On Sun 17-03-13 13:06:59, David Howells wrote: > Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > > Export do_splice_direct() to modules. Needed by overlay filesystem. > > Apparently you cannot call this from any function that is holding an i_mutex > if the target of the splice uses generic_file_splice_write(). > > The problem is a potential deadlock situation: > > We have places already that do: > > mnt_want_write() > mutex_lock() > > This can be found in do_last() for example. > > However, mnt_want_write() calls sb_start_write() as does > generic_file_splice_write(). So now in ovl_copy_up_locked() you're adding: > > mutex_lock() > sb_start_write() > > which lockdep reports as a potential ABBA deadlock. > > Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock > might operate, so it's possible that this is a false alarm. Maybe Jan Kara can > illuminate further, so I've added him to the cc list. 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() Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
