Implyment read write lock with mutex

Using two mutexes[edit]

Raynal demonstrates how to implement an R/W lock using two mutexes and a single integer counter. The counter, b, tracks the number of blocking readers. One mutex, r, protects b and is only used by readers; the other, g (for "global") ensures mutual exclusion of writers. This requires that a mutex acquired by one thread can be released by another. The following is pseudocode for the operations:

Begin Read

• Lock r.
• Increment b.
• If b = 1, lock g.
• Unlock r.

End Read

• Lock r.
• Decrement b.
• If b = 0, unlock g.
• Unlock r.

Begin Write

• Lock g.

End Write

• Unlock g.

This implementation is read-preferring.^[

https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Using_two_mutexes