Re: [Boost-users] [thread][repost] recursive shared mutex

Subject: Re: [Boost-users] [thread][repost] recursive shared mutex
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2010-11-28 17:22:41

On Nov 28, 2010, at 5:07 PM, Marsh Ray wrote:

> On 11/28/2010 02:09 PM, Howard Hinnant wrote:
>> 
>> With this "fair" behavior, imagine thread A getting shared ownership.
>> Then thread B requests exclusive ownership and blocks until A
>> relinquishes shared ownership.  Meanwhile thread A recursively
>> requests shared ownership.  Thread A blocks because no more shared
>> ownerships are granted
> 
> That's not what I think of as a 'recursive mutex' then. Seems to me a recursive mutex would not block A at this point. He wouldn't consider it 'granting a new shared ownership' but simply 'granting a redundant ownership for something the thread already owns'.
> 
>> until thread B is granted exclusive ownership.
>> Deadlock.
> 
> So don't design it that way then? Immediately grant any request for shared ownership if the requesting thread already has shared or exclusive ownership. Even if it doesn't match a literal interpretation of the fairness policy.
> 
>> With recursive shared ownership on shared_mutex being a recipe for
>> disaster, it might be confusing if the API offered recursive
>> exclusive ownership on the shared_mutex.
> 
> Recursive mutex, to me, means "grant more of whatever the requesting thread already has, basically as a no-op." The calling thread isn't really acquiring anything new. Blocking him on something he already holds is a guaranteed deadlock. Whatever he's asking is for is probably going to help run his operation to completion, so it probably reduces contention on the exclusive lock too.
> 
> Not saying this is a superior model in every way, just that it's what I grew up with. I think it's probably a little less prone to deadlocks, though perhaps a bit of code that deadlocked before could now corrupt data. But maybe that code would be broken in the single-threaded case too.

Fair enough.  I guess to implement this the shared_mutex would need to keep a vector<pair<thread::id, unsigned>> of shared owners so that it can detect and track the recursion.

-Howard