Lecture: concurrency Q&A

preparation

• read OSPP §6, Multi-Object Synchronization.

spinlocks in xv6

• show spinlock.h, spinlock.c
  • xchg and ->locked
  • asm volatile
  • would cpu or compiler reorder things?
• what’s pushcli()?
• can we use ->locked = 0 in release() instead of xchg?

more on locks

• spinlocks recap
  • spin on locked (e.g., using xchg)
  • hold for very short times
  • don’t yield CPU while holding lock
  • (un)fairness issues: FIFO ordering?
• alternative: ticket spinlocks
  • real world example: restaurants, banks, DMV, …
  • use two variables
    • next ticket
    • “now serving” ticket
  • take a number from next & wait until now_serving reaches next
  • see Linux kernel’s ticket spinlocks

struct lock { _Atomic int next, now_serving; };

void acquire(struct lock *l)
{
  int ticket = atomic_fetch_add(&l->next, 1);
  while (l->now_serving != ticket);
}

void release(struct lock *l)
{
  ++l->now_serving;
}

• many other locks
  • MCS locks
  • “blocking” locks: waiting threads yield the CPU
  • rwlocks (e.g., pthread_rwlock_t)
  • RCU
• alternative: transactional memory
  • example: Intel’s TSX
  • demo using ph.c
    • compile: gcc -mrtm
    • make sure your CPU supports TSX
      • Linux: check for hlm and rtm in /proc/cpuinfo
      • OS X: sysctl machdep.cpu | grep features
      • otherwise, “Illegal instruction”
  • how do you compare this interface to locks
    • easy to use?
    • performance?
  • ideas for lab X?

#include <immintrin.h>
...
if (_xbegin() == _XBEGIN_STARTED) {
  // do a few things
  _xend();
} else {
  // plan B (e.g., fail or retry)
}