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

1	Started by: Ingo Molnar <mingo@redhat.com>
2	
3	Background
4	----------
5	
6	what are robust futexes? To answer that, we first need to understand
7	what futexes are: normal futexes are special types of locks that in the
8	noncontended case can be acquired/released from userspace without having
9	to enter the kernel.
10	
11	A futex is in essence a user-space address, e.g. a 32-bit lock variable
12	field. If userspace notices contention (the lock is already owned and
13	someone else wants to grab it too) then the lock is marked with a value
14	that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
15	syscall is used to wait for the other guy to release it. The kernel
16	creates a 'futex queue' internally, so that it can later on match up the
17	waiter with the waker - without them having to know about each other.
18	When the owner thread releases the futex, it notices (via the variable
19	value) that there were waiter(s) pending, and does the
20	sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have
21	taken and released the lock, the futex is again back to 'uncontended'
22	state, and there's no in-kernel state associated with it. The kernel
23	completely forgets that there ever was a futex at that address. This
24	method makes futexes very lightweight and scalable.
25	
26	"Robustness" is about dealing with crashes while holding a lock: if a
27	process exits prematurely while holding a pthread_mutex_t lock that is
28	also shared with some other process (e.g. yum segfaults while holding a
29	pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
30	to be notified that the last owner of the lock exited in some irregular
31	way.
32	
33	To solve such types of problems, "robust mutex" userspace APIs were
34	created: pthread_mutex_lock() returns an error value if the owner exits
35	prematurely - and the new owner can decide whether the data protected by
36	the lock can be recovered safely.
37	
38	There is a big conceptual problem with futex based mutexes though: it is
39	the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
40	the kernel cannot help with the cleanup: if there is no 'futex queue'
41	(and in most cases there is none, futexes being fast lightweight locks)
42	then the kernel has no information to clean up after the held lock!
43	Userspace has no chance to clean up after the lock either - userspace is
44	the one that crashes, so it has no opportunity to clean up. Catch-22.
45	
46	In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
47	is needed to release that futex based lock. This is one of the leading
48	bugreports against yum.
49	
50	To solve this problem, the traditional approach was to extend the vma
51	(virtual memory area descriptor) concept to have a notion of 'pending
52	robust futexes attached to this area'. This approach requires 3 new
53	syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
54	FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
55	they have a robust_head set. This approach has two fundamental problems
56	left:
57	
58	 - it has quite complex locking and race scenarios. The vma-based
59	   approach had been pending for years, but they are still not completely
60	   reliable.
61	
62	 - they have to scan _every_ vma at sys_exit() time, per thread!
63	
64	The second disadvantage is a real killer: pthread_exit() takes around 1
65	microsecond on Linux, but with thousands (or tens of thousands) of vmas
66	every pthread_exit() takes a millisecond or more, also totally
67	destroying the CPU's L1 and L2 caches!
68	
69	This is very much noticeable even for normal process sys_exit_group()
70	calls: the kernel has to do the vma scanning unconditionally! (this is
71	because the kernel has no knowledge about how many robust futexes there
72	are to be cleaned up, because a robust futex might have been registered
73	in another task, and the futex variable might have been simply mmap()-ed
74	into this process's address space).
75	
76	This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
77	normal kernels can turn it off, but worse than that: the overhead makes
78	robust futexes impractical for any type of generic Linux distribution.
79	
80	So something had to be done.
81	
82	New approach to robust futexes
83	------------------------------
84	
85	At the heart of this new approach there is a per-thread private list of
86	robust locks that userspace is holding (maintained by glibc) - which
87	userspace list is registered with the kernel via a new syscall [this
88	registration happens at most once per thread lifetime]. At do_exit()
89	time, the kernel checks this user-space list: are there any robust futex
90	locks to be cleaned up?
91	
92	In the common case, at do_exit() time, there is no list registered, so
93	the cost of robust futexes is just a simple current->robust_list != NULL
94	comparison. If the thread has registered a list, then normally the list
95	is empty. If the thread/process crashed or terminated in some incorrect
96	way then the list might be non-empty: in this case the kernel carefully
97	walks the list [not trusting it], and marks all locks that are owned by
98	this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
99	any).
100	
101	The list is guaranteed to be private and per-thread at do_exit() time,
102	so it can be accessed by the kernel in a lockless way.
103	
104	There is one race possible though: since adding to and removing from the
105	list is done after the futex is acquired by glibc, there is a few
106	instructions window for the thread (or process) to die there, leaving
107	the futex hung. To protect against this possibility, userspace (glibc)
108	also maintains a simple per-thread 'list_op_pending' field, to allow the
109	kernel to clean up if the thread dies after acquiring the lock, but just
110	before it could have added itself to the list. Glibc sets this
111	list_op_pending field before it tries to acquire the futex, and clears
112	it after the list-add (or list-remove) has finished.
113	
114	That's all that is needed - all the rest of robust-futex cleanup is done
115	in userspace [just like with the previous patches].
116	
117	Ulrich Drepper has implemented the necessary glibc support for this new
118	mechanism, which fully enables robust mutexes.
119	
120	Key differences of this userspace-list based approach, compared to the
121	vma based method:
122	
123	 - it's much, much faster: at thread exit time, there's no need to loop
124	   over every vma (!), which the VM-based method has to do. Only a very
125	   simple 'is the list empty' op is done.
126	
127	 - no VM changes are needed - 'struct address_space' is left alone.
128	
129	 - no registration of individual locks is needed: robust mutexes dont
130	   need any extra per-lock syscalls. Robust mutexes thus become a very
131	   lightweight primitive - so they dont force the application designer
132	   to do a hard choice between performance and robustness - robust
133	   mutexes are just as fast.
134	
135	 - no per-lock kernel allocation happens.
136	
137	 - no resource limits are needed.
138	
139	 - no kernel-space recovery call (FUTEX_RECOVER) is needed.
140	
141	 - the implementation and the locking is "obvious", and there are no
142	   interactions with the VM.
143	
144	Performance
145	-----------
146	
147	I have benchmarked the time needed for the kernel to process a list of 1
148	million (!) held locks, using the new method [on a 2GHz CPU]:
149	
150	 - with FUTEX_WAIT set [contended mutex]: 130 msecs
151	 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
152	
153	I have also measured an approach where glibc does the lock notification
154	[which it currently does for !pshared robust mutexes], and that took 256
155	msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
156	userspace had to do.
157	
158	(1 million held locks are unheard of - we expect at most a handful of
159	locks to be held at a time. Nevertheless it's nice to know that this
160	approach scales nicely.)
161	
162	Implementation details
163	----------------------
164	
165	The patch adds two new syscalls: one to register the userspace list, and
166	one to query the registered list pointer:
167	
168	 asmlinkage long
169	 sys_set_robust_list(struct robust_list_head __user *head,
170	                     size_t len);
171	
172	 asmlinkage long
173	 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
174	                     size_t __user *len_ptr);
175	
176	List registration is very fast: the pointer is simply stored in
177	current->robust_list. [Note that in the future, if robust futexes become
178	widespread, we could extend sys_clone() to register a robust-list head
179	for new threads, without the need of another syscall.]
180	
181	So there is virtually zero overhead for tasks not using robust futexes,
182	and even for robust futex users, there is only one extra syscall per
183	thread lifetime, and the cleanup operation, if it happens, is fast and
184	straightforward. The kernel doesn't have any internal distinction between
185	robust and normal futexes.
186	
187	If a futex is found to be held at exit time, the kernel sets the
188	following bit of the futex word:
189	
190		#define FUTEX_OWNER_DIED        0x40000000
191	
192	and wakes up the next futex waiter (if any). User-space does the rest of
193	the cleanup.
194	
195	Otherwise, robust futexes are acquired by glibc by putting the TID into
196	the futex field atomically. Waiters set the FUTEX_WAITERS bit:
197	
198		#define FUTEX_WAITERS           0x80000000
199	
200	and the remaining bits are for the TID.
201	
202	Testing, architecture support
203	-----------------------------
204	
205	i've tested the new syscalls on x86 and x86_64, and have made sure the
206	parsing of the userspace list is robust [ ;-) ] even if the list is
207	deliberately corrupted.
208	
209	i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
210	tested the new glibc code (on x86_64 and i386), and it works for his
211	robust-mutex testcases.
212	
213	All other architectures should build just fine too - but they won't have
214	the new syscalls yet.
215	
216	Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
217	inline function before writing up the syscalls (that function returns
218	-ENOSYS right now).
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