On Mon, Mar 18, 2013 at 09:53:34PM +0000, Al Viro wrote: > 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...
I'm looking at the existing callers and I really wonder if we ought to push sb_start_write() from ->splice_write()/->aio_write()/etc. into the callers. Something like file_start_write()/file_end_write(), with check for file being regular one might be a good starting point. As it is, copyup is really fucked both in unionmount and overlayfs... -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
