1. COT 5611 Operating Systems Design Principles Spring 2012
Dan C. Marinescu
Office: HEC 304
Office hours: M-Wd 5:00-6:00 PM

2. Lecture 19 – Wednesday March 21, 2012
Reading assignment:
Chapter 9 from the on-line text
Last time –
All-or-nothing and before-or after atomicity
Atomicity and processor management
Processes, threads, and address spaces
Thread coordination with a bounded buffer – the naïve approach
Thread management
Address spaces and multi-level memories
Kernel structures for the management of multiple cores/processors and threads/processes
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3. Today Locks and before-or-after actions; hardware support for locks
YIELD
Conditions for thread coordination – Safety, Liveness, Bounded-Wait, Fairness
Critical sections – a solution to critical section problem
Deadlocks
Signals
Semaphores
Monitors
Thread coordination with a bounded buffer.
WAIT
NOTIFY
AWAIT
ADVANCE
SEQUENCE
TICKET
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4. Locks; Before-or-After actions
Locks shared variables which acts as a flag to coordinate access to a shared data. Manipulated with two primitives
ACQUIRE
RELEASE
Support implementation of before-or-after actions; only one thread can acquire the lock, the others have to wait.
All threads must obey the convention regarding the locks.
The two operations ACQUIRE and RELEASE must be atomic.
Hardware support for implementation of locks
RSM – Read and Set Memory
CMP –Compare and Swap
RSM (mem)
If mem=LOCKED then RSM returns r=LOCKED and sets mem=LOCKED
If mem=UNLOCKED the RSM returns r=LOCKED and sets mem=LOCKED
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7. Important facts to remember
Each thread has a unique ThreadId
Threads save their state on the stack.
The stack pointer of a thread is stored in the thread table.
To activate a thread the registers of the processor are loaded with information from the thread state.
What if no thread is able to run 
create a dummy thread for each processor called a processor_thread which is scheduled to run when no other thread is available
the processor_thread runs in the thread layer
the SCHEDULER runs in the processor layer
We have a processor thread for each processor/core.
We can use spin locks only if the two processes (the producer and the consumer) run on different CPUs; we need an active process to release a spin lock….
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8. Switching threads with dynamic thread creation
Switching from one user thread to another requires two steps
Switch from the thread releasing the processor to the processor thread
Switch from the processor thread to the new thread which is going to have the control of the processor
The last step requires the SCHEDULER to circle through the thread_table until a thread ready to run is found
The boundary between user layer threads and processor layer thread is crossed twice
Example: switch from thread 0 to thread 6 using
YIELD
ENTER_PROCESSOR_LAYER
EXIT_PROCESSOR_LAYER
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10. The control flow when switching from one thread to another
The control flow is not obvious as some of the procedures reload the stack pointer (SP)
When a procedure reloads the stack pointer then the place where it transfers control when it executes a return is the procedure whose SP was saved on the stack and was reloaded before the execution of the return.
ENTER_PROCESSOR_LAYER
Changes the state of the thread calling YIELD from RUNNING to RUNNABLE
Save the state of the procedure calling it , YIELD, on the stack
Loads the processors registers with the state of the processor thread, thus starting the SCHEDULER
EXIT_PROCESSOR_LAYER
Saves the state of processor thread into the corresponding PROCESSOR_TABLE and loads the state of the thread selected by the SCHEDULER to run (in our example of thread 6) in the processor’s registers
Loads the SP with the values saved by the ENTER_PROCESSOR_LAYER
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13. In ENTER PROCESSOR_LAYER instead of SCHEDULER() should be SP processor_table[processor].topstack
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15. Implicit assumptions for the correctness of the implementation
One sending and one receiving thread. Only one thread updates each shared variable.
Sender and receiver threads run on different processors to allow spin locks
in and out are implemented as integers large enough so that they do not overflow (e.g., 64 bit integers)
The shared memory used for the buffer provides read/write coherence
The memory provides before-or-after atomicity for the shared variables in and out
The result of executing a statement becomes visible to all threads in program order. No compiler optimization supported
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16. In practice….. Threads run concurrently  Race conditions may occur
 data in the buffer may be overwritten
 a lock for the bounded buffer
the producer acquires the lock before writing
the consumer acquires the lock before reading
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18. We have to avoid deadlocks
If a producer thread cannot write because the buffer is full it has to release the lock to allow the consumer thread to acquire the lock to read, otherwise we have a deadlock.
If a consumer thread cannot read because the there is no new item in the buffer it has to release the lock to allow the consumer thread to acquire the lock to write, otherwise we have a deadlock.
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20. In practice…
We have to ensure atomicity of some operations, e.g., updating the pointers
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21. One more pitfall of the previous implementation of bounded buffer
If in and out are long integers (64 or 128 bit) then a load requires two registers, e.,g, R1 and R2.
int “ FFFFFFFF”
L R1,int /* R1 
L R2,int /* R2  FFFFFFFF
Race conditions could affect a load or a store of the long integer.
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23. In practice the threads may run on the same system….
We cannot use spinlocks for a thread to wait until an event occurs.
That’s why we have spent time on YIELD…
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