Re: [PATCH v5 01/45] percpu_rwlock: Introduce the globalreader-writer lock backend

[Steven Rostedt]

On Tue, 2013-01-22 at 13:03 +0530, Srivatsa S. Bhat wrote:
> A straight-forward (and obvious) algorithm to implement Per-CPU Reader-Writer
> locks can also lead to too many deadlock possibilities which can make it very
> hard/impossible to use. This is explained in the example below, which helps
> justify the need for a different algorithm to implement flexible Per-CPU
> Reader-Writer locks.
> 
> We can use global rwlocks as shown below safely, without fear of deadlocks:
> 
> Readers:
> 
>          CPU 0                                CPU 1
>          ------                               ------
> 
> 1.    spin_lock(&random_lock);             read_lock(&my_rwlock);
> 
> 
> 2.    read_lock(&my_rwlock);               spin_lock(&random_lock);
> 
> 
> Writer:
> 
>          CPU 2:
>          ------
> 
>        write_lock(&my_rwlock);
> 

I thought global locks are now fair. That is, a reader will block if a
writer is waiting. Hence, the above should deadlock on the current
rwlock_t types.

We need to fix those locations (or better yet, remove all rwlocks ;-)

-- Steve


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