Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

[Linus Torvalds]

On Fri, Apr 3, 2020 at 7:02 PM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> So in term of priority, my current thinking is
>
>     upgrading unfair reader > unfair reader > reader/writer
>
> A higher priority locker will block other lockers from acquiring the lock.

An alternative option might be to have readers normally be 100% normal
(ie with fairness wrt writers), and not really introduce any special
"unfair reader" lock.

Instead, all the unfairness would come into play only when the special
case - execve() - does it's special "lock for reading with intent to
upgrade".

But when it enters that kind of "intent to upgrade" lock state, it
would not only block all subsequent writers, it would also guarantee
that all other readers can continue to go).

So then the new rwsem operations would be

 - read_with_write_intent_lock_interruptible()

   This is the beginning of "execve()", and waits for all writers to
exit, and puts the lock into "all readers can go" mode.

   You could think of it as a "I'm queuing myself for a write lock,
but I'm allowing readers to go ahead" state.

 - read_lock_to_write_upgrade()

   This is the "now this turns into a regular write lock". It needs to
wait for all other readers to exit, of course.

 - read_with_write_intent_unlock()

   This is the "I'm unqueuing myself, I aborted and will not become a
write lock after all" operation.

NOTE! In this model, there may be multiple threads that do that
initial queuing thing. We only guarantee that only one of them will
get to the actual write lock stage, and the others will abort before
that happens.

If that is a more natural state machine, then that should work fine
too. And it has some advantages, in that it keeps the readers normally
fair, and only turns them unfair when we get to that special
read-for-write stage.

But whatever it most natural for the rwsem code. Entirely up to you.

               Linus