get_user_pages + put_task_struct

Hi,

I am writing a (pci) device driver, in which I'm doing userspace DMA with get_user_pages. After some complicated locking, it occurred to me to use reference counting instead, after finding the functions get_task_mm and get_task_struct. However, put_task_struct cannot be used in a module, linker will complain about __put_task_struct_cb being undefined. I saw it explicitly being removed as a symbol, "we do not want device drivers holding references to task structs". Why exactly is this?

mmput is exported, so no problem for the mm. BTW, I'm also not exactly sure why I (conceptually) need a task to get the pages; but I think it's for statistics.

I need to hold on to this task because the DMA transaction may need to be split in multiple pieces, continuing after the initial call by the user; during which time that process may have exited (e.g. unexpected kill).

Micha