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Documentation / robust-futex-ABI.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Started by Paul Jackson <pj@sgi.com>
2	
3	The robust futex ABI
4	--------------------
5	
6	Robust_futexes provide a mechanism that is used in addition to normal
7	futexes, for kernel assist of cleanup of held locks on task exit.
8	
9	The interesting data as to what futexes a thread is holding is kept on a
10	linked list in user space, where it can be updated efficiently as locks
11	are taken and dropped, without kernel intervention.  The only additional
12	kernel intervention required for robust_futexes above and beyond what is
13	required for futexes is:
14	
15	 1) a one time call, per thread, to tell the kernel where its list of
16	    held robust_futexes begins, and
17	 2) internal kernel code at exit, to handle any listed locks held
18	    by the exiting thread.
19	
20	The existing normal futexes already provide a "Fast Userspace Locking"
21	mechanism, which handles uncontested locking without needing a system
22	call, and handles contested locking by maintaining a list of waiting
23	threads in the kernel.  Options on the sys_futex(2) system call support
24	waiting on a particular futex, and waking up the next waiter on a
25	particular futex.
26	
27	For robust_futexes to work, the user code (typically in a library such
28	as glibc linked with the application) has to manage and place the
29	necessary list elements exactly as the kernel expects them.  If it fails
30	to do so, then improperly listed locks will not be cleaned up on exit,
31	probably causing deadlock or other such failure of the other threads
32	waiting on the same locks.
33	
34	A thread that anticipates possibly using robust_futexes should first
35	issue the system call:
36	
37	    asmlinkage long
38	    sys_set_robust_list(struct robust_list_head __user *head, size_t len);
39	
40	The pointer 'head' points to a structure in the threads address space
41	consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
42	bits on 64 bit arch's, and local byte order.  Each thread should have
43	its own thread private 'head'.
44	
45	If a thread is running in 32 bit compatibility mode on a 64 native arch
46	kernel, then it can actually have two such structures - one using 32 bit
47	words for 32 bit compatibility mode, and one using 64 bit words for 64
48	bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
49	compatibility mode, will attempt to process both lists on each task
50	exit, if the corresponding sys_set_robust_list() call has been made to
51	setup that list.
52	
53	  The first word in the memory structure at 'head' contains a
54	  pointer to a single linked list of 'lock entries', one per lock,
55	  as described below.  If the list is empty, the pointer will point
56	  to itself, 'head'.  The last 'lock entry' points back to the 'head'.
57	
58	  The second word, called 'offset', specifies the offset from the
59	  address of the associated 'lock entry', plus or minus, of what will
60	  be called the 'lock word', from that 'lock entry'.  The 'lock word'
61	  is always a 32 bit word, unlike the other words above.  The 'lock
62	  word' holds 3 flag bits in the upper 3 bits, and the thread id (TID)
63	  of the thread holding the lock in the bottom 29 bits.  See further
64	  below for a description of the flag bits.
65	
66	  The third word, called 'list_op_pending', contains transient copy of
67	  the address of the 'lock entry', during list insertion and removal,
68	  and is needed to correctly resolve races should a thread exit while
69	  in the middle of a locking or unlocking operation.
70	
71	Each 'lock entry' on the single linked list starting at 'head' consists
72	of just a single word, pointing to the next 'lock entry', or back to
73	'head' if there are no more entries.  In addition, nearby to each 'lock
74	entry', at an offset from the 'lock entry' specified by the 'offset'
75	word, is one 'lock word'.
76	
77	The 'lock word' is always 32 bits, and is intended to be the same 32 bit
78	lock variable used by the futex mechanism, in conjunction with
79	robust_futexes.  The kernel will only be able to wakeup the next thread
80	waiting for a lock on a threads exit if that next thread used the futex
81	mechanism to register the address of that 'lock word' with the kernel.
82	
83	For each futex lock currently held by a thread, if it wants this
84	robust_futex support for exit cleanup of that lock, it should have one
85	'lock entry' on this list, with its associated 'lock word' at the
86	specified 'offset'.  Should a thread die while holding any such locks,
87	the kernel will walk this list, mark any such locks with a bit
88	indicating their holder died, and wakeup the next thread waiting for
89	that lock using the futex mechanism.
90	
91	When a thread has invoked the above system call to indicate it
92	anticipates using robust_futexes, the kernel stores the passed in 'head'
93	pointer for that task.  The task may retrieve that value later on by
94	using the system call:
95	
96	    asmlinkage long
97	    sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
98	                        size_t __user *len_ptr);
99	
100	It is anticipated that threads will use robust_futexes embedded in
101	larger, user level locking structures, one per lock.  The kernel
102	robust_futex mechanism doesn't care what else is in that structure, so
103	long as the 'offset' to the 'lock word' is the same for all
104	robust_futexes used by that thread.  The thread should link those locks
105	it currently holds using the 'lock entry' pointers.  It may also have
106	other links between the locks, such as the reverse side of a double
107	linked list, but that doesn't matter to the kernel.
108	
109	By keeping its locks linked this way, on a list starting with a 'head'
110	pointer known to the kernel, the kernel can provide to a thread the
111	essential service available for robust_futexes, which is to help clean
112	up locks held at the time of (a perhaps unexpectedly) exit.
113	
114	Actual locking and unlocking, during normal operations, is handled
115	entirely by user level code in the contending threads, and by the
116	existing futex mechanism to wait for, and wakeup, locks.  The kernels
117	only essential involvement in robust_futexes is to remember where the
118	list 'head' is, and to walk the list on thread exit, handling locks
119	still held by the departing thread, as described below.
120	
121	There may exist thousands of futex lock structures in a threads shared
122	memory, on various data structures, at a given point in time. Only those
123	lock structures for locks currently held by that thread should be on
124	that thread's robust_futex linked lock list a given time.
125	
126	A given futex lock structure in a user shared memory region may be held
127	at different times by any of the threads with access to that region. The
128	thread currently holding such a lock, if any, is marked with the threads
129	TID in the lower 29 bits of the 'lock word'.
130	
131	When adding or removing a lock from its list of held locks, in order for
132	the kernel to correctly handle lock cleanup regardless of when the task
133	exits (perhaps it gets an unexpected signal 9 in the middle of
134	manipulating this list), the user code must observe the following
135	protocol on 'lock entry' insertion and removal:
136	
137	On insertion:
138	 1) set the 'list_op_pending' word to the address of the 'lock entry'
139	    to be inserted,
140	 2) acquire the futex lock,
141	 3) add the lock entry, with its thread id (TID) in the bottom 29 bits
142	    of the 'lock word', to the linked list starting at 'head', and
143	 4) clear the 'list_op_pending' word.
144	
145	On removal:
146	 1) set the 'list_op_pending' word to the address of the 'lock entry'
147	    to be removed,
148	 2) remove the lock entry for this lock from the 'head' list,
149	 3) release the futex lock, and
150	 4) clear the 'lock_op_pending' word.
151	
152	On exit, the kernel will consider the address stored in
153	'list_op_pending' and the address of each 'lock word' found by walking
154	the list starting at 'head'.  For each such address, if the bottom 29
155	bits of the 'lock word' at offset 'offset' from that address equals the
156	exiting threads TID, then the kernel will do two things:
157	
158	 1) if bit 31 (0x80000000) is set in that word, then attempt a futex
159	    wakeup on that address, which will waken the next thread that has
160	    used to the futex mechanism to wait on that address, and
161	 2) atomically set  bit 30 (0x40000000) in the 'lock word'.
162	
163	In the above, bit 31 was set by futex waiters on that lock to indicate
164	they were waiting, and bit 30 is set by the kernel to indicate that the
165	lock owner died holding the lock.
166	
167	The kernel exit code will silently stop scanning the list further if at
168	any point:
169	
170	 1) the 'head' pointer or an subsequent linked list pointer
171	    is not a valid address of a user space word
172	 2) the calculated location of the 'lock word' (address plus
173	    'offset') is not the valid address of a 32 bit user space
174	    word
175	 3) if the list contains more than 1 million (subject to
176	    future kernel configuration changes) elements.
177	
178	When the kernel sees a list entry whose 'lock word' doesn't have the
179	current threads TID in the lower 29 bits, it does nothing with that
180	entry, and goes on to the next entry.
181	
182	Bit 29 (0x20000000) of the 'lock word' is reserved for future use.
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