<title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>
<para>
- There are two main types of kernel locks. The fundamental type
+ There are three main types of kernel locks. The fundamental type
is the spinlock
(<filename class="headerfile">include/asm/spinlock.h</filename>),
which is a very simple single-holder lock: if you can't get the
very small and fast, and can be used anywhere.
</para>
<para>
- The second type is a semaphore
+ The second type is a mutex
+ (<filename class="headerfile">include/linux/mutex.h</filename>): it
+ is like a spinlock, but you may block holding a mutex.
+ If you can't lock a mutex, your task will suspend itself, and be woken
+ up when the mutex is released. This means the CPU can do something
+ else while you are waiting. There are many cases when you simply
+ can't sleep (see <xref linkend="sleeping-things"/>), and so have to
+ use a spinlock instead.
+ </para>
+ <para>
+ The third type is a semaphore
(<filename class="headerfile">include/asm/semaphore.h</filename>): it
can have more than one holder at any time (the number decided at
initialization time), although it is most commonly used as a
- single-holder lock (a mutex). If you can't get a semaphore,
- your task will put itself on the queue, and be woken up when the
- semaphore is released. This means the CPU will do something
- else while you are waiting, but there are many cases when you
- simply can't sleep (see <xref linkend="sleeping-things"/>), and so
- have to use a spinlock instead.
+ single-holder lock (a mutex). If you can't get a semaphore, your
+ task will be suspended and later on woken up - just like for mutexes.
</para>
<para>
Neither type of lock is recursive: see
--- /dev/null
+Generic Mutex Subsystem
+
+started by Ingo Molnar <mingo@redhat.com>
+
+ "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 <mutex_lock>:
+ 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 <mutex_unlock>:
+ 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 irq contexts
+
+ 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);
+
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