【OLK-5.10】拔网线cifs写文件的进程stuck住无法退出

【严重程度】 一般
【特性】 
【重现类型】 有条件概率重现
【定位分析】 </p><p><br></p><p>该问题的根因是CIFS在writepages 时 lock_page ，等待其cifsd 进程清理PG_writeback标记后unlock_page。但是由于cifsd 清理PG_writeback 发生reconnect，导致进入cifs_writev_requeue() 函数中，基线lock_page导致死锁。</p><p><br></p><div class="ql-code-block-container" spellcheck="false"><div class="ql-code-block" data-language="plain">进程1(dd)              进程2(cifsd)        进程3(cifsiod worker)</div><div class="ql-code-block" data-language="plain">cifs_writepages</div><div class="ql-code-block" data-language="plain">  lock_page   - [1]</div><div class="ql-code-block" data-language="plain">  wait_on_page_writeback - [2] 等待write标记被清理，被 [4] 阻塞</div><div class="ql-code-block" data-language="plain">    wait_on_page_bit</div><div class="ql-code-block" data-language="plain">                cifs_demultiplex_thread</div><div class="ql-code-block" data-language="plain">                cifs_read_from_socket</div><div class="ql-code-block" data-language="plain">                cifs_readv_from_socket</div><div class="ql-code-block" data-language="plain">                - 如果此时其他进程触发了reconnect</div><div class="ql-code-block" data-language="plain">                  cifs_reconnect</div><div class="ql-code-block" data-language="plain">                  - mid-&gt;mid_state 被更新为 MID_RETRY_NEEDED</div><div class="ql-code-block" data-language="plain">                  smb2_writev_callback mid_entry-&gt;callback()</div><div class="ql-code-block" data-language="plain">                   - mid_state 进而导致wdata-&gt;result = -EAGAIN</div><div class="ql-code-block" data-language="plain">                   wdata-&gt;result = -EAGAIN</div><div class="ql-code-block" data-language="plain">                   queue_work(cifsiod_wq, &amp;wdata-&gt;work);</div><div class="ql-code-block" data-language="plain">                                         cifs_writev_complete - worker函数</div><div class="ql-code-block" data-language="plain">                                           - 条件满足</div><div class="ql-code-block" data-language="plain">                                           else if (..&amp;&amp; wdata-&gt;result == -EAGAIN)</div><div class="ql-code-block" data-language="plain">                                           cifs_writev_requeue</div><div class="ql-code-block" data-language="plain">                                             lock_page  - [3] 被[1]阻塞</div><div class="ql-code-block" data-language="plain">                                           end_page_writeback  - [4] 不会诶执行，被 [3]阻塞</div><div class="ql-code-block" data-language="plain">  unlock_page</div></div><p><br></p><p><br></p><p>这就导致了cifs_writepages 和 cifsiod 的worker 陷入循环等待中，导致死锁</p><p><br></p><p>    主线重构修补程序d08089f649a0（“cifs：将I/O路径更改为使用迭代器而不是页列表”）在等待写回完成时解锁页，从而避免在重新连接期间由锁顺序问题导致的潜在死锁。</p><p><br></p><p>    由于主线的大重构，补丁不能直接backport。因此，本补丁只是借鉴了主线补丁的部分思路来修复死锁。</p><p><br></p><p>

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