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SubjectRe: Towards eliminating the freezer
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.)

The total number of iterations should never be large. At the end the
PM core would own all the device semaphores and no more devices could
be added. Then it would be safe to call each device's suspend()
method.

This will remove one of the barriers to eliminating the freezer.

Alan Stern

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