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    SubjectRe: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator
    On 2020-07-16 16:47:28 [+0200], Uladzislau Rezki wrote:
    > On Thu, Jul 16, 2020 at 04:25:37PM +0200, Sebastian Andrzej Siewior wrote:
    > > On 2020-07-16 11:19:13 [+0200], Uladzislau Rezki wrote:
    > > > Sebastian, could you please confirm that if that patch that is in
    > > > question fixes it?
    > > >
    > > > It would be appreciated!
    > >
    > > So that preempt disable should in terms any warnings. However I don't
    > > think that it is strictly needed and from scheduling point of view you
    > > forbid a CPU migration which might be good otherwise.
    > >
    > Please elaborate your point regarding "i do not think it is strictly needed".
    >
    > Actually i can rework the patch to remove even such preempt_enable/disable
    > to stay on the same CPU, but i do not see the point of doing it.
    >
    > Do you see the point?

    You disable preemption for what reason? It is not documented, it is not
    obvious - why is it required?

    > As for scheduling point of view. Well, there are many places when there
    > is a demand in memory or pages from atomic context. Also, getting a page
    > is not considered as a hot path in the kfree_rcu().

    If you disable preemption than you assume that you wouldn't be atomic
    otherwise. You say that at this point it is not a hot path so if this is
    not *that* important why not allow preemption and allow the schedule to
    place you somewhere else if the scheduler decides that it is a good idea.

    > > Also if interrupts and everything is enabled then someone else might
    > > invoke kfree_rcu() from BH context for instance.
    > >
    > And what? What is a problem here, please elaborate if you see any
    > issues.

    That the kfree_rcu() caller from BH context will end up here as well,
    asking for a page.

    Sebastian

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