On Thurs, Nov 07, 2013 at 03:51PM +0000, Joe Thornber wrote: > On Wed, Nov 06, 2013 at 08:59:38PM +0000, Mears, Morgan wrote: > > Add the ability to set the era counter maintained by the dm-cache era > > policy shim to an arbitrary 32-bit value, to allow era rollback after > > the underlying device is restored from a snapshot. > > I wonder if we should pass in the old value, and have the call fail if the old > value is incorrect. This would allow applications to spot if they were > competing to set the era. Some thing like: > > set_era_counter <old value>:<new value> Yes, I like this. My inclination is to make the <old value>: portion optional so that the counter value can be forced if desired (for example, to set it to a saved value during a create); objections? > > + era->era_counter = new_era_counter; > > + smp_wmb(); > > Please stop using smp_rmb() and smp_wmb(). Every time I see it used I find > bugs (and this is no exception). Use higher level locking abstractions (eg, spin > locks), and only optimise if we have a performance issue. > > In general alarm bells ring if you use one of smp_*() without the other. See > linux/Documentation/memory-barriers.txt for lots of discussion. Thanks Joe, will fix. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
