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<a href="https://gitlab.rtems.org/mithileshm">MITHILESH MATTAPALLI</a>
commented on a
discussion on <a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/954#note_141999">cpukit/posix/src/aio_suspend.c</a>:
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128
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<td class="new_line diff-line-num" data-linenumber="128" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
128
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<td class="line_content" style="padding: inherit;"><pre style="display: block; font-size: 14px; color: #3a383f; 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: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;"> <span class="line" data-lang="c"> <span class="n" style="color: #333333;">suspendcbp</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333333;">requests_left</span><span class="o" style="font-weight: 600;">++</span><span class="p">;</span></span>
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129
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129
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<td class="line_content" style="padding: inherit;"><pre style="display: block; font-size: 14px; color: #3a383f; 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: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;"> <span class="line" data-lang="c"></span>
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<td class="old_line diff-line-num" data-linenumber="130" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
130
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<td class="new_line diff-line-num" data-linenumber="130" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
130
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<td class="line_content" style="padding: inherit;"><pre style="display: block; font-size: 14px; color: #3a383f; 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: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;"> <span class="line" data-lang="c"> <span class="p">}</span> <span class="k" style="font-weight: 600;">else</span> <span class="p">{</span></span>
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<td class="old_line diff-line-num new" data-linenumber="131" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
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<td class="new_line diff-line-num new" data-linenumber="131" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: inherit;" align="right" bgcolor="#ddfbe6">
131
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style="display: block; font-size: 14px; color: #3a383f; 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: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;">+<span class="line" data-lang="c"> <span class="n" style="color: #333333;">pthread_mutex_unlock</span><span class="p">(</span> <span class="o" style="font-weight: 600;">&</span><span class="n" style="color: #333333;">aio_request_queue</span><span class="p">.</span><span class="n" style="color: #333333;">mutex</span> <span class="p">);</span></span>
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<td class="old_line diff-line-num" data-linenumber="131" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
131
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<td class="new_line diff-line-num" data-linenumber="132" style="width: 35px; color: rgba(5,5,6,.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
132
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<td class="line_content" style="padding: inherit;"><pre style="display: block; font-size: 14px; color: #3a383f; 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: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;"> <span class="line" data-lang="c"> <span class="n" style="color: #333333;">pthread_mutex_lock</span><span class="p">(</span> <span class="o" style="font-weight: 600;">&</span><span class="n" style="color: #333333;">request</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333333;">suspendcbp</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333333;">mutex</span> <span class="p">);</span></span>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">You are absolutely right about the iteration safety dropping the lock inside the loop is unsafe because the queue could change while the lock is released. Thank you for catching that.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">To answer your question about the locking order: The deadlock (CID 1667285) arises from a 'Hold and Wait' on the same lock. aio_suspend holds aio_request_queue.mutex while waiting for a request to complete. However, the completion handler often needs to acquire that same aio_request_queue.mutex to remove the request from the queue. If aio_suspend waits while holding the lock, the handler blocks forever trying to acquire it.</p>
<p dir="auto" style="color: #3a383f; margin: 0;" align="initial">Since dropping the lock inside the loop is unsafe, would the correct approach be to restart the iteration from the beginning after we wake up and re-acquire the lock?</p>
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