ put_device(dev);
}
mutex_unlock(&dpm_list_mtx);
if (error)

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