site_archiver@lists.apple.com Delivered-To: darwin-dev@lists.apple.com Today, for some reason or another, I've been pondering how locks work. After locating the pthread source (in libc, available at opensource.apple.com, for those curious) I've found the source for the pthread_mutex_lock, in pthread_mutex.c It seems the meat of this function are the macros 'LOCK' and 'PLOCKSTAT_MUTEX_ACQUIRE'. The 'LOCK' macro is defined (indirectly) as _spin_lock. _spin_lock is implemented in various places based on the architecture you're compiling for; in my case I'm interested in i386. For that, _spin_lock is defined as: _OSSpinLockLock: _spin_lock: __spin_lock: movl $(_COMM_PAGE_SPINLOCK_LOCK), %eax jmpl *%eax .align 2, 0x90 .globl _OSSpinLockUnlock .globl _spin_unlock .globl __spin_unlock But I can't seem to find what _COMM_PAGE_SPINLOCK_LOCK is. I'm taking a shot in the dark that it's a function address in the kernel's address space. Is this correct? (I'm no assembly expert! :)) If so, where is the value for _COMM_PAGE_SPINLOCK_LOCK set? Secondly we have PLOCKSTAT_MUTEX_ACQUIRE. I can't seem to find where it's defined either, and it seems to be the real heavy-lifter for pthread_mutex_lock. Any ideas? Lastly and perhaps most important, is there a good low-level OS book that might explain the nitty gritty when it comes to locks and everything else deep under-the-hood? -- Greg Parker gparker@apple.com Runtime Wrangler _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... On Feb 12, 2009, at 6:34 PM, Dave Keck wrote: The "comm page" is an area of memory in user space which is shared across all processes and whose contents are controlled by the kernel. It's used for various low-level bits of code like memcpy and spinlock that are tuned carefully for machine-specific details such as CPU type and processor count. Code for at least some of the comm page's functions is in xnu/osfmk/ <arch>/commpage/ . spinlock for i386 is xnu/osfmk/i386/commpage/ spinlocks.s, where there are versions optimized for single- and multi- processor machines, both 32- and 64-bit. Ignore anything plockstat-related. That's just a hook for lock statistics via dtrace, and I don't even know if it's enabled by default. pthread_mutex is based on spinlocks plus either Mach semaphores or kernel mutexes, depending on the pthread mutex attributes requested. Amit Singh's Mac OS X Internals (http://osxbook.com) contains more nitty gritty than you can shake a stick at, but I don't know what his pthreads discussion looks like. Any OS textbook should be a good source for the general ideas of how to write synchronization primitives like spinlock or compare-and-swap and then build higher- level constructs with them. This email sent to site_archiver@lists.apple.com