linux-kernel - Re: [PATCH] Revert "async: simplify lowest_in

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Message-ID: <20171127193637.GK983427@devbig577.frc2.facebook.com>
Date:   Mon, 27 Nov 2017 11:36:37 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>, stable@...r.kernel.org,
        Lai Jiangshan <laijs@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "async: simplify lowest_in_progress()"

On Mon, Nov 20, 2017 at 11:51:47PM +0100, Rasmus Villemoes wrote:
> This reverts commit 92266d6ef60c2381c980c6cdcb2a5c1667b36b49, which
> was simply wrong: In the case where domain is NULL, we now use the
> wrong offsetof() in the list_first_entry macro, so we don't actually
> fetch the ->cookie value, but rather the eight bytes located
> sizeof(struct list_head) further into the struct async_entry.
> 
> On 64 bit, that's the data member, while on 32 bit, we get a u64 built
> from func and data in some order.
> 
> I think the bug happens to be harmless in practice: It obviously only
> affects callers which pass a NULL domain, and AFAICT the only such
> caller is
> 
>   async_synchronize_full() ->
>   async_synchronize_full_domain(NULL) ->
>   async_synchronize_cookie_domain(ASYNC_COOKIE_MAX, NULL)
> 
> and the ASYNC_COOKIE_MAX means that in practice we end up waiting for
> the async_global_pending list to be empty - but it would break if
> somebody happened to pass (void*)-1 as the data element to
> async_schedule, and of course also if somebody ever does a
> async_synchronize_cookie_domain(, NULL) with a "finite" cookie value.
> 
> Cc: stable@...r.kernel.org # 3.10+
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

Ughh... indeed.

Acked-by: Tejun Heo <tj@...nel.org>

Sorry about that.  Can you please resend the patch w/ Andrew Morton
cc'd?  I think it'd be best to route this through -mm.

Thanks.

-- 
tejun