linux-kernel - Re: [PATCH REPOST REPOST v2] fscache: Use only one fscache_object_cong

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Date:   Mon, 3 Jan 2022 14:54:31 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>, linux-kernel@...r.kernel.org,
        linux-cachefs@...hat.com, linux-fsdevel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Gregor Beck <gregor.beck@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH REPOST REPOST v2] fscache: Use only one
 fscache_object_cong_wait.

On 2021-12-26 16:20:30 [-0800], Andrew Morton wrote:
> On Thu, 23 Dec 2021 19:15:09 +0100 Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On 2021-12-23 17:17:09 [+0000], David Howells wrote:
> > > Thanks, but this is gone in the upcoming fscache rewrite.  I'm hoping that
> > > will get in the next merge window.
> > 
> > Yes, I noticed that. What about current tree, v5.16-rc6 and less?
> > Shouldn't this be addressed?
> 
> If the bug is serious enough to justify a -stable backport then yes, we
> should merge a fix such as this ahead of the fscache rewrite, so we
> have something suitable for backporting.
> 
> Is the bug serious enough?
> 
> Or is the bug in a not-yet-noticed state?  In other words, is it
> possible that four years from now, someone will hit this bug in a
> 5.15-based kernel and will then wish we'd backported a fix?

I can't answer how serious it is but:
- with CONFIG_DEBUG_PREEMPT enabled there has to be a visible backtrace
  due this_cpu_ptr() usage.
- because of schedule_timeout(60 * HZ) there is no visible hang. It
  should be either woken up properly (via the waitqueue) or after a
  minute due to the timeout.

both things don't look good in general.

Sebastian