get_user_pages + put_task_struct

[Micha Nelissen]

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
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