Re: [PATCH v9 4/4] xfs: replace mrlock_t with rw_semaphores

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Oct 07, 2020 at 09:17:13AM -0500, Eric Sandeen wrote:
> On 10/6/20 8:21 PM, Darrick J. Wong wrote:
> > On Tue, Oct 06, 2020 at 09:15:41PM +0200, Pavel Reichl wrote:
> >> Remove mrlock_t as it does not provide any extra value over
> >> rw_semaphores. Make i_lock and i_mmaplock native rw_semaphores and
> >> replace mr*() functions with native rwsem calls.
> >>
> >> Release the lock in xfs_btree_split() just before the work-queue
> >> executing xfs_btree_split_worker() is scheduled and make
> >> xfs_btree_split_worker() to acquire the lock as a first thing and
> >> release it just before returning from the function. This it done so the
> >> ownership of the lock is transfered between kernel threads and thus
> >> lockdep won't complain about lock being held by a different kernel
> >> thread.
> >>
> >> Signed-off-by: Pavel Reichl <preichl@redhat.com>
> >> ---
> >>  fs/xfs/libxfs/xfs_btree.c | 14 +++++++
> >>  fs/xfs/mrlock.h           | 78 ---------------------------------------
> >>  fs/xfs/xfs_inode.c        | 36 ++++++++++--------
> >>  fs/xfs/xfs_inode.h        |  4 +-
> >>  fs/xfs/xfs_iops.c         |  4 +-
> >>  fs/xfs/xfs_linux.h        |  2 +-
> >>  fs/xfs/xfs_super.c        |  6 +--
> >>  7 files changed, 41 insertions(+), 103 deletions(-)
> >>  delete mode 100644 fs/xfs/mrlock.h
> >>
> >> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> >> index 2d25bab68764..1d1bb8423688 100644
> >> --- a/fs/xfs/libxfs/xfs_btree.c
> >> +++ b/fs/xfs/libxfs/xfs_btree.c
> >> @@ -2816,6 +2816,7 @@ xfs_btree_split_worker(
> >>  	unsigned long		pflags;
> >>  	unsigned long		new_pflags = PF_MEMALLOC_NOFS;
> >>  
> >> +	rwsem_acquire(&args->cur->bc_ino.ip->i_lock.dep_map, 0, 0, _RET_IP_);
> > These calls also need a comment explaining just what they're doing.
> > 
> >>  	/*
> >>  	 * we are in a transaction context here, but may also be doing work
> >>  	 * in kswapd context, and hence we may need to inherit that state
> >> @@ -2832,6 +2833,7 @@ xfs_btree_split_worker(
> >>  	complete(args->done);
> >>  
> >>  	current_restore_flags_nested(&pflags, new_pflags);
> >> +	rwsem_release(&args->cur->bc_ino.ip->i_lock.dep_map, _THIS_IP_);
> > Note that as soon as you call complete(), xfs_btree_split can wake up
> > and return, which means that *args could now point to reclaimed stack
> > space.  This leads to crashes and memory corruption in generic/562 on
> > a 1k block filesystem (though in principle this can happen anywhere):
> 
> 
> What's the right way out of this; store *ip when we enter the function
> and use that to get to the map, rather than args i guess?

Er, no, because the worker could also get preempted right after
complete() and take so long to get rescheduled that the the inode have
been reclaimed.  Think about it -- the original thread is waiting on the
completion that it passed to the worker through $args, and therefore the
worker cannot touch any of the resources it was accessing through $args
after calling complete()....

--D

> Thanks,
> -Eric