Date: Fri, 27 Jan 2012 10:56:56 +0200 From: Kostik Belousov <kostikbel@gmail.com> To: Doug Ambrisko <ambrisko@ambrisko.com> Cc: freebsd-current@freebsd.org Subject: Re: knlist_empty locking fix Message-ID: <20120127085656.GY2726@deviant.kiev.zoral.com.ua> In-Reply-To: <201201262103.q0QL3QWr083496@ambrisko.com> References: <201201262103.q0QL3QWr083496@ambrisko.com>
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On Thu, Jan 26, 2012 at 01:03:26PM -0800, Doug Ambrisko wrote:
> Ran into problems with running kqueue/aio with WITNESS etc. Sometimes
> things are locked sometimes not. knlist_remove is called telling it
> whether it is locked or not ie:
> extern void knlist_remove(struct knlist *knl, struct knote *kn, int islocked);
> so I changed:
> extern int knlist_empty(struct knlist *knl);
> to:
> extern int knlist_empty(struct knlist *knl, int islocked);
>
> and then updated things to reflect that following what that state of the
> lock for knlist_remove. If it is not locked, it gets a lock and
> frees it after.
>
> This now fixes a panic when a process using kqueue/aio is killed on
> shutdown with WITNESS.
>
> It changes an API/ABI so it probably can't merged back. If there are
> no objections then I'll commit it.
>
Change to knlist_init() does not make sense at all, the knlist shall
not be exposed to other consumers during initialization, so no need
to exclude the parallel access.
Regarding the knlist_empty(), I propose to keep it as is. Locking
the knlist inside knlist_empty() does not make sense, because lock
is immediately dropped afterward, and relocked for remove. This way,
the entry could be removed from the list meantime (can it, really ?).
I think that you should take a lock around the whole if() {} statement,
and call knlist_remove with locked == 1.
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