On Thu, Feb 05, 2015 at 04:11:00PM +0000, Russell King - ARM Linux wrote: > On Thu, Feb 05, 2015 at 06:29:18AM -0800, Paul E. McKenney wrote: > > Works for me, assuming no hidden uses of RCU in the IPI code. ;-) > > Sigh... I kind'a new it wouldn't be this simple. The gic code which > actually raises the IPI takes a raw spinlock, so it's not going to be > this simple - there's a small theoretical window where we have taken > this lock, written the register to send the IPI, and then dropped the > lock - the update to the lock to release it could get lost if the > CPU power is quickly cut at that point. > > Also, we _do_ need the second cache flush in place to ensure that the > unlock is seen to other CPUs. > > We could work around that by taking and releasing the lock in the IPI > processing function... but this is starting to look less attractive > as the lock is private to irq-gic.c. > > Well, we're very close to 3.19, we're too close to be trying to sort > this out, so I'm hoping that your changes which cause this RCU error > are *not* going in during this merge window, because we seem to have > something of a problem right now which needs more time to resolve.
Most likely into the 3.20 merge window. But please keep in mind that RCU is just the messenger here -- the current code will break if any CPU for whatever reason takes more than a jiffy to get from its _stop_machine() handler to the end of its last RCU read-side critical section on its way out. A jiffy may sound like a lot, but it is not hard to exceed this limit, especially in virtualized environments. So not like to go into v3.19, but it does need to be resolved. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/