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Documentation / lockdep-design.txt

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Based on kernel version 3.17.3. Page generated on 2014-11-14 22:19 EST.

1	Runtime locking correctness validator
2	=====================================
4	started by Ingo Molnar <mingo@redhat.com>
5	additions by Arjan van de Ven <arjan@linux.intel.com>
7	Lock-class
8	----------
10	The basic object the validator operates upon is a 'class' of locks.
12	A class of locks is a group of locks that are logically the same with
13	respect to locking rules, even if the locks may have multiple (possibly
14	tens of thousands of) instantiations. For example a lock in the inode
15	struct is one class, while each inode has its own instantiation of that
16	lock class.
18	The validator tracks the 'state' of lock-classes, and it tracks
19	dependencies between different lock-classes. The validator maintains a
20	rolling proof that the state and the dependencies are correct.
22	Unlike an lock instantiation, the lock-class itself never goes away: when
23	a lock-class is used for the first time after bootup it gets registered,
24	and all subsequent uses of that lock-class will be attached to this
25	lock-class.
27	State
28	-----
30	The validator tracks lock-class usage history into 4n + 1 separate state bits:
32	- 'ever held in STATE context'
33	- 'ever held as readlock in STATE context'
34	- 'ever held with STATE enabled'
35	- 'ever held as readlock with STATE enabled'
37	Where STATE can be either one of (kernel/lockdep_states.h)
38	 - hardirq
39	 - softirq
40	 - reclaim_fs
42	- 'ever used'                                       [ == !unused        ]
44	When locking rules are violated, these state bits are presented in the
45	locking error messages, inside curlies. A contrived example:
47	   modprobe/2287 is trying to acquire lock:
48	    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
50	   but task is already holding lock:
51	    (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24
54	The bit position indicates STATE, STATE-read, for each of the states listed
55	above, and the character displayed in each indicates:
57	   '.'  acquired while irqs disabled and not in irq context
58	   '-'  acquired in irq context
59	   '+'  acquired with irqs enabled
60	   '?'  acquired in irq context with irqs enabled.
62	Unused mutexes cannot be part of the cause of an error.
65	Single-lock state rules:
66	------------------------
68	A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
69	following states are exclusive, and only one of them is allowed to be
70	set for any lock-class:
72	 <hardirq-safe> and <hardirq-unsafe>
73	 <softirq-safe> and <softirq-unsafe>
75	The validator detects and reports lock usage that violate these
76	single-lock state rules.
78	Multi-lock dependency rules:
79	----------------------------
81	The same lock-class must not be acquired twice, because this could lead
82	to lock recursion deadlocks.
84	Furthermore, two locks may not be taken in different order:
86	 <L1> -> <L2>
87	 <L2> -> <L1>
89	because this could lead to lock inversion deadlocks. (The validator
90	finds such dependencies in arbitrary complexity, i.e. there can be any
91	other locking sequence between the acquire-lock operations, the
92	validator will still track all dependencies between locks.)
94	Furthermore, the following usage based lock dependencies are not allowed
95	between any two lock-classes:
97	   <hardirq-safe>   ->  <hardirq-unsafe>
98	   <softirq-safe>   ->  <softirq-unsafe>
100	The first rule comes from the fact the a hardirq-safe lock could be
101	taken by a hardirq context, interrupting a hardirq-unsafe lock - and
102	thus could result in a lock inversion deadlock. Likewise, a softirq-safe
103	lock could be taken by an softirq context, interrupting a softirq-unsafe
104	lock.
106	The above rules are enforced for any locking sequence that occurs in the
107	kernel: when acquiring a new lock, the validator checks whether there is
108	any rule violation between the new lock and any of the held locks.
110	When a lock-class changes its state, the following aspects of the above
111	dependency rules are enforced:
113	- if a new hardirq-safe lock is discovered, we check whether it
114	  took any hardirq-unsafe lock in the past.
116	- if a new softirq-safe lock is discovered, we check whether it took
117	  any softirq-unsafe lock in the past.
119	- if a new hardirq-unsafe lock is discovered, we check whether any
120	  hardirq-safe lock took it in the past.
122	- if a new softirq-unsafe lock is discovered, we check whether any
123	  softirq-safe lock took it in the past.
125	(Again, we do these checks too on the basis that an interrupt context
126	could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which
127	could lead to a lock inversion deadlock - even if that lock scenario did
128	not trigger in practice yet.)
130	Exception: Nested data dependencies leading to nested locking
131	-------------------------------------------------------------
133	There are a few cases where the Linux kernel acquires more than one
134	instance of the same lock-class. Such cases typically happen when there
135	is some sort of hierarchy within objects of the same type. In these
136	cases there is an inherent "natural" ordering between the two objects
137	(defined by the properties of the hierarchy), and the kernel grabs the
138	locks in this fixed order on each of the objects.
140	An example of such an object hierarchy that results in "nested locking"
141	is that of a "whole disk" block-dev object and a "partition" block-dev
142	object; the partition is "part of" the whole device and as long as one
143	always takes the whole disk lock as a higher lock than the partition
144	lock, the lock ordering is fully correct. The validator does not
145	automatically detect this natural ordering, as the locking rule behind
146	the ordering is not static.
148	In order to teach the validator about this correct usage model, new
149	versions of the various locking primitives were added that allow you to
150	specify a "nesting level". An example call, for the block device mutex,
151	looks like this:
153	enum bdev_bd_mutex_lock_class
154	{
155	       BD_MUTEX_NORMAL,
156	       BD_MUTEX_WHOLE,
158	};
160	 mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION);
162	In this case the locking is done on a bdev object that is known to be a
163	partition.
165	The validator treats a lock that is taken in such a nested fashion as a
166	separate (sub)class for the purposes of validation.
168	Note: When changing code to use the _nested() primitives, be careful and
169	check really thoroughly that the hierarchy is correctly mapped; otherwise
170	you can get false positives or false negatives.
172	Proof of 100% correctness:
173	--------------------------
175	The validator achieves perfect, mathematical 'closure' (proof of locking
176	correctness) in the sense that for every simple, standalone single-task
177	locking sequence that occurred at least once during the lifetime of the
178	kernel, the validator proves it with a 100% certainty that no
179	combination and timing of these locking sequences can cause any class of
180	lock related deadlock. [*]
182	I.e. complex multi-CPU and multi-task locking scenarios do not have to
183	occur in practice to prove a deadlock: only the simple 'component'
184	locking chains have to occur at least once (anytime, in any
185	task/context) for the validator to be able to prove correctness. (For
186	example, complex deadlocks that would normally need more than 3 CPUs and
187	a very unlikely constellation of tasks, irq-contexts and timings to
188	occur, can be detected on a plain, lightly loaded single-CPU system as
189	well!)
191	This radically decreases the complexity of locking related QA of the
192	kernel: what has to be done during QA is to trigger as many "simple"
193	single-task locking dependencies in the kernel as possible, at least
194	once, to prove locking correctness - instead of having to trigger every
195	possible combination of locking interaction between CPUs, combined with
196	every possible hardirq and softirq nesting scenario (which is impossible
197	to do in practice).
199	[*] assuming that the validator itself is 100% correct, and no other
200	    part of the system corrupts the state of the validator in any way.
201	    We also assume that all NMI/SMM paths [which could interrupt
202	    even hardirq-disabled codepaths] are correct and do not interfere
203	    with the validator. We also assume that the 64-bit 'chain hash'
204	    value is unique for every lock-chain in the system. Also, lock
205	    recursion must not be higher than 20.
207	Performance:
208	------------
210	The above rules require _massive_ amounts of runtime checking. If we did
211	that for every lock taken and for every irqs-enable event, it would
212	render the system practically unusably slow. The complexity of checking
213	is O(N^2), so even with just a few hundred lock-classes we'd have to do
214	tens of thousands of checks for every event.
216	This problem is solved by checking any given 'locking scenario' (unique
217	sequence of locks taken after each other) only once. A simple stack of
218	held locks is maintained, and a lightweight 64-bit hash value is
219	calculated, which hash is unique for every lock chain. The hash value,
220	when the chain is validated for the first time, is then put into a hash
221	table, which hash-table can be checked in a lockfree manner. If the
222	locking chain occurs again later on, the hash table tells us that we
223	dont have to validate the chain again.
225	Troubleshooting:
226	----------------
228	The validator tracks a maximum of MAX_LOCKDEP_KEYS number of lock classes.
229	Exceeding this number will trigger the following lockdep warning:
233	By default, MAX_LOCKDEP_KEYS is currently set to 8191, and typical
234	desktop systems have less than 1,000 lock classes, so this warning
235	normally results from lock-class leakage or failure to properly
236	initialize locks.  These two problems are illustrated below:
238	1.	Repeated module loading and unloading while running the validator
239		will result in lock-class leakage.  The issue here is that each
240		load of the module will create a new set of lock classes for
241		that module's locks, but module unloading does not remove old
242		classes (see below discussion of reuse of lock classes for why).
243		Therefore, if that module is loaded and unloaded repeatedly,
244		the number of lock classes will eventually reach the maximum.
246	2.	Using structures such as arrays that have large numbers of
247		locks that are not explicitly initialized.  For example,
248		a hash table with 8192 buckets where each bucket has its own
249		spinlock_t will consume 8192 lock classes -unless- each spinlock
250		is explicitly initialized at runtime, for example, using the
251		run-time spin_lock_init() as opposed to compile-time initializers
252		such as __SPIN_LOCK_UNLOCKED().  Failure to properly initialize
253		the per-bucket spinlocks would guarantee lock-class overflow.
254		In contrast, a loop that called spin_lock_init() on each lock
255		would place all 8192 locks into a single lock class.
257		The moral of this story is that you should always explicitly
258		initialize your locks.
260	One might argue that the validator should be modified to allow
261	lock classes to be reused.  However, if you are tempted to make this
262	argument, first review the code and think through the changes that would
263	be required, keeping in mind that the lock classes to be removed are
264	likely to be linked into the lock-dependency graph.  This turns out to
265	be harder to do than to say.
267	Of course, if you do run out of lock classes, the next thing to do is
268	to find the offending lock classes.  First, the following command gives
269	you the number of lock classes currently in use along with the maximum:
271		grep "lock-classes" /proc/lockdep_stats
273	This command produces the following output on a modest system:
275		 lock-classes:                          748 [max: 8191]
277	If the number allocated (748 above) increases continually over time,
278	then there is likely a leak.  The following command can be used to
279	identify the leaking lock classes:
281		grep "BD" /proc/lockdep
283	Run the command and save the output, then compare against the output from
284	a later run of this command to identify the leakers.  This same output
285	can also help you find situations where runtime lock initialization has
286	been omitted.
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