1 ================
2 Futex Requeue PI
3 ================
4
5 Requeueing of tasks from a non-PI futex to a PI futex requires
6 special handling in order to ensure the underlying rt_mutex is never
7 left without an owner if it has waiters; doing so would break the PI
8 boosting logic [see rt-mutex-design.rst] For the purposes of
9 brevity, this action will be referred to as "requeue_pi" throughout
10 this document. Priority inheritance is abbreviated throughout as
11 "PI".
12
13 Motivation
14 ----------
15
16 Without requeue_pi, the glibc implementation of
17 pthread_cond_broadcast() must resort to waking all the tasks waiting
18 on a pthread_condvar and letting them try to sort out which task
19 gets to run first in classic thundering-herd formation. An ideal
20 implementation would wake the highest-priority waiter, and leave the
21 rest to the natural wakeup inherent in unlocking the mutex
22 associated with the condvar.
23
24 Consider the simplified glibc calls::
25
26 /* caller must lock mutex */
27 pthread_cond_wait(cond, mutex)
28 {
29 lock(cond->__data.__lock);
30 unlock(mutex);
31 do {
32 unlock(cond->__data.__lock);
33 futex_wait(cond->__data.__futex);
34 lock(cond->__data.__lock);
35 } while(...)
36 unlock(cond->__data.__lock);
37 lock(mutex);
38 }
39
40 pthread_cond_broadcast(cond)
41 {
42 lock(cond->__data.__lock);
43 unlock(cond->__data.__lock);
44 futex_requeue(cond->data.__futex, cond->mutex);
45 }
46
47 Once pthread_cond_broadcast() requeues the tasks, the cond->mutex
48 has waiters. Note that pthread_cond_wait() attempts to lock the
49 mutex only after it has returned to user space. This will leave the
50 underlying rt_mutex with waiters, and no owner, breaking the
51 previously mentioned PI-boosting algorithms.
52
53 In order to support PI-aware pthread_condvar's, the kernel needs to
54 be able to requeue tasks to PI futexes. This support implies that
55 upon a successful futex_wait system call, the caller would return to
56 user space already holding the PI futex. The glibc implementation
57 would be modified as follows::
58
59
60 /* caller must lock mutex */
61 pthread_cond_wait_pi(cond, mutex)
62 {
63 lock(cond->__data.__lock);
64 unlock(mutex);
65 do {
66 unlock(cond->__data.__lock);
67 futex_wait_requeue_pi(cond->__data.__futex);
68 lock(cond->__data.__lock);
69 } while(...)
70 unlock(cond->__data.__lock);
71 /* the kernel acquired the mutex for us */
72 }
73
74 pthread_cond_broadcast_pi(cond)
75 {
76 lock(cond->__data.__lock);
77 unlock(cond->__data.__lock);
78 futex_requeue_pi(cond->data.__futex, cond->mutex);
79 }
80
81 The actual glibc implementation will likely test for PI and make the
82 necessary changes inside the existing calls rather than creating new
83 calls for the PI cases. Similar changes are needed for
84 pthread_cond_timedwait() and pthread_cond_signal().
85
86 Implementation
87 --------------
88
89 In order to ensure the rt_mutex has an owner if it has waiters, it
90 is necessary for both the requeue code, as well as the waiting code,
91 to be able to acquire the rt_mutex before returning to user space.
92 The requeue code cannot simply wake the waiter and leave it to
93 acquire the rt_mutex as it would open a race window between the
94 requeue call returning to user space and the waiter waking and
95 starting to run. This is especially true in the uncontended case.
96
97 The solution involves two new rt_mutex helper routines,
98 rt_mutex_start_proxy_lock() and rt_mutex_finish_proxy_lock(), which
99 allow the requeue code to acquire an uncontended rt_mutex on behalf
100 of the waiter and to enqueue the waiter on a contended rt_mutex.
101 Two new system calls provide the kernel<->user interface to
102 requeue_pi: FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI.
103
104 FUTEX_WAIT_REQUEUE_PI is called by the waiter (pthread_cond_wait()
105 and pthread_cond_timedwait()) to block on the initial futex and wait
106 to be requeued to a PI-aware futex. The implementation is the
107 result of a high-speed collision between futex_wait() and
108 futex_lock_pi(), with some extra logic to check for the additional
109 wake-up scenarios.
110
111 FUTEX_CMP_REQUEUE_PI is called by the waker
112 (pthread_cond_broadcast() and pthread_cond_signal()) to requeue and
113 possibly wake the waiting tasks. Internally, this system call is
114 still handled by futex_requeue (by passing requeue_pi=1). Before
115 requeueing, futex_requeue() attempts to acquire the requeue target
116 PI futex on behalf of the top waiter. If it can, this waiter is
117 woken. futex_requeue() then proceeds to requeue the remaining
118 nr_wake+nr_requeue tasks to the PI futex, calling
119 rt_mutex_start_proxy_lock() prior to each requeue to prepare the
120 task as a waiter on the underlying rt_mutex. It is possible that
121 the lock can be acquired at this stage as well, if so, the next
122 waiter is woken to finish the acquisition of the lock.
123
124 FUTEX_CMP_REQUEUE_PI accepts nr_wake and nr_requeue as arguments, but
125 their sum is all that really matters. futex_requeue() will wake or
126 requeue up to nr_wake + nr_requeue tasks. It will wake only as many
127 tasks as it can acquire the lock for, which in the majority of cases
128 should be 0 as good programming practice dictates that the caller of
129 either pthread_cond_broadcast() or pthread_cond_signal() acquire the
130 mutex prior to making the call. FUTEX_CMP_REQUEUE_PI requires that
131 nr_wake=1. nr_requeue should be INT_MAX for broadcast and 0 for
132 signal.
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