Question

当进程正在等待IO时，如何将任务状态更新为TASK_INTERRUPTABLE（即阻塞）？

想象一下这种情况，一个进程向块设备发出IO请求。根据我以前的thread，该过程最终调用elv_add_request()将请求添加到IO队列。所以我想在这个elv_add_request()调用中，实现将类似于：

elv_add_request(){
   // Register IO_CALLBACK() 
   set_task_state(task, TASK_INTERRUPTABLE); // blocked
   // flush IO request to disk
   ...
}

IO_CALLBACK(){
    set_task_state(task, TASK_RUNNING); // IO completed, ready to run
}

逻辑如下：当IO请求完成时，它将使用回调函数通知内核现在该进程已准备就绪。是否有意义？

如果是这样的话，回调机制是如何实现的？它是CPU /硬件功能吗？

Answer 1

它的行为与你描述的相似，只是在调用elv_add_request()之前设置了io回调。如果我们从前一个线程中获取堆栈：

 [<c027fac4>] error_code+0x74/0x7c
 [<c019ed65>] elv_next_request+0x6b/0x116
 [<e08335db>] scsi_request_fn+0x5e/0x26d [scsi_mod]
 [<c019ee6a>] elv_insert+0x5a/0x134
 [<c019efc1>] __elv_add_request+0x7d/0x82
 [<c019f0ab>] elv_add_request+0x16/0x1d
 [<e0e8d2ed>] pkt_generic_packet+0x107/0x133 [pktcdvd]
 [<e0e8d772>] pkt_get_disc_info+0x42/0x7b [pktcdvd]
 [<e0e8eae3>] pkt_open+0xbf/0xc56 [pktcdvd]
 [<c0168078>] do_open+0x7e/0x246
 [<c01683df>] blkdev_open+0x28/0x51
 [<c014a057>] __dentry_open+0xb5/0x160
 [<c014a183>] nameidata_to_filp+0x27/0x37
 [<c014a1c6>] do_filp_open+0x33/0x3b
 [<c014a211>] do_sys_open+0x43/0xc7
 [<c014a2cd>] sys_open+0x1c/0x1e
 [<c0102b82>] sysenter_past_esp+0x5f/0x85

调用堆栈（我正在查看4.1-rc1源代码）如下：

pkt_generic_packet() 
  blk_execute_rq()
    initialize a 'struct completion' object
    set it as 'struct request->end_io_data'
    blk_execute_rq_nowait(..., blk_end_sync_rq) // that's the io callback
    wait_for_completion_io() //sets task to TASK_UNINTERRUPTIBLE, waits on the 'struct completion' object to be complete

 ...
 the io then happens, a possible scenario would then be:
   blk_end_request()
     blk_end_bidi_request()
       blk_finish_request()
         req->end_io() // This is blk_end_sync_rq
           blk_end_sync_rq()
             complete() // sets task to TASK_NORMAL, signals completion

进程状态如何更新为阻塞状态（TASK_INTERRUPTABLE）？

1 个答案: