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