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Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linu

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Daniel Bristot de Oliveira
Daniel Bristot de Oliveira

Slides of the presentation about the nested locking needs for real-time Linux.

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Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux Daniel B. de Oliveira1,2,3 , Daniel Casini2 , Rômulo S. de Oliveira3 , Tommaso Cucinotta2 , Alessandro Biondi2 , and Giorgio Buttazzo2 Email: bristot@redhat.com, romulo.deoliveira@ufsc.br, { daniel.casini,tommaso.cucinotta, alessandro.biondi, giorgio.buttazzo } @santannapisa.it Real-time Scheduling Open Problems Seminar (RTSOPS 2018)
- Linux is a GPOS with RTOS ambitions - Preemptive - FIFO (TLFP) and Deadline (JLFP) schedulers - User-space locks with PIP & PCP - PREEMPT-RT improves Linux’s predictability by: - Making system as preemptive/schedulable as possible - Bounding priority inversions using PIP on kernel locks - Max (activation delay?) latency of 150 µs RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux2 Real-time Linux
Due to Linux’s GPOS nature, RT Linux developers are challenged to provide the predictability required for an RTOS, while not causing regressions on the general purpose benchmarks. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux3 Real-time Linux howevers
- Developers cannot cause performance regressions - Throughput: - Two implementations: RT and non RT - A newer algorithm cannot cause - much - regression compared to the older one - Predictability: - Cannot increase the latency - e.g, cannot disable the preemption for a long period RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux4 In practice it means
As a consequence, the implementation of some well known algorithms, like read/write semaphores, has been done using approaches that were exhaustively explored in academic papers. IOW: it works, but... RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux5 Consequences...
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux6 Read/write semaphores on Linux Read-side down_read(&rw_semapore) { /* enters in the read-side */ } /* * Read-side critical section * Parallel with other readers * No writers */ up_read(&rw_semapore) { /* leaves the read-side */ } Write-side down_write(&rw_semapore) { /* enters in the write-side */ } /* * Write-side critical section * Exclusive access */ up_write(&rw_semapore) { /* leaves the write-side */ }
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux7 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; };
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux8 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; }; struct rt_mutex { raw_spinlock_t wait_lock; struct rb_root_cached waiters; struct task_struct *owner; int save_state; };
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux9 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; }; struct rt_mutex { raw_spinlock_t wait_lock; struct rb_root_cached waiters; struct task_struct *owner; int save_state; };
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux10 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: ... Spin lock: ... Concurrent down operations
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux11 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux12 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux13 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux14 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Mutex: held Spin lock: ... Concurrent down operations
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux15 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux16 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux17 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux18 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux19 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: held Mutex: ... Spin lock: ...
RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux20 A task taking a write lock, With a nested mutex With a nested spin-lock. down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Mutex: held Spin lock: held Concurrent down operations
Open Issues
1) Implementing in Linux state-of-the-art protocols for heterogeneous nested locks and developing novel analysis techniques
- B. C. Ward and J. H. Anderson, “Supporting nested locking in multiprocessor real-time systems,” in Real-Time Systems (ECRTS), 2012 24th Euromicro Conference on, 2012, pp. 223–232. - Proposed real-time nested locking protocol (RNLP), with the related asymptotic analysis. - B. C. Ward and J. H. Anderson, “Fine-grained multiprocessor real-time locking with improved blocking,” in Proceedings of the 21st International Conference on Real-Time Networks and Systems, ser. RTNS ’13, 2013. - Conceived to deal with heterogeneous nested critical sections: Block + Spinning ( short-on-long) RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux23 Shared memory nested critical sections
- C. E. Nemitz, T. Amert, and J. H. Anderson, “Real-time multiprocessor locks with nesting: Optimizing the common case,” in Proceedings of the 25th International Conference on Real-Time and Network Systems (RTNS 2017), 2017 - nested read/write spin lock with fast path! - A. Biondi, A. Weider, and B. Brandenburg, “A blocking bound for nested fifo spin locks,” in Real-Time Systems Symposium (RTSS), 2016, pp. 291–302. - Graph abstraction is introduced to derive a fine-grained analysis, not based on asymptotic bounds for FIFO non-preemptive spin locks. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux24 Shared memory nested critical sections
- Sleeping: - Nested blocking (rt mutexes) - Nested read/write (rw semaphores) - Busy-wait: - Nested read/write spin (rw lock) - Nested spinlock (raw spin lock) - Fast path is important - Schedulers: TLFP, JLFP & IRQ/NMI - Arbitrary affinities RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux25 Linux’s locking needs
2) The design of specialized analysis techniques accounting for specific implementations of complex types of locks (e.g., the aforementioned read/write lock in Linux).
3) finding more efficient locking protocols, accounting for both general purpose benchmark performance (i.e., average-case behavior, needed by the GPOS nature of Linux) and predictability.
Questions?
Thanks!

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Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linu

  • 1. Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux Daniel B. de Oliveira1,2,3 , Daniel Casini2 , Rômulo S. de Oliveira3 , Tommaso Cucinotta2 , Alessandro Biondi2 , and Giorgio Buttazzo2 Email: bristot@redhat.com, romulo.deoliveira@ufsc.br, { daniel.casini,tommaso.cucinotta, alessandro.biondi, giorgio.buttazzo } @santannapisa.it Real-time Scheduling Open Problems Seminar (RTSOPS 2018)
  • 2. - Linux is a GPOS with RTOS ambitions - Preemptive - FIFO (TLFP) and Deadline (JLFP) schedulers - User-space locks with PIP & PCP - PREEMPT-RT improves Linux’s predictability by: - Making system as preemptive/schedulable as possible - Bounding priority inversions using PIP on kernel locks - Max (activation delay?) latency of 150 µs RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux2 Real-time Linux
  • 3. Due to Linux’s GPOS nature, RT Linux developers are challenged to provide the predictability required for an RTOS, while not causing regressions on the general purpose benchmarks. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux3 Real-time Linux howevers
  • 4. - Developers cannot cause performance regressions - Throughput: - Two implementations: RT and non RT - A newer algorithm cannot cause - much - regression compared to the older one - Predictability: - Cannot increase the latency - e.g, cannot disable the preemption for a long period RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux4 In practice it means
  • 5. As a consequence, the implementation of some well known algorithms, like read/write semaphores, has been done using approaches that were exhaustively explored in academic papers. IOW: it works, but... RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux5 Consequences...
  • 6. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux6 Read/write semaphores on Linux Read-side down_read(&rw_semapore) { /* enters in the read-side */ } /* * Read-side critical section * Parallel with other readers * No writers */ up_read(&rw_semapore) { /* leaves the read-side */ } Write-side down_write(&rw_semapore) { /* enters in the write-side */ } /* * Write-side critical section * Exclusive access */ up_write(&rw_semapore) { /* leaves the write-side */ }
  • 7. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux7 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; };
  • 8. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux8 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; }; struct rt_mutex { raw_spinlock_t wait_lock; struct rb_root_cached waiters; struct task_struct *owner; int save_state; };
  • 9. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux9 Read/write semaphores on Linux struct rw_semaphore { atomic_t readers; struct rt_mutex rtmutex; }; struct rt_mutex { raw_spinlock_t wait_lock; struct rb_root_cached waiters; struct task_struct *owner; int save_state; };
  • 10. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux10 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: ... Spin lock: ... Concurrent down operations
  • 11. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux11 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
  • 12. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux12 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
  • 13. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux13 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Atomic operation Mutex: held Spin lock: ... Concurrent down operations
  • 14. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux14 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Mutex: held Spin lock: ... Concurrent down operations
  • 15. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux15 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
  • 16. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux16 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
  • 17. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux17 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
  • 18. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux18 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: block Mutex: ... Spin lock: held
  • 19. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux19 down_read(rw_sem) if (++rw_sem->readers > 1) return /* enter the critical section */ else rw_sem->readers-- take rw_sem->rtmutex.wait_lock if (WRITER BIAS is not set) { rw_sem->readers++ release rw_sem->rtmutex.wait_lock return /* enter in the critical section */ } release rw_sem->rtmutex.wait_lock take rw_sem->rt_mutex rw_sem->readers++ release the rw_sem->rt_mutex return /* enter in the critical section */ } down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Concurrent down operations Mutex: held Spin lock: held Mutex: ... Spin lock: ...
  • 20. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux20 A task taking a write lock, With a nested mutex With a nested spin-lock. down_write(rw_sem) { take rw_sem->rtmutex clear READER BIAS if (rw_sem->readers != 0) suspend waiting for the last reader while(1) { take sem->rtmutex->wait_lock if (sem->readers == 0) { set WRITER BIAS release rw_sem->rtmutex->wait_lock return } release rw_sem->rtmutex->wait_lock. suspend waiting for the last reader } return } Mutex: held Spin lock: held Concurrent down operations
  • 22. 1) Implementing in Linux state-of-the-art protocols for heterogeneous nested locks and developing novel analysis techniques
  • 23. - B. C. Ward and J. H. Anderson, “Supporting nested locking in multiprocessor real-time systems,” in Real-Time Systems (ECRTS), 2012 24th Euromicro Conference on, 2012, pp. 223–232. - Proposed real-time nested locking protocol (RNLP), with the related asymptotic analysis. - B. C. Ward and J. H. Anderson, “Fine-grained multiprocessor real-time locking with improved blocking,” in Proceedings of the 21st International Conference on Real-Time Networks and Systems, ser. RTNS ’13, 2013. - Conceived to deal with heterogeneous nested critical sections: Block + Spinning ( short-on-long) RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux23 Shared memory nested critical sections
  • 24. - C. E. Nemitz, T. Amert, and J. H. Anderson, “Real-time multiprocessor locks with nesting: Optimizing the common case,” in Proceedings of the 25th International Conference on Real-Time and Network Systems (RTNS 2017), 2017 - nested read/write spin lock with fast path! - A. Biondi, A. Weider, and B. Brandenburg, “A blocking bound for nested fifo spin locks,” in Real-Time Systems Symposium (RTSS), 2016, pp. 291–302. - Graph abstraction is introduced to derive a fine-grained analysis, not based on asymptotic bounds for FIFO non-preemptive spin locks. RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux24 Shared memory nested critical sections
  • 25. - Sleeping: - Nested blocking (rt mutexes) - Nested read/write (rw semaphores) - Busy-wait: - Nested read/write spin (rw lock) - Nested spinlock (raw spin lock) - Fast path is important - Schedulers: TLFP, JLFP & IRQ/NMI - Arbitrary affinities RTSOPS 2018: Nested Locks in the Lock Implementation: The Real-Time Read-Write Semaphores on Linux25 Linux’s locking needs
  • 26. 2) The design of specialized analysis techniques accounting for specific implementations of complex types of locks (e.g., the aforementioned read/write lock in Linux).
  • 27. 3) finding more efficient locking protocols, accounting for both general purpose benchmark performance (i.e., average-case behavior, needed by the GPOS nature of Linux) and predictability.