Re: hugetlbfs lockdep spew revisited.
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On 2012-02-17 00:49:22, Al Viro wrote: > On Thu, Feb 16, 2012 at 06:38:49PM -0600, Tyler Hicks wrote: > > On 2012-02-16 19:16:34, Josh Boyer wrote: > > > On Thu, Feb 16, 2012 at 07:08:57PM -0500, Dave Jones wrote: > > > > Remember this ? https://lkml.org/lkml/2011/4/15/272 > > > > Josh took a stab at fixing it in e096d0c7e2e4e5893792db865dd065ac73cf1f00, > > > > but it seems to still be there. > > > > > > I think Tyler Hicks actually noticed this a while ago, but his patch has > > > been waiting on comment from Al and Christoph: > > > > > > http://thread.gmane.org/gmane.linux.file-systems/58795/focus=59565 > > > > > > I've been hesitant to comment because I obviously screwed up once > > > already. We could try this patch in Fedora for a while if Al and > > > company don't speak up soon. > > > > I'm pretty confident that my patch that Josh linked to would "fix" the > > lockdep warning below. According to the backtrace, it is barking about a > > directory inode and a regular inode having a circular locking > > dependency, so deadlock is not possible in this case. > > Sigh... That patch is correct, but it has nothing to do with the locking > order violation that really *is* there. The only benefit would be to > get rid of the "deadlock is not possible" nonsense, since you would see > read/write vs. mmap instead of readdir vs. mmap in the traces. Locking > order is the *same* for directories and nondirectories; both can have > pagefaults under ->i_mutex on their respective inodes. And while mmap > cannot happen for directories, it certainly can happen for regular files, > so taking ->i_mutex in ->mmap() is a plain and simple bug. Should never > be done; in particular, hugetlbfs has ->i_mutex held in read() around > pagefaults, which gives you an obvious deadlock with its ->mmap(). > > Folks, this is not a false positive and it has nothing to do with misannotation > for directories. Deadlock is real; I have no idea WTF do we what ->i_mutex > held over that area in hugetlbfs ->mmap(), but doing that is really, really > wrong, whatever the reason. Thanks for clearing that up, Al. I only knew that the inodes were being incorrectly annotated, but I wasn't sure about the correct locking order. Tyler
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