Messages in this thread | | | Date | Sun, 11 Sep 2011 17:57:36 +0200 | From | Marco Stornelli <> | Subject | Re: [PATCH, RFC] xip: use i_mutex for xip_file_fault |
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Il 11/09/2011 13:25, Al Viro ha scritto: > On Sun, Sep 11, 2011 at 12:15:04PM +0100, Al Viro wrote: > >> write() grabs ->i_mutex on the file it's going to write to. It uses >> copy_from_user() while holding ->i_mutex; that can end up calling ->fault(). >> If your data comes from the same file mmapped in your address space, you >> have xip_write_fault() called while you are in xip_file_write() and *already* >> are holding ->i_mutex on the same inode. With your patch it will, AFAICS, >> cheerfully deadlock. > > Oh, wait... You are only doing that to write side of pagefault? That's > better, but not much: > > thread 1: mmap the file, modify mapping > thread 2: write() to file > > The former will do xip_write_fault() while holding ->mmap_sem. > The latter will do copy_from_user() from xip_file_write(), getting > pagefaults while holding ->i_mutex. > > Note that we are grabbing ->mmap_sem and ->i_mutex in opposite orders. > I.e. that will deadlock on you - all you need is threads sharing the > address space. >
Ok, thank you very much for the on-line debug :) So i_mutex is not a good lock to use in this situation. It was a common sync point, but it has some collateral effect on the write path that we must avoid. At this point, what can be a good strategy? Any opinion is welcome.
Marco
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