Re: [PATCH RFC] Btrfs: fix deadlock between sys_sync and freeze

Il 14/08/2012 07:01, liub.liubo@gmail.com ha scritto:

From: Liu Bo <redacted>

I found this while testing xfstests 068, the story is

     t1                                            t2
   sys_sync                                    thaw_super
     iterate_supers
       down_read(sb->s_umount)                   down_write(sb->s_umount) --->wait for t1
       sync_fs (with wait mode)
         start_transaction
           sb_start_intwrite --------------------> wait for t2 to set s_writers.frozen to SB_UNFROZEN

In this patch, I add an helper sb_start_intwrite_trylock() and use it before we
start_transaction in sync_fs() with wait mode so that we won't hit the deadlock.

IMHO, we should avoid to call the sync operation on a frozen fs. The 
freeze operation, indeed, already include a sync operation. According to 
man page, no other operation should modify the fs after the freeze. So 
for me the modification is inside sync_filesystem (and sync_one_sb).

Marco