Re: [PATCH] xfs: use atomic to provide buffer I/O accounting serialization

[Brian Foster <bfoster@redhat.com>]

On Mon, May 22, 2017 at 12:05:10PM -0700, Christoph Hellwig wrote:
> On Mon, May 22, 2017 at 02:29:11PM -0400, Brian Foster wrote:
> > We've had user reports of unmount hangs in xfs_wait_buftarg() that
> > analysis shows is due to btp->bt_io_count == -1. bt_io_count
> > represents the count of in-flight asynchronous buffers and thus
> > should always be >= 0. xfs_wait_buftarg() waits for this value to
> > stabilize to zero in order to ensure that all untracked (with
> > respect to the lru) buffers have completed I/O processing before
> > unmount proceeds to tear down in-core data structures.
> > 
> > The value of -1 implies an I/O accounting decrement race. Indeed,
> > the fact that xfs_buf_ioacct_dec() is called from xfs_buf_rele()
> > (where the buffer lock is no longer held) means that bp->b_flags can
> > be updated from an unsafe context. While a user-level reproducer is
> > currently not available, some intrusive hacks to run racing buffer
> > lookups/ioacct/releases from multiple threads was used to
> > successfully manufacture this problem.
> > 
> > Existing callers do not expect to acquire the buffer lock from
> > xfs_buf_rele(). Therefore, we can not safely update ->b_flags from
> > this context. To close the race, replace the in-flight buffer flag
> > with a per-buffer atomic for tracking accounting against the
> > buftarg. This field resides in a hole in the existing data structure
> > and thus does not increase the size of xfs_buf.
> 
> I hate these uses of atomic_t as binary flags.  Can you use
> test_and_set_bit and friends wit a bitop?  This would require
> an unsigned long which an actually be larger than an atomic_t,
> but it's both cleaner and provides headroom for additional atomic flags
> in the future.

I thought it may be a little confusing to have multiple sets of flags
for a buffer, hence the counter (even though it is logically a flag).
But I'm fine with it for now if we don't mind wasting the extra space.

Though I suppose we could also add a smaller field and use cmpxchg() to
set and clear it... thoughts?

Brian