About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / pi-futex.txt




Custom Search

Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

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

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.