Why can't semaphores be used in interrupts and why can't interrupt context sleep

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1、 interrupt Why not use semaphores in ？

answer ：  Semaphore It can lead to sleep .

2、 Why can't I sleep when I interrupt ？

（ The following is a reprint of ： http://blog.chinaunix.net/uid-7332782-id-3213376.html）

         After the interrupt occurs ,CPU Jump to the interrupt handling code set by the kernel , This part of the kernel code handles interrupts . The context in this process is the interrupt context .
     Why can't functions that may cause sleep be used in the context of interrupts ？ First, sleep means putting a process in “ sleep ” state , In this state The process of cannot be scheduled for execution . then , At a certain time , This process may be reset to “ function ” state , Thus, it may be scheduled to execute . so ,“ sleep ” And “ function ” It is for the process , Representing the process task_struct Structure records the status of the process . Kernel “ Scheduler ” adopt task_struct Schedule processes .
     however , Interrupt context is not a process , It doesn't exist task_struct, So it It's not schedulable . therefore , You can't sleep in interrupt context .
     that , Why does the interrupt context have no corresponding task_struct Structure ？
     Interrupts occur very frequently （ At least every millisecond （ See configuration , Probably 10 Milliseconds or other values ） A clock interrupt is generated ）, And interrupt processing will be very fast . If a corresponding... Is maintained for the interrupt context task_struct structure , Then this structure is frequently allocated 、 Recycling 、 And affect the management of the scheduler , This will affect the throughput of the whole system .
     But in some embedded systems that pursue real-time linux in , Interrupts may also be assigned task_struct structure . This is to avoid a large number of interrupts and continuous nesting , Lead to a period of time CPU Always run in interrupt context , So that some very high priority processes can not be run . This approach can improve the real-time performance of the system , But the cost is a reduction in throughput .