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<a href="https://gitlab.rtems.org/sebhub">Sebastian Huber</a>
<a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5311#note_130878">commented</a>:
</p>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The commit <a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/commit/0cf6de01df734c11212e63134551ee344a10ecec" data-reference-type="commit" data-original="0cf6de01df734c11212e63134551ee344a10ecec" data-link="false" data-link-reference="false" data-commit="0cf6de01df734c11212e63134551ee344a10ecec" data-project="26" data-container="body" data-placement="top" title="cpukit/libio: Support close with IOP references held" class="gfm gfm-commit has-tooltip" style='font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; font-size: 95%; margin-top: 0;'>0cf6de01</a> introduced a couple of issues:</p>
<ul dir="auto" style="text-align: initial; list-style-type: disc; margin: 0 0 1rem; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">ABI changes on a release branch</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">Redundant LIBIO_FLAGS_FREE flag: an iop is free if and only if it is on the free list.</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">When the iop is freed, its reference count was set to zero. The iop reference count must never be set to a specific value after initialization at system start. The only valid operations are increments and decrements.</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">In the iop free handling, there was a deadlock potential by obtaining the libio lock while owning a file system lock.</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">The reference counting did not work at all under concurrent access. This was indicated by the failing spintrcritical24 test program.</li>
</ul>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">The spintrcritical24 test was specifically added to test the file descriptor reference counting. It fails now on at least the sparc/gr740 and arm/xilinx_zynq_zc702 simulators. It fails since <code style='font-size: 90%; color: #18171d; word-wrap: break-word; border-radius: .25rem; background-color: #ececef; margin-top: 0; font-weight: inherit; white-space: break-spaces; word-break: break-all; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; overflow-wrap: break-word; padding: 0.125rem 0.25rem;'>rtems_libio_free()</code> may free the iop by looking at the current flags. This is plainly wrong, actions must be carried out when certain state changes happen. Consider this sequence of events:</p>
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<pre data-canonical-lang="c" class="code highlight js-syntax-highlight language-c" v-pre="true" style='display: block; font-size: 14px; color: #3a383f; line-height: 1.6em; overflow-x: auto; border-radius: .25rem; position: relative; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 1rem; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; border-radius: .25rem; background-color: inherit; white-space: pre; margin-top: 0; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="c" style="margin-top: 0;"><span class="kt" style="margin-top: 0;">int</span> <span class="nf">close</span><span class="p">(</span></span>
<span id="LC2" class="line" lang="c"> <span class="kt" style="margin-top: 0;">int</span> <span class="n">fd</span></span>
<span id="LC3" class="line" lang="c"><span class="p" style="margin-top: 0;">)</span></span>
<span id="LC4" class="line" lang="c"><span class="p" style="margin-top: 0;">{</span></span>
<span id="LC5" class="line" lang="c"> <span class="n" style="margin-top: 0;">rtems_libio_t</span> <span class="o">*</span><span class="n">iop</span><span class="p">;</span></span>
<span id="LC6" class="line" lang="c"> <span class="kt" style="margin-top: 0;">unsigned</span> <span class="kt">int</span> <span class="n">flags</span><span class="p">;</span></span>
<span id="LC7" class="line" lang="c"> <span class="kt" style="margin-top: 0;">int</span> <span class="n">rc</span><span class="p">;</span></span>
<span id="LC8" class="line" lang="c"></span>
<span id="LC9" class="line" lang="c"> <span class="k" style="margin-top: 0;">if</span> <span class="p">(</span> <span class="p">(</span><span class="kt">uint32_t</span><span class="p">)</span> <span class="n">fd</span> <span class="o">>=</span> <span class="n">rtems_libio_number_iops</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC10" class="line" lang="c"> <span class="n" style="margin-top: 0;">rtems_set_errno_and_return_minus_one</span><span class="p">(</span> <span class="n">EBADF</span> <span class="p">);</span></span>
<span id="LC11" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC12" class="line" lang="c"></span>
<span id="LC13" class="line" lang="c"> <span class="n" style="margin-top: 0;">iop</span> <span class="o">=</span> <span class="n">rtems_libio_iop</span><span class="p">(</span> <span class="n">fd</span> <span class="p">);</span></span>
<span id="LC14" class="line" lang="c"> <span class="n" style="margin-top: 0;">flags</span> <span class="o">=</span> <span class="n">rtems_libio_iop_flags</span><span class="p">(</span> <span class="n">iop</span> <span class="p">);</span></span>
<span id="LC15" class="line" lang="c"></span>
<span id="LC16" class="line" lang="c"> <span class="k" style="margin-top: 0;">while</span> <span class="p">(</span> <span class="nb">true</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC17" class="line" lang="c"> <span class="kt" style="margin-top: 0;">unsigned</span> <span class="kt">int</span> <span class="n">desired</span><span class="p">;</span></span>
<span id="LC18" class="line" lang="c"> <span class="n" style="margin-top: 0;">bool</span> <span class="n">success</span><span class="p">;</span></span>
<span id="LC19" class="line" lang="c"></span>
<span id="LC20" class="line" lang="c"> <span class="k" style="margin-top: 0;">if</span> <span class="p">(</span> <span class="p">(</span> <span class="n">flags</span> <span class="o">&</span> <span class="n">LIBIO_FLAGS_OPEN</span> <span class="p">)</span> <span class="o">==</span> <span class="mi">0</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC21" class="line" lang="c"> <span class="n" style="margin-top: 0;">rtems_set_errno_and_return_minus_one</span><span class="p">(</span> <span class="n">EBADF</span> <span class="p">);</span></span>
<span id="LC22" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC23" class="line" lang="c"></span>
<span id="LC24" class="line" lang="c"> <span class="cm" style="margin-top: 0;">/* The expected flags depends on close when busy flag. If set</span></span>
<span id="LC25" class="line" lang="c"><span class="cm" style="margin-top: 0;"> * there can be references held when calling the close handler */</span></span>
<span id="LC26" class="line" lang="c"> <span class="k" style="margin-top: 0;">if</span> <span class="p">(</span> <span class="p">(</span> <span class="n">flags</span> <span class="o">&</span> <span class="n">LIBIO_FLAGS_CLOSE_BUSY</span> <span class="p">)</span> <span class="o">==</span> <span class="mi">0</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC27" class="line" lang="c"> <span class="n" style="margin-top: 0;">flags</span> <span class="o">&=</span> <span class="n">LIBIO_FLAGS_FLAGS_MASK</span><span class="p">;</span></span>
<span id="LC28" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC29" class="line" lang="c"></span>
<span id="LC30" class="line" lang="c"> <span class="n" style="margin-top: 0;">desired</span> <span class="o">=</span> <span class="n">flags</span> <span class="o">&</span> <span class="o">~</span><span class="n">LIBIO_FLAGS_OPEN</span><span class="p">;</span></span>
<span id="LC31" class="line" lang="c"> <span class="n" style="margin-top: 0;">success</span> <span class="o">=</span> <span class="n">_Atomic_Compare_exchange_uint</span><span class="p">(</span></span>
<span id="LC32" class="line" lang="c"> <span class="o" style="margin-top: 0;">&</span><span class="n">iop</span><span class="o">-></span><span class="n">flags</span><span class="p">,</span></span>
<span id="LC33" class="line" lang="c"> <span class="o" style="margin-top: 0;">&</span><span class="n">flags</span><span class="p">,</span></span>
<span id="LC34" class="line" lang="c"> <span class="n" style="margin-top: 0;">desired</span><span class="p">,</span></span>
<span id="LC35" class="line" lang="c"> <span class="n" style="margin-top: 0;">ATOMIC_ORDER_ACQ_REL</span><span class="p">,</span></span>
<span id="LC36" class="line" lang="c"> <span class="n" style="margin-top: 0;">ATOMIC_ORDER_RELAXED</span></span>
<span id="LC37" class="line" lang="c"> <span class="p" style="margin-top: 0;">);</span></span>
<span id="LC38" class="line" lang="c"></span>
<span id="LC39" class="line" lang="c"> <span class="k" style="margin-top: 0;">if</span> <span class="p">(</span> <span class="n">success</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC40" class="line" lang="c"> <span class="k" style="margin-top: 0;">break</span><span class="p">;</span></span>
<span id="LC41" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC42" class="line" lang="c"></span>
<span id="LC43" class="line" lang="c"> <span class="k" style="margin-top: 0;">if</span> <span class="p">(</span> <span class="p">(</span> <span class="n">flags</span> <span class="o">&</span> <span class="n">LIBIO_FLAGS_REFERENCE_MASK</span> <span class="p">)</span> <span class="o">!=</span> <span class="mi">0</span> <span class="p">)</span> <span class="p">{</span></span>
<span id="LC44" class="line" lang="c"> <span class="n" style="margin-top: 0;">rtems_set_errno_and_return_minus_one</span><span class="p">(</span> <span class="n">EBUSY</span> <span class="p">);</span></span>
<span id="LC45" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC46" class="line" lang="c"> <span class="p" style="margin-top: 0;">}</span></span>
<span id="LC47" class="line" lang="c"></span>
<span id="LC48" class="line" lang="c"><span class="o" style="margin-top: 0;"><---</span> <span class="n">Lets</span> <span class="n">suppose</span> <span class="n">LIBIO_FLAGS_CLOSE_BUSY</span> <span class="n">is</span> <span class="n">cleared</span><span class="p">.</span> <span class="n">At</span> <span class="n">this</span> <span class="n">point</span> <span class="n">we</span> <span class="n">cleared</span> <span class="n">LIBIO_FLAGS_OPEN</span> <span class="n">above</span> <span class="n">and</span> <span class="n">the</span> <span class="n">reference</span> <span class="n">count</span> <span class="n">is</span> <span class="n">zero</span><span class="p">.</span> <span class="n">Let</span> <span class="n">an</span> <span class="n">interrupt</span> <span class="n">call</span> <span class="k">for</span> <span class="n">example</span> <span class="n">fstat</span><span class="p">(</span><span class="n">fd</span><span class="p">).</span> <span class="n">It</span> <span class="n">will</span> <span class="n">hold</span> <span class="n">and</span> <span class="n">drop</span> <span class="n">the</span> <span class="n">iop</span><span class="p">.</span> <span class="n">The</span> <span class="n">drop</span> <span class="n">will</span> <span class="n">call</span> <span class="n">rtems_libio_free</span><span class="p">()</span> <span class="n">which</span> <span class="n">will</span> <span class="n">calle</span> <span class="n">rtems_libio_free_iop</span><span class="p">()</span> <span class="n">since</span> <span class="n">the</span> <span class="n">iop</span> <span class="n">is</span> <span class="n">no</span> <span class="n">longer</span> <span class="n">open</span> <span class="n">and</span> <span class="n">has</span> <span class="n">a</span> <span class="n">zero</span> <span class="n">reference</span> <span class="n">count</span><span class="p">.</span> <span class="n">This</span> <span class="n">will</span> <span class="n">clear</span> <span class="n">the</span> <span class="n">iop</span><span class="o">-></span><span class="n">pathinfo</span><span class="p">.</span> <span class="n">This</span> <span class="n">will</span> <span class="n">lead</span> <span class="n">to</span> <span class="n">a</span> <span class="nb">NULL</span> <span class="n">pointer</span> <span class="n">access</span> <span class="n">here</span><span class="o">:</span></span>
<span id="LC49" class="line" lang="c"></span>
<span id="LC50" class="line" lang="c"> <span class="n" style="margin-top: 0;">rc</span> <span class="o">=</span> <span class="p">(</span><span class="o">*</span><span class="n">iop</span><span class="o">-></span><span class="n">pathinfo</span><span class="p">.</span><span class="n">handlers</span><span class="o">-></span><span class="n">close_h</span><span class="p">)(</span> <span class="n">iop</span> <span class="p">);</span></span>
<span id="LC51" class="line" lang="c"></span>
<span id="LC52" class="line" lang="c"> <span class="n" style="margin-top: 0;">rtems_libio_free</span><span class="p">(</span> <span class="n">iop</span> <span class="p">);</span></span>
<span id="LC53" class="line" lang="c"></span>
<span id="LC54" class="line" lang="c"> <span class="k" style="margin-top: 0;">return</span> <span class="n">rc</span><span class="p">;</span></span>
<span id="LC55" class="line" lang="c"><span class="p" style="margin-top: 0;">}</span></span></code></pre>
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