* Michal Hocko <[email protected]> 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.
Thanks,
Ingo
