boost::shared_mutex multiple-reader / single-writer mutex

Question

I am trying to use boost::shared_mutex to implement a multiple-reader / single-writer mutex. My question is fairly simple, is it possible for a thread to gain reader access to a shared_mutex, when another thread tries to lock that shared_mutex for writing? For example, I have 10 threads, only one of them can write,

thread 1 has a shared_lock on that shared_mutex and tries to read something
thread 2 has a shared_lock on that shared_mutex and tries to read something
thread 3 has a unique_lock on that shared_mutex and tries to write something
thread 4 has a shared_lock on that shared_mutex and tries to read something
thread 5 has a shared_lock on that shared_mutex and tries to read something

The shared_mutex is currently shared locked by thread 2, my question is whether it is possible that thread 4 can gain read access to that shared_mutex, before thread 3 can write? Is it possible for a reader/writer mutex ever gets into a starvation situation, e.g., 100 reader v.s. 1 writer?

Thanks.

Duplicate: stackoverflow.com/questions/4203467/…

Craig H
– Craig H

2012-02-21 20:18:43 +00:00
Commented Feb 21, 2012 at 20:18 — Craig H
– Craig H, Commented Feb 21, 2012 at 20:18
and here: stackoverflow.com/questions/989795/…

Craig H
– Craig H

2012-02-21 20:19:32 +00:00
Commented Feb 21, 2012 at 20:19 — Craig H
– Craig H, Commented Feb 21, 2012 at 20:19

Tudor · Accepted Answer · 2012-02-21 21:09:01Z

4

Apparently the boost::shared_mutex leaves the fairness policy up to the implementation. It can be either fair, reader-over-writer or writer-over-reader so depending on which it is for your particular version it's possible that the writer can be starved.

answered Feb 21, 2012 at 21:09

Tudor

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3 Comments

2607 Over a year ago

Tudor, can you tell me bit more more about "reader-over-writer" and "writer-over-reader"? Thanks.

Tudor Over a year ago

reader-over-writer means that readers waiting to do shared_lock will have priority over writers waiting to do lock. writer-over-reader is the opposite.

Guss Over a year ago

When you say "boost::shared_mutex leaves the faireness policy up to the implementation" you probably meant to say that boost::shared_lock (and friends) leaves the fairness policy to the mutex implementation - boost::shared_mutex implements "reader-over-writer" policy (AFAIU). Is there a boost implementation for "writer-over-reader" mutex policy, or do I have to implement my own?

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