Re: [Boost-users] [interprocess] named_upgradable_mutex

Subject: Re: [Boost-users] [interprocess] named_upgradable_mutex
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2009-07-01 18:05:48

Moritz escribió:
> Hi everybody,
> 
> I have a question concerning the interprocess::updgradable::mutex.
> 
> My scenario:
> 
> The idea is that the writer locks the upgradable_mutex normally (calling 
> lock()) and the readers lock it sharable (calling lock_sharable()). If 
> the mutex is locked sharable and the writer tries to lock it normally, 
> another attempt to lock it sharable has to wait until the writer 
> succeeded in locking and unlocking the mutex.

An upgradable mutex is efficient if the writer not always writes, but it 
also reads many times. In that case, acquiring upgradable lock still 
allows concurrent readers that have acquired sharable locks. The 
reader-writer can read but can also write *if upgrades the upgradable 
lock to an exclusive lock*. If the reader-writer realizes that a write 
is necessary it should call unlock_upgradable_and_lock() which blocks 
until all readers finish.

There is no guarantee that new readers will not get sharable lock while 
the upgradable lock calls unlock_upgradable_and_lock(), because all of 
them need to grab an internal mutex and this scheduling is done by the 
OS. You can only guarantee that new readers will block *after* 
unlock_upgradable_and_lock() has been completed.

Best,

Ion