Re: [PATCH v2] xfs: fix incorrect log_flushed on fsync

[Christoph Hellwig <hch@lst.de>]

On Tue, Sep 05, 2017 at 11:34:39AM -0400, Brian Foster wrote:
> In xfs_log_force_lsn(), the iclog has to be ACTIVE. We switch it to
> WANT_SYNC, bump the refcnt and call release_iclog(). The latter may or
> may not do anything depending on the refcount. We set log_flushed
> immediately after the release before we know whether an xlog_sync() has
> actually occurred (is that your concern?). If XFS_LOG_SYNC is set, we
> wait on the iclog before _force_lsn() returns, which means the log
> buffer I/O has to have completed before we return either way. If the
> iclog is still WANT_SYNC, the release didn't do anything, but we wait on
> ic_force_wait which is triggered during I/O completion handling. If the
> iclog is already DIRTY/ACTIVE, then the I/O has already submitted and
> completed.
> 
> fsync looks like the only the user of log_flushed and it also uses
> XFS_LOG_SYNC. ISTM that the code is probably still bogus in principle,
> but in practice the fsync code may be safe. Thoughts? Am I missing
> something else?

Yes, I think you're right - the extra XFS_LOG_SYNC code papers over
the issue.  But given that the log_flushed indicator makes very
little sene without XFS_LOG_SYNC it should be ok.