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doc: Add design documentation on interruption of NMI handlers 

Make Requirements.html talk about how NMI handlers can take what appear
to RCU to be normal interrupts.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-06-22 06:22:20 -07:00
parent 5b394b2ddf
commit e77cb32558
1 changed files with 11 additions and 0 deletions
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11
Documentation/RCU/Design/Requirements/Requirements.html
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@ -2275,6 +2275,17 @@ he also kindly surprised me with
<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
that meets this requirement.
<p>
Furthermore, NMI handlers can be interrupted by what appear to RCU
to be normal interrupts.
One way that this can happen is for code that directly invokes
<tt>rcu_irq_enter()</tt> and </tt>rcu_irq_exit()</tt> to be called
from an NMI handler.
This astonishing fact of life prompted the current code structure,
which has <tt>rcu_irq_enter()</tt> invoking <tt>rcu_nmi_enter()</tt>
and <tt>rcu_irq_exit()</tt> invoking <tt>rcu_nmi_exit()</tt>.
And yes, I also learned of this requirement the hard way.
<h3><a name="Loadable Modules">Loadable Modules</a></h3>
<p>