Messages in this thread | | | Date | Mon, 18 Mar 2013 21:53:34 +0000 | From | Al Viro <> | Subject | Re: [PATCH 2/9] vfs: export do_splice_direct() to modules |
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On Mon, Mar 18, 2013 at 04:39:36PM +0100, Jan Kara wrote: > IMO the deadlock is real. In freeze_super() we wait for all writers to > the filesystem to finish while blocking beginning of any further writes. So > we have a deadlock scenario like: > > THREAD1 THREAD2 THREAD3 > mnt_want_write() mutex_lock(&inode->i_mutex); > ... freeze_super() > block on mutex_lock(&inode->i_mutex) > sb_wait_write(sb, SB_FREEZE_WRITE); > block in sb_start_write()
The bug is on fsfreeze side and this is not the only problem related to it. I've missed the implications when I applied "fs: Add freezing handling to mnt_want_write() / mnt_drop_write()" last June ;-/
The thing is, until then mnt_want_write() used to be a counter; it could be nested. Now any such nesting is a deadlock you've just described. This is seriously wrong, IMO.
BTW, having sb_start_write() buried in individual ->splice_write() is asking for trouble; could you describe the rules for that? E.g. where does it nest wrt filesystem-private locks? XFS iolock, for example...
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