On Mon 23-04-18 02:16:19, Amir Goldstein wrote: > On Mon, Apr 23, 2018 at 1:57 AM, Jan Kara <jack@xxxxxxx> wrote: > > On Sun 22-04-18 09:12:10, Amir Goldstein wrote: > >> If fs is frozen after mount and before the first file open, the > >> update of s_last_mounted bypasses freeze protection and prints out > >> a WARNING splat: > >> > >> $ mount /vdf > >> $ fsfreeze -f /vdf > >> $ cat /vdf/foo > >> > >> [ 31.578555] WARNING: CPU: 1 PID: 1415 at > >> fs/ext4/ext4_jbd2.c:53 ext4_journal_check_start+0x48/0x82 > >> > >> [ 31.614016] Call Trace: > >> [ 31.614997] __ext4_journal_start_sb+0xe4/0x1a4 > >> [ 31.616771] ? ext4_file_open+0xb6/0x189 > >> [ 31.618094] ext4_file_open+0xb6/0x189 > >> > >> This fix might not be free of open vs. freeze race, but it closes > >> a big hole. > >> > >> [backport hint: to apply to stable tree, just need to repace > >> sb_rdonly(sb) with (sb->s_flags & MS_RDONLY)] > >> > > > > Thanks for the patch. But a proper (race-free) fix for this is to use > > sb_start_intwrite(), before starting a handle and sb_end_intwrite() after > > stopping it. > > > > The thing is, it doesn't make sense reading a file will block because > fs is frozen, does it? So a proper fix would probably involve a new > helper sb_start_intwrite_trylock(), but I wasn't sure exactly, so posted > this naiive version. So do you think sb_start_intwrite_trylock() > would be good here? Yeah, better than your racy check... ;) Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
