Re: [RFC 0/12] introduce down_write_killable for rw_semaphore

From: Ingo Molnar
Date: Wed Mar 09 2016 - 07:18:58 EST

* Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> Hi,
> the following patchset implements a killable variant of write lock for
> rw_semaphore. My usecase is to turn as many mmap_sem write users to use a
> killable variant which will be helpful for the oom_reaper [1] to asynchronously
> tear down the oom victim address space which requires mmap_sem for read. This
> will reduce a likelihood of OOM livelocks caused by oom victim being stuck on a
> lock or other resource which prevents it to reach its exit path and release the
> memory. [...]

So I'm a tiny bit concerned about this arguments.

AFAICS killability here just makes existing system calls more interruptible -
right? In that sense that's not really a livelock scenario: it just takes shorter
time for resources to be released.

If a livelock is possible (where resources are never released) then I'd like to
see a specific example of such a livelock.

You have the other patch-set:

[PATCH 0/18] change mmap_sem taken for write killable

that makes use of down_write_killable(), and there you argue:

[...] this is a follow up work for oom_reaper [1]. As the async OOM killing
depends on oom_sem for read we would really appreciate if a holder for write
stood in the way. This patchset is changing many of down_write calls to be
killable to help those cases when the writer is blocked and waiting for readers
to release the lock and so help __oom_reap_task to process the oom victim.

there seems to be a misunderstanding: if a writer is blocked waiting for readers
then no new readers are allowed - the writer will get its turn the moment all
existing readers drop the lock.

So there's no livelock scenario - it's "only" about latencies.

And once we realize that it's about latencies (assuming I'm right!), not about
correctness per se, I'm wondering whether it would be a good idea to introduce
down_write_interruptible(), instead of down_write_killable().

I'd love various processes to quit faster on Ctrl-C as well, not just on kill -9!

This would also test the new code paths a lot better: kill -9 is a lot rarer than
regular interruption.


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