On 02/18/2013 09:21 PM, Michel Lespinasse wrote: > On Mon, Feb 18, 2013 at 8:39 PM, Srivatsa S. Bhat > <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote: >> @@ -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. I don't seem to recall any strong reasons to do it this way, so I don't have any strong opinions on doing it this way. But one of the things to note is that, in the CPU Hotplug case, the readers are *way* more hotter than the writer. So avoiding extra checks/'if' conditions/memory barriers in the reader-side is very welcome. (If we slow down the read-side, we get a performance hit even when *not* doing hotplug!). Considering this, the logic used in this patchset seems better, IMHO. Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
