On Mon 26-09-16 18:07:48, Oleg Nesterov wrote: > Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than > frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which > drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering. > > In this case thaw_super() will wrongly call s_op->unfreeze_fs() before > it was actually frozen, and call sb_freeze_unlock() which leads to the > unbalanced percpu_up_write(). Unfortunately lockdep can't detect this, > so this triggers misc BUG_ON()'s in kernel/rcu/sync.c. > > Reported-and-tested-by: Nikolay Borisov <[email protected]> > Signed-off-by: Oleg Nesterov <[email protected]> > Cc: [email protected]
The patch looks good. Thanks! Reviewed-by: Jan Kara <[email protected]> Honza > --- > fs/super.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/fs/super.c b/fs/super.c > index d78b984..2549896c 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -1324,8 +1324,8 @@ int freeze_super(struct super_block *sb) > } > } > /* > - * This is just for debugging purposes so that fs can warn if it > - * sees write activity when frozen is set to SB_FREEZE_COMPLETE. > + * For debugging purposes so that fs can warn if it sees write activity > + * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super(). > */ > sb->s_writers.frozen = SB_FREEZE_COMPLETE; > up_write(&sb->s_umount); > @@ -1344,7 +1344,7 @@ int thaw_super(struct super_block *sb) > int error; > > down_write(&sb->s_umount); > - if (sb->s_writers.frozen == SB_UNFROZEN) { > + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) { > up_write(&sb->s_umount); > return -EINVAL; > } > -- > 2.5.0 > > -- Jan Kara <[email protected]> SUSE Labs, CR
