On Wed, 2009-03-18 at 18:03 -0700, David Miller wrote:

From: Gregory Haskins <redacted>
Date: Wed, 18 Mar 2009 17:54:04 -0400

quoted

Note that -rt doesnt typically context-switch under contention anymore
since we introduced adaptive-locks.  Also note that the contention
against the lock is still contention, regardless of whether you have -rt
or not.  Its just that the slow-path to handle the contended case for
-rt is more expensive than mainline.  However, once you have the
contention as stated, you have already lost.

First, contention is not implicitly a bad thing.

Second, if the -rt kernel is doing adaptive spinning I see no
reason why that adaptive spinning is not kicking in here to
make this problem just go away.

The basic 'problem' with comparing RT adaptive spinning to non-rt
spinlocks is that if the lock owner is !oncpu, all spinners must break
and go to sleep, otherwise we (potentially) deadlock.  This does not
exist for non-rt spinners. 

Best,
-PWM

This lock is held for mere cycles, just to unlink an SKB from
the networking qdisc, and then it is immediately released.
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