Thanks for your comment. > -----Original Message----- > From: Al Viro [mailto:viro@xxxxxxxxxxxxxxxx] On Behalf Of Al Viro > Sent: Monday, September 23, 2013 10:07 PM > To: Inki Dae > Cc: 'YoungJun Cho'; dri-devel@xxxxxxxxxxxxxxxxxxxxx > Subject: Re: [RFC] deadlock in "drm/exynos: fix wrong pointer access at vm > close" > > On Mon, Sep 23, 2013 at 04:49:30PM +0900, Inki Dae wrote: > > Hi, > > > > > -----Original Message----- > > > From: Al Viro [mailto:viro@xxxxxxxxxxxxxxxx] On Behalf Of Al Viro > > > Sent: Monday, September 23, 2013 6:29 AM > > > To: YoungJun Cho > > > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx; Inki Dae > > > Subject: [RFC] deadlock in "drm/exynos: fix wrong pointer access at vm > > > close" > > > > > > You have drm_dev->struct_mutex grabbed before ->mmap_sem in > > > exynos_drm_gem_mmap_ioctl() and after - in exynos_drm_gem_fault() > > > (since ->fault() is always called with ->mmap_sem held). Looks like > > > a garden-variety AB-BA deadlock... > > > > > > > Right, if mmap system call is requested by another process between - > >f_op > > pointer changing and restoring, the deadlock can be incurred. > > > > For this, I think we can resolve this issue like below, > > > > At exynos_drm_gem_mmap_ioctl() > > down_write(&mm->mmap_sem); > > mutex_lock(&dev->struct_mutex); > > ... > > Umm... If you do it that way, why bother with changing ->private_data > at all? You can as well stash obj in dev->dev_private->something after > you've grabbed the mutex and have ->mmap() pick it there... I changed ->private_data to a gem object so that mmap_buffer function can get the gem object. But yes, we can use dev->dev_private->something instead, and that seems like better way. For this, I missed that we can get drm_device object from file pointer. > > Said that, I really don't like that approach - both playing with ->f_op > and the games with private vmas; Agree. But is there other better way to support direct mapping; not needing page fault handler to map, and indirect mapping interfaces; needing the page fault handler? > exynos_gem_get_vma(), AFAICS, calls > find_vma() (without bothering to hold ->mmap_sem, BTW - there's nothing > to prevent the result of find_vma() being freed just as it's returned I can't see to hold ->mmap_sem when it calls find_vma() anywhere else. > to caller) and clones it manually, regardless of whether that vma allows > to clone itself or not. Quite a few drivers rely on that not happening... > I think that has no any problem because exynos_gem_get_vma() takes a reference to vma, and also v4l2 side is using same way. I and v4l2 guys might be missing something what you are concerning. So Could you give us more comments? And, I found a bug that we never call exynos_gem_put_vma() when userptr is freed. That should be fixed. > IOW, you are already digging deep inside VM guts and this only makes > it deeper ;-/ > > And no, the deadlock doesn't depend on race between ioctl() and mmap() > from another process; all it takes is Yes, there was my wrong comments. That is a mmap callback(exynos_drm_gem_mmap_buffer) we don't want can be called if we called mmap system call when ->f_op points to exynos_drm_gem_fops. > 1) thread A does clone(), creating thread B that shares address space with > it. > 2) thread A does that ioctl, creating a mapping > 3) thread A does that ioctl again, creating another mapping, while thread > B dereferences an address in the first mapping and triggers a page fault. > > The only race is on step 3 in the above; the question about mmap() vs. > ioctl() was about mmap(2) _during_ that ioctl() hitting > exynos_drm_gem_mmap_buffer() instead of exynos_drm_gem_mmap() it would've > called before or after ioctl(). Here the interesting case is when > callers of mmap() and ioctl() do *not* share the address space, since > in that case mmap(2) won't notice ->mmap_sem held by you - it's on the > different mm_struct, so mmap(2) will get to calling the ->f_op->mmap() > without waiting for you to restore ->f_op... Yes, it's a problem. When two more threads are running, and when thread A called mmap system call, exynos_drm_gem_mmap_buffer() will be called if thread B called ioctl() first and f_op was not restored yet. I have no idea to resolve this issue yet. So do you have any good idea? At any rate, we should fix this issue. > > For another bug in the same area, try building that driver modular and > watch what happens to use count after a round of this switching ->f_op and > restoring it back to original; fops_get() in there is wrong and AFAICS > pointless. Yes, it would also be another bug. Will check it out. It seems that it's not good way to change and restore ->f_op, and this way makes mmap part to be complicated. I will try to find a way to resolve this issue or to find another better way to support direct and indirect mapping interfaces. I'd appreciate all the advices I can get. Thanks, Inki Dae _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
