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#Readwrite org how to#
#Readwrite org upgrade#

The counter, b, tracks the number of blocking readers.

#Readwrite org how to#

Raynal demonstrates how to implement an R/W lock using two mutexes and a single integer counter. Several implementation strategies for readers–writer locks exist, reducing them to synchronization primitives that are assumed to pre-exist. Unspecified priority can in some situations be preferable if it allows for a more efficient implementation. Unspecified priority RW locks does not provide any guarantees with regards read vs.This variation is sometimes also known as "write-biased" readers–writer lock. Also the lock is less performant because each operation, taking or releasing the lock for either read or write, is more complex, internally requiring taking and releasing two mutexes instead of one. The downside is that write-preferring locks allows for less concurrency in the presence of writer threads, compared to read-preferring RW locks.

Write-preferring RW locks avoid the problem of writer starvation by preventing any new readers from acquiring the lock if there is a writer queued and waiting for the lock the writer will acquire the lock as soon as all readers which were already holding the lock have completed.

Priority to readers may be weak, as just described, or strong, meaning that whenever a writer releases the lock, any blocking readers always acquire it next. Since multiple reader threads may hold the lock at once, this means that a writer thread may continue waiting for the lock while new reader threads are able to acquire the lock, even to the point where the writer may still be waiting after all of the readers which were holding the lock when it first attempted to acquire it have released the lock. This is because writer threads will not be able to acquire the lock as long as at least one reading thread holds it.

Read-preferring RW locks allow for maximum concurrency, but can lead to write-starvation if contention is high.

These policies lead to different tradeoffs with regards to concurrency and starvation. The lock can either be designed to always give priority to readers ( read-preferring), to always give priority to writers ( write-preferring) or be unspecified with regards to priority. RW locks can be designed with different priority policies for reader vs.

#Readwrite org upgrade#

The deadlock can be avoided by allowing only one thread to acquire the lock in "read-mode with intent to upgrade to write" while there are no threads in write mode and possibly non-zero threads in read-mode. Upgradable RW locks can be tricky to use safely, since whenever two threads holding reader locks both attempt to upgrade to writer locks, a deadlock is created that can only be broken by one of the threads releasing its reader lock. Some RW locks allow the lock to be atomically upgraded from being locked in read-mode to write-mode, as well as being downgraded from write-mode to read-mode.

3.2 Using a condition variable and a mutex.