On Fri, Apr 10, 2009 at 10:21:22PM +0200, Krzysztof Helt wrote:

On Thu, 9 Apr 2009 14:58:51 +0200
Andrea Righi [off-list ref] wrote:

quoted

On Thu, Apr 09, 2009 at 07:36:24PM +0800, Eric Miao wrote:

quoted

This happens on my Marvell PXA310-based Littleton platform with
Angstrom Distribution. The offending paths are many:

FBIOPUT_VSCREENINFO:
  lock_fb_info()
  --> fb_set_var()
    --> fb_notifier_call_chain() [FBINFO_MISC_USEREVENT]
      --> fbcon_event_notifier() [FB_EVENT_MODE_CHANGE]
        --> lock_fb_info()

OK, now hang. I'd suggest a clean fix to the original assumption of
circular locking
issue and revert this commit first.

-- 
Cheers
- eric

I can agree to revert 66c1ca019078220dc1bf968f2bb18421100ef147, since I
don't have a clean fix for this. Pushing down fb_info->lock in
fb_set_var() excluding to call fb_notifier_call_chain with fb_info->lock
held doesn't seem to be so trivial...

However, reverting this will re-introduce the circular locking
dependency fb_info->lock => mm->mmap_sem => fb_info->lock.

If anybody is interested I have looked into the code of the fb_mmap()
and have found that the problem Andrea tried to fix is caused
only by drivers which implements their own fb_mmap().

Krzysztof,

first of all thanks for looking into the code!

The correct solution is to push the info->lock mutex into
the fb_mmap() implemented inside the drivers. Some drivers
may not need it (the ones which only calculate offsets
and do not "real" iommaping).

mmmh... I may have missed something, but the common fb_mmap() should
acquire mm->mmap_sem and then info->lock, while fb_ioctl() can do that
in reverse order (info->lock first and then mm->mmap_sem) causing the
circular locking dependency. Are you sure that pushing info->lock down
each driver's fb_mmap will fix the problem?

I'll try to prepare a patch after the Easters. This will require a revert
 of the 66c1ca commit.

Thanks and happy Easter,
-Andrea

Happy Easters,
Krzysztof

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