Re: [PATCH 11/15] ptwalk: copy_pte_range hang

[Nick Piggin]

Hugh Dickins wrote:
> This patch is the odd-one-out of the sequence.  The one before adjusted
> copy_pte_range from a for loop to a do while loop, and it was therefore
> simplest to check for lockbreak before copying pte: possibility that it
> might keep getting preempted without making progress under some loads.
>
> Some loads such as startup: 2*HT*P4 with preemption cannot even reach
> multiuser login.  Suspect needs_lockbreak is broken, can get in a state
> when it remains forever true.  Investigate that later: for now, and for
> all time, it makes sense to aim for a little progress before breaking
> out; and we can manage more pte_nones than copies.

(Just to reiterate a private mail sent to Hugh earlier)

Yeah I think lockbreak is broken. Because the inner spinlock never
has a cond_resched_lock performed on it, so its break_lock is
never set to 0, but need_lockbreak still always returns 1 for it.

IMO, spin_lock should set break_lock to 0, then cond_resched_lock
need not bother with it.

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