operating systems [b05008d3]

Syscalls transition the privilage level

• syscalls are like any regular procedure calls, but with a trap instruction
• kernel agrees to put syscalls on well known locations

Classical deadlock bugs

• Conditions for a deadlock

  1. Mutual exclusion
  2. Hold and wait
  3. No preemption
  4. Circular wait

Similar to a linux process that always run in the same memory

• own instruction pointer, and can be scheduled on different CPU's and executing different things on the same program
• shares the process structure (execpt stacks)
• linux threads have their own pid
• solaris is completely thread based
• in linux, a thread is a prococess without a thread
• created with the clone() system call
• treat as one process in some games, uses the thread group
• getpid() will tell you some info
• syn and locks!
• threads tend to be pretty fast, usually an async that does stuff in the background, and never switches off the task running state, spinning is TASK_RUN, but it's not waiting on IO but rather a lock
• biggest reason to use over a fork is because you can do the same memory

linux kernel only uses lightweight processes, or threads.

• Threads use libc to transition to kernel space, but posix threads originate from libc and libpthread. Note that the kernel only uses LWPs. However, at the kernel layer, since a userspace thread maps to a lwp, it doesn't really matter

Debugger commands

• GDB

  • info threads -> list threads
  • thread <n> -> switches between threads
  • thread apply all bt -> backtraces

• LLDB

  • thread list
  • thread select 1
  • thread backtrace all~/~bt all

Thread stacks

• each thread has its own stack to store data, and we can dump it like normal

• Stack tracing a thread is when a function A calls function B, and function B has a return address to where function A called it.

  

Symbol files

• Symbols provide mappings between memory address ranges and associated symbol names, if you don't have the symbols, you just have the current function