Re: [Threads-devel] shared mutex locking discipline

From: Anthony Williams (anthony_at_[hidden])
Date: 2008-04-14 03:40:25

Quoting Frank Mori Hess <fmhess_at_[hidden]>:

> I'm curious if there is any theory on how to avoid deadlock when using a
> shared mutex?  Having a well-defined locking order works well for an
> ordinary mutex, but things get messy with a shared mutex.  For example, if
> we have mutexes A and B then the locking sequence
>
> 1) shared lock A
> 2) unique lock B
> 3) upgrade to unique lock A

My advice: don't do that. By sandwiching the lock to B within the two  
halves of the lock on A, you've exposed yourself to deadlocks on both  
sides, as you point out. This only happens if you start with an  
upgrade_lock on A, since otherwise you have to unlock the shared lock  
before you can take the unique lock, and taking an upgrade lock should  
be rare anyway, as there can be only one.

Anthony
-- 
Anthony Williams            | Just Software Solutions Ltd
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