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SubjectRe: [PATCH] opp: fix a missing check on list iterator
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On Mon, 28 Mar 2022 15:09:33 +0530, Viresh Kumar wrote:
> On 28-03-22, 17:13, Xiaomeng Tong wrote:
> > On Mon, 28 Mar 2022 14:20:57 +0530, Viresh Kumar wrote:
> > > On 28-03-22, 15:43, Xiaomeng Tong wrote:
> > > > No. the conditon to call opp_migrate_dentry(opp_dev, opp_table); is:
> > > > if (!list_is_singular(&opp_table->dev_list)),
> > > >
> > > > while list_is_singlular is: !list_empty(head) && (head->next == head->prev);
> > > >
> > > > so the condition is: list_empty(head) || (head->next != head->prev)
> > > >
> > > > if the list is empty, the bug can be triggered.
> > >
> > > List can't be empty here by design. It will be a huge bug in that
> > > case, which should lead to crash somewhere.
> > >
> >
> > There is anther condition to trigger this bug: the list is not empty and
> > no element found (if (iter != opp_dev)).
>
> I suggest reading the code again, considering opp_debug_unregister()
> as well.
>
> What's happening here is this:
>
> - Several devices share the OPP table.
> - One of them (devX) is going away and opp_debug_unregister() is called for this device.
> - If devX is the last device for this OPP table, then we don't migrate
> and just release all resources.
> - Otherwise, we migrate it to the next element in the list. i.e. any
> device which != devX.
>
> Please tell based on this where do you see a possibility of a bug.
> Surely there can be one, but I fail to see it at the moment and need
> more detail of the same.
>

Perhaps you are right. Anyway, It is a good choise to use list iterator
only inside the loop as linus suggested [1], to avoid potential risk.
I have also repost another patch with changed commit message. Please
check it, thank you.

[1]:https://lore.kernel.org/lkml/20220301075839.4156-1-xiam0nd.tong@gmail.com/

--
Xiaomeng Tong

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