Shared/Exclusive Locks
Shared/exclusive locks (sx locks) are locks that threads can hold while asleep. As the name implies, multiple threads can have a shared hold on an sx lock, but only one thread can have an exclusive hold on an sx lock. When a thread has an exclusive hold on an sx lock, other threads cannot have a shared hold on that lock.
sx locks do not support priority propagation and are inefficient compared to mutexes. The main reason for using sx locks is that threads can sleep while holding one.
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