Re: [PATCH v6 07/46] percpu_rwlock: Allow writers to be readers, andadd lockdep annotations
From: Michel Lespinasse
Date: Mon Feb 18 2013 - 10:51:48 EST
On Mon, Feb 18, 2013 at 8:39 PM, Srivatsa S. Bhat
> @@ -200,6 +217,16 @@ void percpu_write_lock_irqsave(struct percpu_rwlock *pcpu_rwlock,
> smp_mb(); /* Complete the wait-for-readers, before taking the lock */
> write_lock_irqsave(&pcpu_rwlock->global_rwlock, *flags);
> + /*
> + * It is desirable to allow the writer to acquire the percpu-rwlock
> + * for read (if necessary), without deadlocking or getting complaints
> + * from lockdep. To achieve that, just increment the reader_refcnt of
> + * this CPU - that way, any attempt by the writer to acquire the
> + * percpu-rwlock for read, will get treated as a case of nested percpu
> + * reader, which is safe, from a locking perspective.
> + */
> + this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt);
I find this quite disgusting, but once again this may be because I
don't like unfair recursive rwlocks.
In my opinion, the alternative of explicitly not taking the read lock
when one already has the write lock sounds *much* nicer.
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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