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[XFS] Fix race condition in xfs_write().
This change addresses a race in xfs_write() where, for direct I/O, the flags need_i_mutex and need_flush are setup before the iolock is acquired. The logic used to setup the flags may change between setting the flags and acquiring the iolock resulting in these flags having incorrect values. For example, if a file is not currently cached then need_i_mutex is set to zero and then if the file is cached before the iolock is acquired we will fail to do the flushinval before the direct write. The flush (and also the call to xfs_zero_eof()) need to be done with the iolock held exclusive so we need to acquire the iolock before checking for cached data (or if the write begins after eof) to prevent this state from changing. For direct I/O I've chosen to always acquire the iolock in shared mode initially and if there is a need to promote it then drop it and reacquire it. There's also some other tidy-ups including removing the O_APPEND offset adjustment since that work is done in generic_write_checks() (and we don't use offset as an input parameter anywhere). SGI-PV: 962170 SGI-Modid: xfs-linux-melb:xfs-kern:28319a Signed-off-by:Lachlan McIlroy <lachlan@sgi.com> Signed-off-by: