From: Al Viro <viro@ZenIV.linux.org.uk>
To: NeilBrown <neilb@suse.com>
Cc: linux-fsdevel@vger.kernel.org
Subject: Re: [RFC] bloody odd logics in md_exit()
Date: Sun, 30 Sep 2018 03:00:14 +0100 [thread overview]
Message-ID: <20180930020010.GJ32577@ZenIV.linux.org.uk> (raw)
In-Reply-To: <87k1n3gb2s.fsf@notabene.neil.brown.name>
On Sun, Sep 30, 2018 at 09:04:11AM +1000, NeilBrown wrote:
> Hi Al,
> I don't think wake_up_all() does remove anything from the queue.
> It simply wakes up the various processes that are waiting.
> They remain on the queue until they call remove_wait_queue(), which
> could be delayed arbitrarily.
> If it was delayed until after the module was unloaded and
> "md_event_waiters" no longer existed, the unlink attempt would cause an
> invalid memory access.
>
> I think your approach for simplify the code would only work if
> md_event_waiters could be moved out of the module, or if some global
> wait_queue could be used instead.
> Maybe we could use bit_waitqueue(NULL,0) (rather an ugly hack).
> Maybe we could export a general-purpose waitqueue for modules to use.
> Maybe procfs could export something??
>
> I agree that we can remove md_unloading, by moving remove_proc_entry()
> before the wakeup. I'm not yet convinced that we can remove the wakeup
> loop.
>
> Or am I missing something else here?
You are not, unfortunately; I plead a bad braino. remove_wait_queue() will,
indeed, happen only when we get around to poll_freewait(). Bugger...
That's a problem, and I'm not at all sure that it's only md that needs to
be concerned with it. And it's not just procfs either - debugfs users have
exact same problem if they have ->poll() in their file_operations...
next prev parent reply other threads:[~2018-09-30 8:31 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-29 3:33 [RFC] bloody odd logics in md_exit() Al Viro
2018-09-29 23:04 ` NeilBrown
2018-09-30 2:00 ` Al Viro [this message]
2018-09-30 2:13 ` Matthew Wilcox
2018-09-30 3:17 ` Al Viro
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