Re: Linux 2.2.19pre2

[Andrew Morton <andrewm@uow.edu.au>]

Andrea Arcangeli wrote:
> 
> > o     wake_one semantics for accept()                 (Andrew Morton)
> 
> I dislike the implementation. I stick with my faster and nicer implementation
> that was just included in aa kernels for some time (2.2.18aa2 included it for
> example). Andrew, I guess you didn't noticed I just implemented the wakeone for
> accept. (I just ported it on top of 2.2.19pre2 after backing out the wakeone in
> pre2)

Yes, I noticed your implementation a few weeks back.

'fraid I never liked the use of the TASK_EXCLUSIVE bit in
task_struct.state.  It's an enumerated state, not an aggregation
of flags.  Most of the kernel treats it as an enumerated state
and so will happily clear the TASK_EXCLUSIVE bit without masking it
off.  Fragile.

If a task is on two waitqueues at the same time it becomes a bug:
if the outer waitqueue is non-exclusive and the inner is exclusive,
the task suddenly becomes exclusive on the outer one and converts
it from wake-all to wake-some!

Is it faster?  Not sure.  You've saved a cacheline read in __wake_up
(which was in fact a preload, if you look at what comes later) at the
cost of having to mask out the bit in current->state every time
we schedule().

Anyway, it's academic.  davem would prefer that we do it properly
and move the `exclusive' flag into the waitqueue head to avoid the 
task-on-two-waitqueues problem, as was done in 2.4.  I think he's
right.  Do you?

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