Why kernel code/thread executing in interrupt context cannot sleep?

I am reading following article by Robert Love

http://www.linuxjournal.com/article/6916

that says

"...Let's discuss the fact that work queues run in process context. This is in contrast to the other bottom-half mechanisms, which all run in interrupt context. Code running in interrupt context is unable to sleep, or block, because interrupt context does not have a backing process with which to reschedule. Therefore, because interrupt handlers are not associated with a process, there is nothing for the scheduler to put to sleep and, more importantly, nothing for the scheduler to wake up..."

I don't get it. AFAIK, scheduler in the kernel is O(1), that is implemented through the bitmap. So what stops the scehduler from putting interrupt context to sleep and taking next schedulable process and passing it the control?

> So what stops the scehduler from putting interrupt context to sleep and taking next schedulable process and passing it the control?

The problem is that the interrupt context is not a process, and therefore cannot be put to sleep.

When an interrupt occurs, the processor saves the registers onto the stack and jumps to the start of the interrupt service routine. This means that when the interrupt handler is running, it is running in the context of the process that was executing when the interrupt occurred. The interrupt is executing on that process's stack, and when the interrupt handler completes, that process will resume executing.

If you tried to sleep or block inside an interrupt handler, you would wind up not only stopping the interrupt handler, but also the process it interrupted. This could be dangerous, as the interrupt handler has no way of knowing what the interrupted process was doing, or even if it is safe for that process to be suspended.

A simple scenario where things could go wrong would be a deadlock between the interrupt handler and the process it interrupts.

1. Process1 enters kernel mode.
2. Process1 acquires LockA.
3. Interrupt occurs.
4. ISR starts executing using Process1's stack.
5. ISR tries to acquire LockA.
6. ISR calls sleep to wait for LockA to be released.

At this point, you have a deadlock. Process1 can't resume execution until the ISR is done with its stack. But the ISR is blocked waiting for Process1 to release LockA.

I think it's a design idea.

Sure, you can design a system that you can sleep in interrupt, but except to make to the system hard to comprehend and complicated(many many situation you have to take into account), that's does not help anything. So from a design view, declare interrupt handler as can not sleep is very clear and easy to implement.

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From Robert Love (a kernel hacker): http://permalink.gmane.org/gmane.linux.kernel.kernelnewbies/1791

You cannot sleep in an interrupt handler because interrupts do not have a backing process context, and thus there is nothing to reschedule back into. In other words, interrupt handlers are not associated with a task, so there is nothing to "put to sleep" and (more importantly) "nothing to wake up". They must run atomically.

This is not unlike other operating systems. In most operating systems, interrupts are not threaded. Bottom halves often are, however.

The reason the page fault handler can sleep is that it is invoked only by code that is running in process context. Because the kernel's own memory is not pagable, only user-space memory accesses can result in a page fault. Thus, only a few certain places (such as calls to copy_{to,from}_user()) can cause a page fault within the kernel. Those places must all be made by code that can sleep (i.e., process context, no locks, et cetera).

Because the thread switching infrastructure is unusable at that point. When servicing an interrupt, only stuff of higher priority can execute - See the Intel Software Developer's Manual on interrupt, task and processor priority. If you did allow another thread to execute (which you imply in your question that it would be easy to do), you wouldn't be able to let it do anything - if it caused a page fault, you'd have to use services in the kernel that are unusable while the interrupt is being serviced (see below for why).

Typically, your only goal in an interrupt routine is to get the device to stop interrupting and queue something at a lower interrupt level (in unix this is typically a non-interrupt level, but for Windows, it's dispatch, apc or passive level) to do the heavy lifting where you have access to more features of the kernel/os. See - Implementing a handler.

It's a property of how O/S's have to work, not something inherent in Linux. An interrupt routine can execute at any point so the state of what you interrupted is inconsistent. If you interrupted the thread scheduling code, its state is inconsistent so you can't be sure you can "sleep" and switch threads. Even if you protect the thread switching code from being interrupted, thread switching is a very high level feature of the O/S and if you protected everything it relies on, an interrupt becomes more of a suggestion than the imperative implied by its name.

> So what stops the scehduler from putting interrupt context to sleep and taking next schedulable process and passing it the control?

Scheduling happens on timer interrupts. The basic rule is that only one interrupt can be open at a time, so if you go to sleep in the "got data from device X" interrupt, the timer interrupt cannot run to schedule it out.

Interrupts also happen many times and overlap. If you put the "got data" interrupt to sleep, and then get more data, what happens? It's confusing (and fragile) enough that the catch-all rule is: no sleeping in interrupts. You will do it wrong.

Even if you could put an ISR to sleep, you wouldn't want to do it. You want your ISRs to be as fast as possible to reduce the risk of missing subsequent interrupts.

Disallowing an interrupt handler to block is a design choice. When some data is on the device, the interrupt handler intercepts the current process, prepares the transfer of the data and enables the interrupt; before the handler enables the current interrupt, the device has to hang. We want keep our I/O busy and our system responsive, then we had better not block the interrupt handler.

I don't think the "unstable states" are an essential reason. Processes, no matter they are in user-mode or kernel-mode, should be aware that they may be interrupted by interrupts. If some kernel-mode data structure will be accessed by both interrupt handler and the current process, and race condition exists, then the current process should disable local interrupts, and moreover for multi-processor architectures, spinlocks should be used to during the critical sections.

I also don't think if the interrupt handler were blocked, it cannot be waken up. When we say "block", basically it means that the blocked process is waiting for some event/resource, so it links itself into some wait-queue for that event/resource. Whenever the resource is released, the releasing process is responsible for waking up the waiting process(es).

However, the really annoying thing is that the blocked process can do nothing during the blocking time; it did nothing wrong for this punishment, which is unfair. And nobody could surely predict the blocking time, so the innocent process has to wait for unclear reason and for unlimited time.

The linux kernel has two ways to allocate interrupt stack. One is on the kernel stack of the interrupted process, the other is a dedicated interrupt stack per CPU. If the interrupt context is saved on the dedicated interrupt stack per CPU, then indeed the interrupt context is completely not associated with any process. The "current" macro will produce an invalid pointer to current running process, since the "current" macro with some architecture are computed with the stack pointer. The stack pointer in the interrupt context may point to the dedicated interrupt stack, not the kernel stack of some process.

By nature, the question is whether in interrupt handler you can get a valid "current" (address to the current process task_structure), if yes, it's possible to modify the content there accordingly to make it into "sleep" state, which can be back by scheduler later if the state get changed somehow. The answer may be hardware-dependent.

But in ARM, it's impossible since 'current' is irrelevant to process under interrupt mode. See the code below:

#linux/arch/arm/include/asm/thread_info.h 
94 static inline struct thread_info *current_thread_info(void)
95 {
96	register unsigned long sp asm ("sp");
97	return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
98 }

sp in USER mode and SVC mode are the "same" ("same" here not mean they're equal, instead, user mode's sp point to user space stack, while svc mode's sp r13_svc point to the kernel stack, where the user process's task_structure was updated at previous task switch, When a system call occurs, the process enter kernel space again, when the sp (sp_svc) is still not changed, these 2 sp are associated with each other, in this sense, they're 'same'), So under SVC mode, kernel code can get the valid 'current'. But under other privileged modes, say interrupt mode, sp is 'different', point to dedicated address defined in cpu_init(). The 'current' calculated under these mode will be irrelevant to the interrupted process, accessing it will result in unexpected behaviors. That's why it's always said that system call can sleep but interrupt handler can't, system call works on process context but interrupt not.

High-level interrupt handlers mask the operations of all lower-priority interrupts, including those of the system timer interrupt. Consequently, the interrupt handler must avoid involving itself in an activity that might cause it to sleep. If the handler sleeps, then the system may hang because the timer is masked and incapable of scheduling the sleeping thread. Does this make sense?

If a higher-level interrupt routine gets to the point where the next thing it must do has to happen after a period of time, then it needs to put a request into the timer queue, asking that another interrupt routine be run (at lower priority level) some time later.

When that interrupt routine runs, it would then raise priority level back to the level of the original interrupt routine, and continue execution. This has the same effect as a sleep.

It is just a design/implementation choices in Linux OS. The advantage of this design is simple, but it may not be good for real time OS requirements.

Other OSes have other designs/implementations.

For example, in Solaris, the interrupts could have different priorities, that allows most of devices interrupts are invoked in interrupt threads. The interrupt threads allows sleep because each of interrupt threads has separate stack in the context of the thread. The interrupt threads design is good for real time threads which should have higher priorities than interrupts.