Documentation: preempt-locking: Use better example

The existing wording implies that the use of spin_unlock whilst irqs are
disabled might trigger a reschedule. However the preemptible() test in
preempt_schedule will prevent a reschedule if irqs are disabled.

Lets improve the clarity of this wording to change the example from
spin_unlock to cond_resched() and cond_resched_lock() as these are
functions that will trigger a reschedule if the preempt count is 0 without
testing that irqs are disabled.

Also remove the 'Last Updated' line as this is not up to date and better
tracked via GIT.

Signed-off-by: Andrew Murray <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/preempt-locking.txt

···
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 :Author: Robert Love <[e-mail]>
-:Last Updated: 28 Aug 2002
 
 
 Introduction
···
 in those cases.
 
 But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
-disabling preemption - any spin_unlock() decreasing the preemption count
-to 0 might trigger a reschedule. A simple printk() might trigger a reschedule.
-So use this implicit preemption-disabling property only if you know that the
-affected codepath does not do any of this. Best policy is to use this only for
-small, atomic code that you wrote and which calls no complex functions.
+disabling preemption - any cond_resched() or cond_resched_lock() might trigger
+a reschedule if the preempt count is 0. A simple printk() might trigger a
+reschedule. So use this implicit preemption-disabling property only if you
+know that the affected codepath does not do any of this. Best policy is to use
+this only for small, atomic code that you wrote and which calls no complex
+functions.
 
 Example::