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SubjectRe: Towards eliminating the freezer
On Tuesday, 24 July 2007 18:06, Alan Stern wrote:
> On Tue, 24 Jul 2007, Rafael J. Wysocki wrote:
> > > As with Oliver's suggestion, this would create a locking order
> > > violation. Drivers registering children (and thus acquiring
> > > dpm_list_mtx) will often already hold the parent's sem. But
> > > device_suspend() needs to acquire device sems while holding
> > > dpm_list_mtx.
> >
> > Hmm, but this is done already (ie. device_suspend() acquires device sems
> > while holding dpm_list_mtx in the current code).
> >
> > What I'm suggesting is not to let device_suspend() release dpm_list_mtx
> > when it's finished. The appended patch illustrates that I mean.
> Oh, okay, I see what you mean.
> I should have explained earlier that my proposal was meant to be in the
> context of a previous discussion, where I suggested that
> device_suspend() should go through a preliminary step of acquiring all
> the device semaphores. This would have the beneficial effect of
> blocking all attempts at driver binding or unbinding while a suspend is
> underway.
> Still, this isn't a bad approach. Maybe the following algorithm could
> be used:
> get_more:
> For each device on dpm_list
> Acquire dev->sem
> Move dev from dpm_list to a temporary list
> Lock dpm_list_mutex
> If (!list_empty(dpm_list)) {
> Unlock dpm_list_mutex
> Goto get_more
> }
> (The "For each" loop would have to be written carefully to allow for
> device removal.)

Hmm, I still don't understand why we can't lock dpm_list_mutex before the
"For each" loop (we already do something like this in device_suspend() and
device_resume()) and that would simplify things.

It seems that we can do something like this:

Lock dpm_list_mutex (from now on, new devices cannot be added)
For each device on dpm_active, reverse
acquire dev->sem (from now on, no new drivers can bind to dev)
move dev to dpm_off

For each device on dpm_off
move dev to dpm_active
resume(dev) (this cannot fail)
release dev->sem (allow new drivers to bind to dev)
Unlock dpm_list_mutex (allow new devices to be added)


"Premature optimization is the root of all evil." - Donald Knuth
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