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<a href="https://gitlab.rtems.org/chris">Chris Johns</a>
<a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5201#note_123181">commented</a>:
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<a href="#reason" aria-hidden="true" class="anchor" id="user-content-reason" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Reason</h1>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">This behavior is also present on <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>6-freebsd-14</code> and <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>7-freebsd-14</code> and I suspect <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>main</code> (FreeBSD 15). The reason is an <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> IOP is held when referenced by <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>kqueue</code> event sets. The <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>kqueue</code> <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> is also held at the same time resulting in a deadlock because RTEMS's <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>libio</code> extended the errors <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>close</code> returns. If you attempt to close an <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> while being held <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>EBUSY</code> is returned. The approach is a little naive performing the check at the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>libio</code> layer rather than letting the file system decide. You cannot call close on either the event <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> or <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>kqueue</code> to unblock the call.</p>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">The close <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>EBUSY</code> check was added then the reference counting change was made. My guess is the implementation simplifies the free logic for an <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code>. We cannot change the current implementation without checking all users and possibly breaking them.</p>
<h1 dir="auto" style="font-size: 1.75em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #28272d; margin: 24px 0 16px;" align="initial">
<a href="#libio" aria-hidden="true" class="anchor" id="user-content-libio" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>LibIO</h1>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">The proper fix is to clean up <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>libio</code>. There are other issues around the referencing counting in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>libio</code>. The base design is good and sound with lock-less atomic access to the IOP flags giving good performance however it does not deal with more complex <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> relationships such as FreeBSD's <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>kqueue</code> and the event <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code>'s with close.</p>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">The IOP flags use the open bit in the IOP as a reference to say a user has the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code>. The reference counts indicate if the IOP is held in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>libio</code> or a file system call however the real count is plus one for the open state.</p>
<h1 dir="auto" style="font-size: 1.75em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #28272d; margin: 24px 0 16px;" align="initial">
<a href="#solution" aria-hidden="true" class="anchor" id="user-content-solution" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Solution</h1>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">The solution is to let <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>close</code> call the close handler if references to the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code> are held. The default behavior is to return <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>EBADF</code> if a reference is held. It is what we have now. There is too much work to audit and check all the existing file systems and users of these handler interfaces to know handling close with reference held is OK.</p>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">A file system that can support close with held references can set an <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code> flag called <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>LIBIO_FLAGS_CLOSE_BUSY</code> in open and <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>close</code> checks for the flag and does raise error if set.</p>
<p dir="auto" style="color: #28272d; margin: 0 0 16px;" align="initial">This approach complicates when the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code> can be freed as the free is currently done in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>close</code> as there are no held references. Adding checks to free the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code> to all handler paths is not great because it complicates that code and a case might be missed.</p>
<p dir="auto" style="color: #28272d; margin: 0;" align="initial">FreeBSD's descriptor code has a reference for the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>fd</code> allocation and implements standard reference counting logic for a shared resource. If the hold count drops to zero the resource is freed. Adding the free to the <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; vertical-align: bottom; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>iop</code> drop can work.</p>
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