1=======================
2Generic Mutex Subsystem
3=======================
4
5started by Ingo Molnar <mingo@redhat.com>
6
7updated by Davidlohr Bueso <davidlohr@hp.com>
8
9What are mutexes?
10-----------------
11
12In the Linux kernel, mutexes refer to a particular locking primitive
13that enforces serialization on shared memory systems, and not only to
14the generic term referring to 'mutual exclusion' found in academia
15or similar theoretical text books. Mutexes are sleeping locks which
16behave similarly to binary semaphores, and were introduced in 2006[1]
17as an alternative to these. This new data structure provided a number
18of advantages, including simpler interfaces, and at that time smaller
19code (see Disadvantages).
20
21[1] https://lwn.net/Articles/164802/
22
23Implementation
24--------------
25
26Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
27and implemented in kernel/locking/mutex.c. These locks use an atomic variable
28(->owner) to keep track of the lock state during its lifetime.  Field owner
29actually contains `struct task_struct *` to the current lock owner and it is
30therefore NULL if not currently owned. Since task_struct pointers are aligned
31to at least L1_CACHE_BYTES, low bits (3) are used to store extra state (e.g.,
32if waiter list is non-empty).  In its most basic form it also includes a
33wait-queue and a spinlock that serializes access to it. Furthermore,
34CONFIG_MUTEX_SPIN_ON_OWNER=y systems use a spinner MCS lock (->osq), described
35below 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 cmpxchg()ing the owner with
41    the current task. This only works in the uncontended case (cmpxchg() checks
42    against 0UL, so all 3 state bits above have to be 0). If the lock is
43    contended it goes to the next possible path.
44
45(ii) midpath: aka optimistic spinning, tries to spin for acquisition
46     while the lock owner is running and there are no other tasks ready
47     to run that have higher priority (need_resched). The rationale is
48     that if the lock owner is running, it is likely to release the lock
49     soon. The mutex spinners are queued up using MCS lock so that only
50     one spinner can compete for the mutex.
51
52     The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
53     with the desirable properties of being fair and with each cpu trying
54     to acquire the lock spinning on a local variable. It avoids expensive
55     cacheline bouncing that common test-and-set spinlock implementations
56     incur. An MCS-like lock is specially tailored for optimistic spinning
57     for sleeping lock implementation. An important feature of the customized
58     MCS lock is that it has the extra property that spinners are able to exit
59     the MCS spinlock queue when they need to reschedule. This further helps
60     avoid situations where MCS spinners that need to reschedule would continue
61     waiting to spin on mutex owner, only to go directly to slowpath upon
62     obtaining the MCS lock.
63
64
65(iii) slowpath: last resort, if the lock is still unable to be acquired,
66      the task is added to the wait-queue and sleeps until woken up by the
67      unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
68
69While formally kernel mutexes are sleepable locks, it is path (ii) that
70makes them more practically a hybrid type. By simply not interrupting a
71task and busy-waiting for a few cycles instead of immediately sleeping,
72the performance of this lock has been seen to significantly improve a
73number of workloads. Note that this technique is also used for rw-semaphores.
74
75Semantics
76---------
77
78The mutex subsystem checks and enforces the following rules:
79
80    - Only one task can hold the mutex at a time.
81    - Only the owner can unlock the mutex.
82    - Multiple unlocks are not permitted.
83    - Recursive locking/unlocking is not permitted.
84    - A mutex must only be initialized via the API (see below).
85    - A task may not exit with a mutex held.
86    - Memory areas where held locks reside must not be freed.
87    - Held mutexes must not be reinitialized.
88    - Mutexes may not be used in hardware or software interrupt
89      contexts such as tasklets and timers.
90
91These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
92In addition, the mutex debugging code also implements a number of other
93features that make lock debugging easier and faster:
94
95    - Uses symbolic names of mutexes, whenever they are printed
96      in debug output.
97    - Point-of-acquire tracking, symbolic lookup of function names,
98      list of all locks held in the system, printout of them.
99    - Owner tracking.
100    - Detects self-recursing locks and prints out all relevant info.
101    - Detects multi-task circular deadlocks and prints out all affected
102      locks and tasks (and only those tasks).
103
104Mutexes - and most other sleeping locks like rwsems - do not provide an
105implicit reference for the memory they occupy, which reference is released
106with mutex_unlock().
107
108[ This is in contrast with spin_unlock() [or completion_done()], which
109  APIs can be used to guarantee that the memory is not touched by the
110  lock implementation after spin_unlock()/completion_done() releases
111  the lock. ]
112
113mutex_unlock() may access the mutex structure even after it has internally
114released the lock already - so it's not safe for another context to
115acquire the mutex and assume that the mutex_unlock() context is not using
116the structure anymore.
117
118The mutex user must ensure that the mutex is not destroyed while a
119release operation is still in progress - in other words, callers of
120mutex_unlock() must ensure that the mutex stays alive until mutex_unlock()
121has returned.
122
123Interfaces
124----------
125Statically define the mutex::
126
127   DEFINE_MUTEX(name);
128
129Dynamically initialize the mutex::
130
131   mutex_init(mutex);
132
133Acquire the mutex, uninterruptible::
134
135   void mutex_lock(struct mutex *lock);
136   void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
137   int  mutex_trylock(struct mutex *lock);
138
139Acquire the mutex, interruptible::
140
141   int mutex_lock_interruptible_nested(struct mutex *lock,
142				       unsigned int subclass);
143   int mutex_lock_interruptible(struct mutex *lock);
144
145Acquire the mutex, interruptible, if dec to 0::
146
147   int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
148
149Unlock the mutex::
150
151   void mutex_unlock(struct mutex *lock);
152
153Test if the mutex is taken::
154
155   int mutex_is_locked(struct mutex *lock);
156
157Disadvantages
158-------------
159
160Unlike its original design and purpose, 'struct mutex' is among the largest
161locks in the kernel. E.g: on x86-64 it is 32 bytes, where 'struct semaphore'
162is 24 bytes and rw_semaphore is 40 bytes. Larger structure sizes mean more CPU
163cache and memory footprint.
164
165When to use mutexes
166-------------------
167
168Unless the strict semantics of mutexes are unsuitable and/or the critical
169region prevents the lock from being shared, always prefer them to any other
170locking primitive.
171