Generic Mutex Subsystem started by Ingo Molnar "Why on earth do we need a new mutex subsystem, and what's wrong with semaphores?" firstly, there's nothing wrong with semaphores. But if the simpler mutex semantics are sufficient for your code, then there are a couple of advantages of mutexes: - 'struct mutex' is smaller on most architectures: E.g. on x86, 'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes. A smaller structure size means less RAM footprint, and better CPU-cache utilization. - tighter code. On x86 i get the following .text sizes when switching all mutex-alike semaphores in the kernel to the mutex subsystem: text data bss dec hex filename 3280380 868188 396860 4545428 455b94 vmlinux-semaphore 3255329 865296 396732 4517357 44eded vmlinux-mutex that's 25051 bytes of code saved, or a 0.76% win - off the hottest codepaths of the kernel. (The .data savings are 2892 bytes, or 0.33%) Smaller code means better icache footprint, which is one of the major optimization goals in the Linux kernel currently. - the mutex subsystem is slightly faster and has better scalability for contended workloads. On an 8-way x86 system, running a mutex-based kernel and testing creat+unlink+close (of separate, per-task files) in /tmp with 16 parallel tasks, the average number of ops/sec is: Semaphores: Mutexes: $ ./test-mutex V 16 10 $ ./test-mutex V 16 10 8 CPUs, running 16 tasks. 8 CPUs, running 16 tasks. checking VFS performance. checking VFS performance. avg loops/sec: 34713 avg loops/sec: 84153 CPU utilization: 63% CPU utilization: 22% i.e. in this workload, the mutex based kernel was 2.4 times faster than the semaphore based kernel, _and_ it also had 2.8 times less CPU utilization. (In terms of 'ops per CPU cycle', the semaphore kernel performed 551 ops/sec per 1% of CPU time used, while the mutex kernel performed 3825 ops/sec per 1% of CPU time used - it was 6.9 times more efficient.) the scalability difference is visible even on a 2-way P4 HT box: Semaphores: Mutexes: $ ./test-mutex V 16 10 $ ./test-mutex V 16 10 4 CPUs, running 16 tasks. 8 CPUs, running 16 tasks. checking VFS performance. checking VFS performance. avg loops/sec: 127659 avg loops/sec: 181082 CPU utilization: 100% CPU utilization: 34% (the straight performance advantage of mutexes is 41%, the per-cycle efficiency of mutexes is 4.1 times better.) - there are no fastpath tradeoffs, the mutex fastpath is just as tight as the semaphore fastpath. On x86, the locking fastpath is 2 instructions: c0377ccb : c0377ccb: f0 ff 08 lock decl (%eax) c0377cce: 78 0e js c0377cde <.text..lock.mutex> c0377cd0: c3 ret the unlocking fastpath is equally tight: c0377cd1 : c0377cd1: f0 ff 00 lock incl (%eax) c0377cd4: 7e 0f jle c0377ce5 <.text..lock.mutex+0x7> c0377cd6: c3 ret - 'struct mutex' semantics are well-defined and are enforced if CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have virtually no debugging code or instrumentation. The mutex subsystem checks and enforces the following rules: * - only one task can hold the mutex at a time * - only the owner can unlock the mutex * - multiple unlocks are not permitted * - recursive locking is not permitted * - a mutex object must be initialized via the API * - a mutex object must not be initialized via memset or copying * - task may not exit with mutex held * - memory areas where held locks reside must not be freed * - held mutexes must not be reinitialized * - mutexes may not be used in hardware or software interrupt * contexts such as tasklets and timers furthermore, there are also convenience features in the debugging code: * - uses symbolic names of mutexes, whenever they are printed in debug output * - point-of-acquire tracking, symbolic lookup of function names * - list of all locks held in the system, printout of them * - owner tracking * - detects self-recursing locks and prints out all relevant info * - detects multi-task circular deadlocks and prints out all affected * locks and tasks (and only those tasks) Disadvantages ------------- The stricter mutex API means you cannot use mutexes the same way you can use semaphores: e.g. they cannot be used from an interrupt context, nor can they be unlocked from a different context that which acquired it. [ I'm not aware of any other (e.g. performance) disadvantages from using mutexes at the moment, please let me know if you find any. ] Implementation of mutexes ------------------------- 'struct mutex' is the new mutex type, defined in include/linux/mutex.h and implemented in kernel/mutex.c. It is a counter-based mutex with a spinlock and a wait-list. The counter has 3 states: 1 for "unlocked", 0 for "locked" and negative numbers (usually -1) for "locked, potential waiters queued". the APIs of 'struct mutex' have been streamlined: DEFINE_MUTEX(name); mutex_init(mutex); void mutex_lock(struct mutex *lock); int mutex_lock_interruptible(struct mutex *lock); int mutex_trylock(struct mutex *lock); void mutex_unlock(struct mutex *lock); int mutex_is_locked(struct mutex *lock); void mutex_lock_nested(struct mutex *lock, unsigned int subclass); int mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass); int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);