On 02/09/2013 05:17 AM, Paul E. McKenney wrote: > On Tue, Jan 22, 2013 at 01:04:23PM +0530, Srivatsa S. Bhat wrote: >> CPU hotplug (which will be the first user of per-CPU rwlocks) has a special >> requirement with respect to locking: the writer, after acquiring the per-CPU >> rwlock for write, must be allowed to take the same lock for read, without >> deadlocking and without getting complaints from lockdep. In comparison, this >> is similar to what get_online_cpus()/put_online_cpus() does today: it allows >> a hotplug writer (who holds the cpu_hotplug.lock mutex) to invoke it without >> locking issues, because it silently returns if the caller is the hotplug >> writer itself. >> >> This can be easily achieved with per-CPU rwlocks as well (even without a >> "is this a writer?" check) by incrementing the per-CPU refcount of the writer >> immediately after taking the global rwlock for write, and then decrementing >> the per-CPU refcount before releasing the global rwlock. >> This ensures that any reader that comes along on that CPU while the writer is >> active (on that same CPU), notices the non-zero value of the nested counter >> and assumes that it is a nested read-side critical section and proceeds by >> just incrementing the refcount. Thus we prevent the reader from taking the >> global rwlock for read, which prevents the writer from deadlocking itself. >> >> Add that support and teach lockdep about this special locking scheme so >> that it knows that this sort of usage is valid. Also add the required lockdep >> annotations to enable it to detect common locking problems with per-CPU >> rwlocks. > > Very nice! The write-side interrupt disabling ensures that the task > stays on CPU, as required. > > One request: Could we please have a comment explaining the reasons for > the writer incrementing and decrementing the reader reference count? > > It looked really really strange to me until I came back and read the > commit log. ;-) > Sure :-) 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
