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Documentation / pi-futex.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	Lightweight PI-futexes
2	----------------------
3	
4	We are calling them lightweight for 3 reasons:
5	
6	 - in the user-space fastpath a PI-enabled futex involves no kernel work
7	   (or any other PI complexity) at all. No registration, no extra kernel
8	   calls - just pure fast atomic ops in userspace.
9	
10	 - even in the slowpath, the system call and scheduling pattern is very
11	   similar to normal futexes.
12	
13	 - the in-kernel PI implementation is streamlined around the mutex
14	   abstraction, with strict rules that keep the implementation
15	   relatively simple: only a single owner may own a lock (i.e. no
16	   read-write lock support), only the owner may unlock a lock, no
17	   recursive locking, etc.
18	
19	Priority Inheritance - why?
20	---------------------------
21	
22	The short reply: user-space PI helps achieving/improving determinism for
23	user-space applications. In the best-case, it can help achieve
24	determinism and well-bound latencies. Even in the worst-case, PI will
25	improve the statistical distribution of locking related application
26	delays.
27	
28	The longer reply:
29	-----------------
30	
31	Firstly, sharing locks between multiple tasks is a common programming
32	technique that often cannot be replaced with lockless algorithms. As we
33	can see it in the kernel [which is a quite complex program in itself],
34	lockless structures are rather the exception than the norm - the current
35	ratio of lockless vs. locky code for shared data structures is somewhere
36	between 1:10 and 1:100. Lockless is hard, and the complexity of lockless
37	algorithms often endangers to ability to do robust reviews of said code.
38	I.e. critical RT apps often choose lock structures to protect critical
39	data structures, instead of lockless algorithms. Furthermore, there are
40	cases (like shared hardware, or other resource limits) where lockless
41	access is mathematically impossible.
42	
43	Media players (such as Jack) are an example of reasonable application
44	design with multiple tasks (with multiple priority levels) sharing
45	short-held locks: for example, a highprio audio playback thread is
46	combined with medium-prio construct-audio-data threads and low-prio
47	display-colory-stuff threads. Add video and decoding to the mix and
48	we've got even more priority levels.
49	
50	So once we accept that synchronization objects (locks) are an
51	unavoidable fact of life, and once we accept that multi-task userspace
52	apps have a very fair expectation of being able to use locks, we've got
53	to think about how to offer the option of a deterministic locking
54	implementation to user-space.
55	
56	Most of the technical counter-arguments against doing priority
57	inheritance only apply to kernel-space locks. But user-space locks are
58	different, there we cannot disable interrupts or make the task
59	non-preemptible in a critical section, so the 'use spinlocks' argument
60	does not apply (user-space spinlocks have the same priority inversion
61	problems as other user-space locking constructs). Fact is, pretty much
62	the only technique that currently enables good determinism for userspace
63	locks (such as futex-based pthread mutexes) is priority inheritance:
64	
65	Currently (without PI), if a high-prio and a low-prio task shares a lock
66	[this is a quite common scenario for most non-trivial RT applications],
67	even if all critical sections are coded carefully to be deterministic
68	(i.e. all critical sections are short in duration and only execute a
69	limited number of instructions), the kernel cannot guarantee any
70	deterministic execution of the high-prio task: any medium-priority task
71	could preempt the low-prio task while it holds the shared lock and
72	executes the critical section, and could delay it indefinitely.
73	
74	Implementation:
75	---------------
76	
77	As mentioned before, the userspace fastpath of PI-enabled pthread
78	mutexes involves no kernel work at all - they behave quite similarly to
79	normal futex-based locks: a 0 value means unlocked, and a value==TID
80	means locked. (This is the same method as used by list-based robust
81	futexes.) Userspace uses atomic ops to lock/unlock these mutexes without
82	entering the kernel.
83	
84	To handle the slowpath, we have added two new futex ops:
85	
86	  FUTEX_LOCK_PI
87	  FUTEX_UNLOCK_PI
88	
89	If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to
90	TID fails], then FUTEX_LOCK_PI is called. The kernel does all the
91	remaining work: if there is no futex-queue attached to the futex address
92	yet then the code looks up the task that owns the futex [it has put its
93	own TID into the futex value], and attaches a 'PI state' structure to
94	the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware,
95	kernel-based synchronization object. The 'other' task is made the owner
96	of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the
97	futex value. Then this task tries to lock the rt-mutex, on which it
98	blocks. Once it returns, it has the mutex acquired, and it sets the
99	futex value to its own TID and returns. Userspace has no other work to
100	perform - it now owns the lock, and futex value contains
101	FUTEX_WAITERS|TID.
102	
103	If the unlock side fastpath succeeds, [i.e. userspace manages to do a
104	TID -> 0 atomic transition of the futex value], then no kernel work is
105	triggered.
106	
107	If the unlock fastpath fails (because the FUTEX_WAITERS bit is set),
108	then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the
109	behalf of userspace - and it also unlocks the attached
110	pi_state->rt_mutex and thus wakes up any potential waiters.
111	
112	Note that under this approach, contrary to previous PI-futex approaches,
113	there is no prior 'registration' of a PI-futex. [which is not quite
114	possible anyway, due to existing ABI properties of pthread mutexes.]
115	
116	Also, under this scheme, 'robustness' and 'PI' are two orthogonal
117	properties of futexes, and all four combinations are possible: futex,
118	robust-futex, PI-futex, robust+PI-futex.
119	
120	More details about priority inheritance can be found in
121	Documentation/rt-mutex.txt.
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