Understanding the FreeBSD locking mechanism

Yubin Ruan ablacktshirt at gmail.com
Sun Apr 2 08:30:38 UTC 2017

I am reading the FreeBSD source code related to its locking mechanism. I
have done some researches but still cannot understand some codes.

Let's take `spinlock' for example. I know there are different kinds of
mutex in FreeBSD: spin mutex and other kinds of mutex. I try to locate
the source of spin mutex but what I find all look very weird to me. For
example, the `spinlock_enter()`

 1816 void
 1817 spinlock_enter(void)
 1818 {
 1819         struct thread *td;
 1820         register_t flags;
 1822         td = curthread;
 1823         if (td->td_md.md_spinlock_count == 0) {
 1824                 flags = intr_disable();
 1825                 td->td_md.md_spinlock_count = 1;
 1826                 td->td_md.md_saved_flags = flags;
 1827         } else
 1828                 td->td_md.md_spinlock_count++;
 1829         critical_enter();
 1830 }

Does this function provides the ordinary "spinlock" functionality? There
is no special "test-and-set" instruction, and neither any extra locking
to protect internal data structure manipulation. Isn't this subjected to
race condition?

I also checked the `mtx_lock()`, but neither can't find a seemingly
correct implementation.

Do I miss anything? Which is the real implementation of the spin lock in

Yubin Ruan

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