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