On Tue, Apr 30, 2019 at 03:45:48PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-04-30 15:28:11 [+0200], Peter Zijlstra wrote: > > On Tue, Apr 30, 2019 at 02:51:31PM +0200, Sebastian Andrzej Siewior wrote: > > > On 2019-04-19 10:56:27 [+0200], Juri Lelli wrote: > > > > On 26/03/19 10:34, Juri Lelli wrote: > > > > > Hi, > > > > > > > > > > Running this reproducer on a 4.19.25-rt16 kernel (with lock debugging > > > > > turned on) produces warning below. > > > > > > > > And I now think this might lead to an actual crash. > > > > > > Peter, could you please take a look at the thread: > > > https://lkml.kernel.org/r/20190419085627.GI4742@localhost.localdomain > > > > > > I assumed that returning to userland with acquired locks is something we > > > did not want… > > > > Yeah, but AFAIK fs freezing code has a history of doing exactly that.. > > This is just the latest incarnation here. > > > > So the immediate problem here is that the task doing thaw isn't the same > > that did freeze, right? The thing is, I'm not seeing how that isn't a > > problem with upstream either. > > > > The freeze code seems to do: percpu_down_write() for the various states, > > and then frobs lockdep state. > > > > Thaw then does the reverse, frobs lockdep and then does: percpu_up_write(). > > > > percpu_down_write() directly relies on down_write(), and > > percpu_up_write() on up_write(). And note how __up_write() has: > > > > DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem); > > > > So why isn't this same code coming unstuck in mainline? > > I have to re-route most of this questions to Juri Lelli. > Lockdep has these gems: > lockdep_sb_freeze_release() / lockdep_sb_freeze_acquire() Yeah, saw those, but irrespective of them, the rwsem code (not percpu_rwsem) should complain about freeze and thaw not being the same process. Anyway; it's Oleg and Jan who put this together. I simply don't see how upstream is correct here.
