[PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend
Steven Rostedt
rostedt at goodmis.org
Tue Jan 22 14:32:32 EST 2013
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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