1Generic Mutex Subsystem
2
3started by Ingo Molnar <mingo@redhat.com>
4updated by Davidlohr Bueso <davidlohr@hp.com>
5
6What are mutexes?
7-----------------
8
9In the Linux kernel, mutexes refer to a particular locking primitive
10that enforces serialization on shared memory systems, and not only to
11the generic term referring to 'mutual exclusion' found in academia
12or similar theoretical text books. Mutexes are sleeping locks which
13behave similarly to binary semaphores, and were introduced in 2006[1]
14as an alternative to these. This new data structure provided a number
15of advantages, including simpler interfaces, and at that time smaller
16code (see Disadvantages).
17
18[1] http://lwn.net/Articles/164802/
19
20Implementation
21--------------
22
23Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
24and implemented in kernel/locking/mutex.c. These locks use a three
25state atomic counter (->count) to represent the different possible
26transitions that can occur during the lifetime of a lock:
27
28	  1: unlocked
29	  0: locked, no waiters
30   negative: locked, with potential waiters
31
32In its most basic form it also includes a wait-queue and a spinlock
33that serializes access to it. CONFIG_SMP systems can also include
34a pointer to the lock task owner (->owner) as well as a spinner MCS
35lock (->osq), both described below in (ii).
36
37When acquiring a mutex, there are three possible paths that can be
38taken, depending on the state of the lock:
39
40(i) fastpath: tries to atomically acquire the lock by decrementing the
41    counter. If it was already taken by another task it goes to the next
42    possible path. This logic is architecture specific. On x86-64, the
43    locking fastpath is 2 instructions:
44
45    0000000000000e10 <mutex_lock>:
46    e21:   f0 ff 0b                lock decl (%rbx)
47    e24:   79 08                   jns    e2e <mutex_lock+0x1e>
48
49   the unlocking fastpath is equally tight:
50
51    0000000000000bc0 <mutex_unlock>:
52    bc8:   f0 ff 07                lock incl (%rdi)
53    bcb:   7f 0a                   jg     bd7 <mutex_unlock+0x17>
54
55
56(ii) midpath: aka optimistic spinning, tries to spin for acquisition
57     while the lock owner is running and there are no other tasks ready
58     to run that have higher priority (need_resched). The rationale is
59     that if the lock owner is running, it is likely to release the lock
60     soon. The mutex spinners are queued up using MCS lock so that only
61     one spinner can compete for the mutex.
62
63     The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
64     with the desirable properties of being fair and with each cpu trying
65     to acquire the lock spinning on a local variable. It avoids expensive
66     cacheline bouncing that common test-and-set spinlock implementations
67     incur. An MCS-like lock is specially tailored for optimistic spinning
68     for sleeping lock implementation. An important feature of the customized
69     MCS lock is that it has the extra property that spinners are able to exit
70     the MCS spinlock queue when they need to reschedule. This further helps
71     avoid situations where MCS spinners that need to reschedule would continue
72     waiting to spin on mutex owner, only to go directly to slowpath upon
73     obtaining the MCS lock.
74
75
76(iii) slowpath: last resort, if the lock is still unable to be acquired,
77      the task is added to the wait-queue and sleeps until woken up by the
78      unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
79
80While formally kernel mutexes are sleepable locks, it is path (ii) that
81makes them more practically a hybrid type. By simply not interrupting a
82task and busy-waiting for a few cycles instead of immediately sleeping,
83the performance of this lock has been seen to significantly improve a
84number of workloads. Note that this technique is also used for rw-semaphores.
85
86Semantics
87---------
88
89The mutex subsystem checks and enforces the following rules:
90
91    - Only one task can hold the mutex at a time.
92    - Only the owner can unlock the mutex.
93    - Multiple unlocks are not permitted.
94    - Recursive locking/unlocking is not permitted.
95    - A mutex must only be initialized via the API (see below).
96    - A task may not exit with a mutex held.
97    - Memory areas where held locks reside must not be freed.
98    - Held mutexes must not be reinitialized.
99    - Mutexes may not be used in hardware or software interrupt
100      contexts such as tasklets and timers.
101
102These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
103In addition, the mutex debugging code also implements a number of other
104features that make lock debugging easier and faster:
105
106    - Uses symbolic names of mutexes, whenever they are printed
107      in debug output.
108    - Point-of-acquire tracking, symbolic lookup of function names,
109      list of all locks held in the system, printout of them.
110    - Owner tracking.
111    - Detects self-recursing locks and prints out all relevant info.
112    - Detects multi-task circular deadlocks and prints out all affected
113      locks and tasks (and only those tasks).
114
115
116Interfaces
117----------
118Statically define the mutex:
119   DEFINE_MUTEX(name);
120
121Dynamically initialize the mutex:
122   mutex_init(mutex);
123
124Acquire the mutex, uninterruptible:
125   void mutex_lock(struct mutex *lock);
126   void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
127   int  mutex_trylock(struct mutex *lock);
128
129Acquire the mutex, interruptible:
130   int mutex_lock_interruptible_nested(struct mutex *lock,
131				       unsigned int subclass);
132   int mutex_lock_interruptible(struct mutex *lock);
133
134Acquire the mutex, interruptible, if dec to 0:
135   int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
136
137Unlock the mutex:
138   void mutex_unlock(struct mutex *lock);
139
140Test if the mutex is taken:
141   int mutex_is_locked(struct mutex *lock);
142
143Disadvantages
144-------------
145
146Unlike its original design and purpose, 'struct mutex' is larger than
147most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice
148as large as 'struct semaphore' (24 bytes) and tied, along with rwsems,
149for the largest lock in the kernel. Larger structure sizes mean more
150CPU cache and memory footprint.
151
152When to use mutexes
153-------------------
154
155Unless the strict semantics of mutexes are unsuitable and/or the critical
156region prevents the lock from being shared, always prefer them to any other
157locking primitive.
158