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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 | 2) release the futex lock, and |
150 | 2) 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. |
183 |
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