Messages in this thread | | | Date | Wed, 30 Apr 2008 15:43:05 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH 5/8] cpu: cpu-hotplug deadlock |
| |
On 04/30, Gautham R Shenoy wrote: > > On Tue, Apr 29, 2008 at 06:33:50PM +0400, Oleg Nesterov wrote: > > > > Could you explain what is the semantics difference? The current code allows > > read-in-write recursion too. > > How about the following ? > ---------------------------------------------------------------------------- > cpu_hotplug: split the cpu_hotplug.lock mutex into a spinlock and a waitqueue. > > Consider the following callpath: > get_online_cpus() > . > . > . > some_fn()--> takes some_lock; /* lock_acquire(some_lock) [1] */ > . > . > . > get_online_cpus(); /* lock_acquire(cpu_hotplug.lock) [2] */ > > and on the cpu_hotplug callback path, we have > cpu_hotplug.lock /* lock_acquire(cpu_hotplug.lock) [3]*/ > . > . > . > some_other_fn() ----> take some_lock /* lock_acquire(some_lock) [4]*/ > > lockdep will treat this as a ABBA possiblity since on the write path we > currently hold the lock so that the readers are blocked on it. > > However, the write path won't be activated until the refcount goes > to zero. Which means that lockdep yelling at [2] is a false positive.
Thanks...
So, we need these complications to avoid a false positive from lockdep... I wonder if it is possible to simply hide this internal mutex from lockdep.
OK, at least get_online_cpus() doesn't suffer from the performance pov, and lockdep is really important. (I didn't actually understand this until it found the bug introduced by me ;)
BTW, not that it matters, but get_online_cpus/put_online_cpus can check "cpu_hotplug.active_writer == current" lockless.
Thanks!
Oleg.
| |