On Thu, 2009-02-19 at 11:33 +0100, Peter Zijlstra wrote: > On Thu, 2009-02-19 at 10:19 +0100, Peter Zijlstra wrote: > > On Wed, 2009-02-18 at 11:38 -0500, k...@bitplanet.net wrote: > > > From: Kristian Høgsberg <k...@redhat.com> > > > > > > A number of GEM operations (and legacy drm ones) want to copy data to > > > or from userspace while holding the struct_mutex lock. However, the > > > fault handler calls us with the mmap_sem held and thus enforces the > > > opposite locking order. This patch downs the mmap_sem up front for > > > those operations that access userspace data under the struct_mutex > > > lock to ensure the locking order is consistent. > > > > > > Signed-off-by: Kristian Høgsberg <k...@redhat.com> > > > --- > > > > > > Here's a different and simpler attempt to fix the locking order > > > problem. We can just down_read() the mmap_sem pre-emptively up-front, > > > and the locking order is respected. It's simpler than the > > > mutex_trylock() game, avoids introducing a new mutex. > > > > > OK let me try that again -- my initial response was a tad curt :/
No that's fair, I was aware that the patch was probably borderline and got the feedback I was looking for ;) > While I appreciate your efforts in fixing GEM (I too have an interest in > seeing it done), I cannot support your patch. > > Firstly, you're using mmap_sem well outside its problem domain, this is > bad form. Furthermore, holding it for extended durations for no good > reason affects all other users. Yup, agree. > Secondly, mmap_sem is not a recursive lock (very few kernel locks are, > and we generally frown upon recursive locking schemes), this means that > the fault handler still cannot function properly. I understand, but we take it twice only as a read lock, so that should work, right? We prevent the deadlock the lockdep validator warned about and as far as I can see, the patch doesn't introduce a new one. But other than that I agree with the frowning on recursive locking, it's too often used to paper over badly thought out locking. cheers, Kristian ------------------------------------------------------------------------------ Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA -OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise -Strategies to boost innovation and cut costs with open source participation -Receive a $600 discount off the registration fee with the source code: SFAD http://p.sf.net/sfu/XcvMzF8H -- _______________________________________________ Dri-devel mailing list Dri-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/dri-devel
