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SubjectRe: do_exit() and lock_kernel() semantics
Kevin Pedretti wrote:
>
> Hello,
> do_exit() does a lock_kernel() before it destroys the dying
> processes mm context (sets task_struct->mm to NULL in 2.4 and &init_mm
> in 2.2). Does lock_kernel() somehow disable interrupts? It doesn't
> look like it does.

Nope.

> Is there anyway from an interrupt context to check if a process is still
> alive (not exiting) and prevent it from exiting until the ISR is over?

See kernel/timer.c:count_active_tasks(). It does
read_lock(&task_list_lock) to pin everything down
while it walks the task list in an interrupt.

And you're in luck - tasklist_lock is exported to modules.

> I guess if lock_kernel disables interrupts globally and waits for
> inprogress interrupts to complete, then this isn't a problem.
>
> More detail:
> The reason I ask is that I'm working on/modifying a set of modules that
> accesses user space from interrupt context. I know this is not a good
> thing to do generally, but for performance reasons the original author
> wanted to copy directly into a mlocked user space buffer from a network
> receive interrupt. Since the buffer is mlocked, it is always guaranteed
> to be there and no page faults will happen (right??? I'm new at this).
> Thus, for each receive we have to convert the virt address of the
> user-land receive buffer to a physical address (in the kernel region)
> before doing the memcpy (copy_to_user doesn't work from interrupt
> context).

That sounds sane. Pin the user pages, set up a kernel virtual
mapping of them. You can't rely on userspace having performed
the mlock of course; you'll need to pin the pages in-kernel.
Probably you can just use map_user_kiobuf().

> This all seems to work fine in practice. However, it seems
> to me that there is a race that can happen if a process is in the middle
> of dying and a receive interrupt happens. task->mm can be set to
> NULL/init_mm out from under me while doing a receive (e.g. on another cpu).
>

I guess that if you've pinned the pages, then you're safe even
if the task exits - the pages won't be going away. But your
interrupt will need to deal with the kiovec, not the process mm.

This could end up meaning that your final page_cache_release()
happens in interrupt context. We may have a problem with that
if the page is still on the global LRU. See the thread
starting at http://www.uwsg.iu.edu/hypermail/linux/kernel/0202.0/1157.html

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