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<concept id="GUID-5F66D3C1-0F7A-478B-AD22-C9848A9DD771" xml:lang="en"><title>SMP
Troubleshooting Guide</title><shortdesc>This document describes a few common error patterns to help you
identify and correct some of the problems you may have in writing SMP-safe
code. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>There are two main classes of programming errors that can occur in code
running on an SMP device:<ul>
<li><p>unsafe assumptions about thread execution order</p></li>
<li><p>unsafe assumptions about memory/data sharing.</p></li>
</ul>These errors can cause deadlocks or panics on a SMP system, or may cause
data corruption or unexpected results. </p>
<section id="GUID-BC147BD0-8A99-44D7-AC73-B265C6C0E285"><title>Execution order
assumptions</title><p>Most of the errors found in code so far are not unique
to SMP, but are more likely to be found on an SMP system. Some existing code
makes assumptions about how threads are scheduled and how thread priority
affects the sequence of operations across multiple threads.</p><p>For example,
one thread makes a request on a server. The developer assumes that the server
will complete the request before the calling thread became active again. This
is a dangerous assumption as the server might not be ready to run, or could
pause while waiting for a resource. This would mean that the scheduler could
continue executing the original calling thread before the server has completed
the request. </p><p>The above example would be even more likely to be an unsafe
assumption on an SMP system because the scheduler could run the server on
one CPU core and the original calling thread on another at the same time.
This is why it is important for the calling thread to be written in a thread-safe
manner which waits for the server to complete the request before continuing
into code that requires that request to be complete.</p><ul>
<li><p><xref href="GUID-93C75E31-6155-48F1-B99C-92ECD0B2C067.dita">Common Error
Patterns - Execution Order</xref> describes a number of common error patterns
that have been found during SMP testing.</p></li>
<li><p><xref href="GUID-314FAEB5-946C-4090-B6AA-1BEEC9BE8EFB.dita">Common Error
Patterns - Case Studies</xref> provides specific examples of common error
patterns.</p></li>
</ul></section>
<section id="GUID-9626880A-4D3A-4C16-8E4C-1256D2D33524"><title>Shared data/memory</title><p>Errors
may occur if multiple threads try to update or delete the same memory areas
at the same time.</p><p>Good programming practice, whether for single core
or multiple core, is to lock objects while they are being updated to prevent
them being read by another thread in the middle of an update. For kernel-side
code use a lock to ensure that a particular thread has exclusive access to
data and interrupts.</p>  </section>
<section id="GUID-6EFA3D71-2254-4231-A375-3519F6A13FBF"><title>Flexible Memory
Model Problems</title><p>One problem that has been found, though not specifically
SMP related, is where the flexible memory model is being used. For global
chunks, memory locations are accessed as an offset to a base address. However,
each thread may have a different base address for the global chunk: if applications
use absolute addresses, they will read the wrong data.</p></section>
</conbody><related-links>
<link href="GUID-16AED228-539F-4BF7-A7FD-9A01FF1A9A84.dita"><linktext>Locking</linktext>
</link>
<link href="GUID-9D93F895-B975-4F2D-A2A3-817033EA5C12.dita"><linktext>Data Integrity
and Memory Barriers</linktext></link>
<link href="GUID-E55F9286-F586-4665-93D8-86F1E7BE2C7C.dita"><linktext>SMP Developer
Tips</linktext></link>
</related-links></concept>