deadlock between mm_sem and tx assign in zfs_write() and page fault
The bug time sequence:
- thread #1, zfs_write assign a txg "n".
- In a same process, thread #2, mmap page fault (which means the mm_sem is hold) occurred, zfs_dirty_inode open a txg failed, and wait previous txg "n" completed.
- thread #1 call uiomove to write, however page fault is occurred in uiomove, which means it need mm_sem, but mm_sem is hold by thread #2, so it stuck and can't complete, then txg "n" will not complete.
So thread #1 and thread #2 are deadlocked.
Reviewed-by: Chunwei Chen <email@example.com>
Reviewed-by: Brian Behlendorf <firstname.lastname@example.org>
Reviewed-by: Matthew Ahrens <email@example.com>
Signed-off-by: Grady Wong <firstname.lastname@example.org>