1. Userspace Synchronization
Chris Gill, Brian Kocoloski
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63143

2. CSE 422S – Operating Systems Organization
Atomic Operations
Today, we will be exploring atomic hardware operations and synchronization in more detail
Important atomic functions:
__atomic_compare_exchange() – set a variable to a new value if its old value is something we expect it to be
__atomic_add_fetch() – add to a variable and return its new value
__atomic_sub_fetch() – subtract from a variable and return its new value
CSE 422S – Operating Systems Organization

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Compare/Exchange
Syntax: cmp_exchg(ptr, expected, desired)
Parameters
ptr : pointer to an integer valued
expected: value that ptr should have now
desired: value that we want to set ptr to
Semantics: (of course, this is executed atomically)
if (*ptr == expected) {
*ptr = desired;
return true;
} else {
return false; }
CSE 422S – Operating Systems Organization

4. Spinlock via Compare/Exchange
Syntax: cmp_exchg(ptr, expected, desired)
Lock:
Repeatedly invoke cmp_exchg(ptr, UNLOCKED, LOCKED)
If it fails, that means the value of ptr is not unlocked, so need to try again
If it succeeds, value is set to LOCKED to prevent concurrent access
Unlock:
Invoke cmp_exchg(ptr, LOCKED, UNLOCKED)
Has to succeed – if not, the lock was never acquired
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Spinlocks vs Futexes
Spinlocks via cmp/xchg function correctly, but waste CPU cycles via spinning
In cases of longer critical sections, it is desirable to be able to put processes/threads to sleep
Similar to kernel-level mutexes/semaphores
Fast userspace mutexes (futexes) are a new feature to improve lock performance
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Futexes
Two components:
(1) an atomic integer variable in userspace
(2) a system call to sleep/wake up contending processes
Can be used to synchronize multiple processes or threads
In multiprocess settings, processes must explicitly share memory via something like mmap()
In multithread settings, threads implicitly share memory
In their simplest form, futexes offer primitive operations on which other (sleep) lock mechanisms can be built
Semaphores, condition variables, readers/writer locks, etc.
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7. Futex Example <On whiteboard>
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8. Futex Example <On whiteboard> Takeaways:
Common case code path – (no lock contention) – is optimized by not requiring any system calls
Uncommon case - (lock contention) – does not waste CPU cycles by spinning, but rather sleeps/wakes up via system calls
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Today’s Studio
Implement userspace spinlocks and mutexes
Some Programming Considerations
Volatile variables are often important
Inform compiler (and someone reading the code) that a variable’s value may change unexpectedly)
Read type declarations right-to-left in C and C++
E.g., volatile int * is a pointer to an int that’s volatile (the int value, not the pointer, may change)
OpenMP is for parallel programming
Pragmas built into gcc, to support concurrent execution of functions, barrier synchronization before and after parallel-for loops, etc.
CSE 422S – Operating Systems Organization