P2L4 Thread Design Considerations - Signals and Interrupts

Question 1

What is an interrupt?

Answer 1

• Interrupts are events to the CPU.
• Interrupts are generated externally by components other than the CPU.

Question 2

Give 3 examples of components that can interrupt a CPU

Answer 2

1. I/O devices
2. Timers
3. Other CPUs

Question 3

What is a signal?

Answer 3

Signals are events that are:

• Triggered by the CPU

​or

• by software running on the CPU

Question 4

Which particular interrupts can occur on a given physical platform depends on:

1. the ___________ of that platform
2. the ___________________ the platform comes with
3. the _________________of the platform itself.

Answer 4

1. the configuration of that platform
2. the types of devices the platform comes with
3. the hardware architecture of the platform itself.

Question 5

Do interrupts appear asynchronously or synchronously?

Answer 5

always asynchronously

Question 6

Two identical hardware platforms, running different operating systems will have:

• Same or different interrupts?
• Same or different signals?

Answer 6

Same interrupts

Different signals

Question 7

State 3 properties of signals that differ from interrupts

Answer 7

1. notification from the CPU itself or from software running on the CPU
2. which interrupts can occur depends on the operating system
3. may be asynchonous or synchronous

Question 8

State 3 properties of interrupts that are not shared with signals

Answer 8

1. notification from outside the CPU, to the CPU
2. which interrupts can occur on platform, depends on the physical platform (configuration, types of devices, hardware architecture)
3. asynchonous

Question 9

List 3 similarities between interrupts and signals.

But for each similarity note the variation between interrupt and signal.

Answer 9

1. unique identifier
   
   • interrupt —> defined by hardware
   • signal —> defined by OS
2. can be masked (to enable/disable)
   
   • interrupt —> mask per-CPU
   • signal —> mask per process
3. handler
   
   • interrupt —> handler specified for whole system
   • signal —> handler specified per process

Question 10

Give a toy shop visual metaphor for signals and interrupts.

1. An interrupt is like ….
2. A signal is like ….
3. Discuss 3 ways of categorizing signals and interrupts

• 1. ________________
• 2. ________________
• 3.__________________

Answer 10

1. An interrupt is like … a snowstorm
2. A signal is like …. a low battery warning
3. Discuss 3 ways of categorizing signals and interrupts

• 1. handle in specific ways, e.g. safety protocols, rules..
• 2. Can be ignored (masking …)
• 3.expected/unexpected - synchronous/asynchronous

Question 11

Explain how an interrupt works

Answer 11

1. Device interrupts the CPU via the wires that connect it.
2. It sends a unique message, i.e. INT-N.
3. The hardware defined message is then looked up in a table
4. Corresponding handler is called. (OS defined!!)
5. The program counter is moved to that address of the handler code
6. Handler routine is then executed.
7. All of this happens within the context of the thread that is interrupted.

Question 12

Interrupt handling always occurs outside of the thread context. True or false?

Answer 12

False

The interrupt interrupts the execution of the thread that was executing on top of the CPU

… The CPU looks up the interrupt number in a table, and executes the handler routine that the interrupt maps to …

All of this happens within the context of the thread that is interrupted.

Question 13

What is SIGSEGV? It’s a signal that ….

Describe how it is handled as an example of signal handling

Answer 13

SIGSEGV - process tries to access memory that has not been allocated

Steps of signal handling:

1. Signals originate from the CPU.
2. For each process, the OS maintains a mapping
3. Keys in map correspond to the signal number (SIGSEGV is signal 11, for example).
4. Values in map point to the starting address of handling routines.
5. When a signal is generated, the program counter is adjusted to point to the handling routine for that signal for that process.

Question 14

Give 3 examples of default actions the OS has for handling signals

Answer 14

The process may specify how a signal can handled, or the operating system default may be used. Some default signal responses include:

• Terminate
• Ignore
• Terminate and Core Dump (like SIGSEGV)
• Stop
• Continue (from stopped)

Question 15

Give 2 examples of asynchronous signals and explain when they are sent.

Answer 15

1. SIGKILL(kill) - from the point of view of targeted process, the kill comes asynchronously out of the blue!
2. SIGALARM - timer timeout

Question 16

Give 3 examples of synchronous signals and explain when they are sent.

Answer 16

1. SIGSEGV - access to protected memory
2. SIGFPE - divide by zero
3. SIGKILL(kill, id) - one process sends signal to kill another process

Question 17

• Can a process install its own custom handling routine?
• Can all signals be caught?

Answer 17

• Yes, for most signals, processes can install its own custom handling routine, usually through a system call like signal() or sigaction()
• No, there are some signals which cannot be caught.

Question 18

Why would you disable interrupts or signals? What problem could arise? How does disabling the interrupt/signal solve the problem?

Answer 18

Because a deadlock situation could arise.

Because interrupts and signals are handled in the same context of the thread being interrupted.

So there is a shared stack

So we need to use a mutex. But …

The interrupted thread is waiting for the handler to return. The handler is waiting for the interrupted thread to release the mutex lock —> deadlock!!

Solution:

• disable the interrupt/signal before acquiring the mutex
• re-enable interrupt/signal after releasing the mutex.

Once the signal is re-enabled, the pending signal is handled by the handler.

Question 19

Fill in the following with ‘interrupt’ or ‘signal’:

________ masks are maintained on a per CPU basis. This means that if a mask disables an ________ , the ________ ________ routing mechanism will not deliver the ________ to the CPU.

On the other hand, ________ masks are maintained on a per ________ ________ (_ _ _ on top of KLT). If a mask disables a ________ , the kernel will see this and will not interrupt the corresponding ________ ________ .

Answer 19

Interrupt masks are maintained on a per CPU basis. This means that if a mask disables an interrupt, the hardware interrupt routing mechanism will not deliver the interrupt to the CPU.

Signal masks are maintained on a per execution context (ULT on top of KLT). If a mask disables a signal, the kernel will see this and will not interrupt the corresponding execution context.

Question 20

True or False?

If the calling process defines a handler for a one-shot signal, each time the signal interrupts, that handler will be executed.

Answer 20

False

• One-shot signals must also be explicitly re-enabled every time.
• If the handler is not re-enabled, the default handler (if it exists) will be executed the subsequent times.

Question 21

One shot signals refer to …

Real time signals refer to …

Answer 21

• One-shot signals refer to signals that will only interrupt once.
• This means that from the perspective of the user level thread, n signals will look exactly like one signal.
• One-shot signals must also be explicitly re-enabled every time.
• Real Time Signals refer to signals that will interrupt as many times are they are raised.
• If n signals occur, the handler will be called n times.

Question 22

When are these signals sent?

1. SIGINT
2. SIGURG
3. SIGTTOU
4. SIGXFSZ

Answer 22

1. terminal interrupt signal
2. high bandwidth data is available on a socket
3. background process attempting to write.
4. file size limit exceeded