Re: [RFC PATCH -RT] epoll: Fix eventpoll read-lock not writer-fair in PREEMPT_RT

From: Sebastian Andrzej Siewior
Date: Fri Aug 27 2021 - 06:07:48 EST

On 2021-08-26 22:36:04 [+0206], John Ogness wrote:
> On 2021-08-26, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> > On 2021-08-25 15:27:54 [+0200], Frederic Weisbecker wrote:

> >> I guess making readlock writer fair is still not the plan so all I can
> >> propose is to make that rwlock build-conditional.

> > It is writer fair in a sense that once a writer attempts to acquire
> > the lock no new reader are allowed in.
> >
> > What you want is that the writer pi-boosts each reader which is what
> > is not done (multi reader boost). Long ago there was an attempt to
> > make this happen (I think with rwsem) but it turned out to be
> > problematic. There was a workaround by only allowing one reader and
> > doing PI as usual.
> This patch is essentially forcing that exact workaround for eventpoll.

Frederic ended the mail with "readlock is not writer fair" so I
explained that it is and he means something else and this is not not
coming. I also suggested between the lines that he might try to move
the reader side to RCU.

> John Ogness