linux-kernel - Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs in cpu_is

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Date:   Fri, 1 Dec 2023 07:06:53 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mrunal Patel <mpatel@...hat.com>,
        Ryan Phillips <rphillips@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>
Subject: Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs
 in cpu_is_isolated() check

Hello,

On Wed, Nov 29, 2023 at 11:01:04AM -0500, Waiman Long wrote:
...
> > > Depending on how the cpumask operators are implemented, we may not have a
> > > guarantee that testing CPU 2, for instance, will always return true. That is
> > Can you please elaborate this part a bit? I'm having a difficult time
> > imagining the sequence of operations where this would matter but that could
> > easily be me not being familiar with the details.
> 
> I may be a bit paranoid about incorrect result due to racing as I had been
> burned before. Just testing a bit in the bitmask may probably be OK. I don't

Setting and clearing a bit is as atomic as it gets, right?

> think it will be a problem for x86, but I am less certain about other more
> exotic architectures like arm64 or PPC which I am less familiar about. I add
> a seqcount for synchronization just for the peace of mind. I can take the
> seqcount out if you don't it is necessary.

I just can't think of a case where this would be broken. The data being read
and written is atomic. There's no way to break a bit operation into multiple
pieces. It is possible to write a really bone-headed bitmask operations
(like, if you shift the bits into place or sth) to make the bits go through
unintended changes but that'd just be a flat-out broken implementation. Even
for a bitmask where write accesses are synchronized through a spinlock, we
should still be able to use test_bit() without holding the lock. This seems
like a pretty basic assumption.

Adding unnecessary synchronization confuses the readers. If we don't need
it, we shouldn't have it.

Thanks.

-- 
tejun