futex-requeue-pi.txt: standardize document format
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - promote level for the document title; - mark literal blocks. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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@ -1,5 +1,6 @@
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================
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Futex Requeue PI
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----------------
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================
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Requeueing of tasks from a non-PI futex to a PI futex requires
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special handling in order to ensure the underlying rt_mutex is never
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@ -20,28 +21,28 @@ implementation would wake the highest-priority waiter, and leave the
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rest to the natural wakeup inherent in unlocking the mutex
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associated with the condvar.
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Consider the simplified glibc calls:
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Consider the simplified glibc calls::
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/* caller must lock mutex */
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pthread_cond_wait(cond, mutex)
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{
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lock(cond->__data.__lock);
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unlock(mutex);
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do {
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unlock(cond->__data.__lock);
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futex_wait(cond->__data.__futex);
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lock(cond->__data.__lock);
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} while(...)
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unlock(cond->__data.__lock);
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lock(mutex);
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}
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/* caller must lock mutex */
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pthread_cond_wait(cond, mutex)
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{
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lock(cond->__data.__lock);
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unlock(mutex);
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do {
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unlock(cond->__data.__lock);
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futex_wait(cond->__data.__futex);
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lock(cond->__data.__lock);
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} while(...)
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unlock(cond->__data.__lock);
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lock(mutex);
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}
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pthread_cond_broadcast(cond)
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{
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lock(cond->__data.__lock);
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unlock(cond->__data.__lock);
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futex_requeue(cond->data.__futex, cond->mutex);
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}
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pthread_cond_broadcast(cond)
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{
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lock(cond->__data.__lock);
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unlock(cond->__data.__lock);
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futex_requeue(cond->data.__futex, cond->mutex);
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}
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Once pthread_cond_broadcast() requeues the tasks, the cond->mutex
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has waiters. Note that pthread_cond_wait() attempts to lock the
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@ -53,29 +54,29 @@ In order to support PI-aware pthread_condvar's, the kernel needs to
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be able to requeue tasks to PI futexes. This support implies that
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upon a successful futex_wait system call, the caller would return to
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user space already holding the PI futex. The glibc implementation
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would be modified as follows:
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would be modified as follows::
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/* caller must lock mutex */
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pthread_cond_wait_pi(cond, mutex)
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{
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lock(cond->__data.__lock);
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unlock(mutex);
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do {
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unlock(cond->__data.__lock);
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futex_wait_requeue_pi(cond->__data.__futex);
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lock(cond->__data.__lock);
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} while(...)
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unlock(cond->__data.__lock);
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/* the kernel acquired the mutex for us */
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}
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/* caller must lock mutex */
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pthread_cond_wait_pi(cond, mutex)
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{
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lock(cond->__data.__lock);
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unlock(mutex);
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do {
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unlock(cond->__data.__lock);
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futex_wait_requeue_pi(cond->__data.__futex);
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lock(cond->__data.__lock);
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} while(...)
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unlock(cond->__data.__lock);
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/* the kernel acquired the mutex for us */
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}
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pthread_cond_broadcast_pi(cond)
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{
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lock(cond->__data.__lock);
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unlock(cond->__data.__lock);
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futex_requeue_pi(cond->data.__futex, cond->mutex);
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}
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pthread_cond_broadcast_pi(cond)
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{
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lock(cond->__data.__lock);
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unlock(cond->__data.__lock);
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futex_requeue_pi(cond->data.__futex, cond->mutex);
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}
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The actual glibc implementation will likely test for PI and make the
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necessary changes inside the existing calls rather than creating new
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