On Fri, Jun 30, 2017 at 03:50:33PM -0400, Alan Stern wrote:

On Fri, 30 Jun 2017, Oleg Nesterov wrote:

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On 06/30, Paul E. McKenney wrote:

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On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote:

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I do not think the overhead will be noticeable in this particular case.

But I am not sure I understand why do we want to unlock_wait. Yes I agree,

                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

if it was not clear, I tried to say "why do we want to _remove_ unlock_wait".

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it has some problems, but still...

The code above looks strange for me. If we are going to repeat this pattern
the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;)

If not, we should probably change this code more:

This looks -much- better than my patch!  May I have your Signed-off-by?

Only if you promise to replace all RCU flavors with a single simple implementation
based on rwlock ;)

Seriously, of course I won't argue, and it seems that nobody except me likes
this primitive, but to me spin_unlock_wait() looks like synchronize_rcu(() and
sometimes it makes sense.

If it looks like synchronize_rcu(), why not actually use 
synchronize_rcu()?

My guess is that the latencies of synchronize_rcu() don't suit his needs.
When the lock is not held, spin_unlock_wait() is quite fast, even
compared to expedited grace periods.

							Thanx, Paul

Alan Stern

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Including this particular case. task_work_run() is going to flush/destroy the
->task_works list, so it needs to wait until all currently executing "readers"
(task_work_cancel()'s which have started before ->task_works was updated) have
completed.