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SubjectRe: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 2)
On Sun, 23 Mar 2008, Rafael J. Wysocki wrote:

> On Sunday, 23 of March 2008, Alan Stern wrote:
> > On Sat, 22 Mar 2008, Rafael J. Wysocki wrote:
> >
> [--snip--]
> >
> > No, you have missed the entire point. The problem doesn't exist in the
> > current code; it exists only if we switch over to using a single list.
> > Routines like dpm_suspend() won't be able to use list_for_each_entry()
> > to traverse the list because entries may be removed by other threads
> > during the traversal. Even list_for_each_entry_safe() won't work
> > correctly without careful attention to details.
> Ah, ok. Thanks for the clarification.
> Doesn't it help that we traverse the list under dpm_list_mtx? Anyone who
> removes an entry is required to take dpm_list_mtx that we're holding while
> the list is traversed except when the callbacks are invoked.

It doesn't help. What _does_ help is the fact that these traversals
are all serialized (since only one thread can carry out a system sleep
at any time).

> The only problem I see is when the device currently being handled is removed
> from the list by a concurrent thread. Is that you were referring to?

Yes, that is the problem. If you try to work around it by using
list_for_each_entry_safe() then you run into a problem when a
concurrent thread removes the device _following_ the one being handled
(or when the device being handled is the last one on the list and a
concurrent thread registers a new device, which can only happen in

It's not hard to fix. Just something to be aware of.

Alan Stern

P.S.: Oh yes, another related issue... We should call get_device() and
put_device() while holding dpm_list_mtx. Otherwise the device
structure might vanish when the callbacks are invoked.

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