linux-kernel - Re: [PATCH AUTOSEL 4.20 028/304] srcu: Prevent __call_srcu() counter wrap with read-side critical section

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Message-ID: <20190128171154.GA3973@sasha-vm>
Date:   Mon, 28 Jan 2019 12:11:54 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.20 028/304] srcu: Prevent __call_srcu() counter
 wrap with read-side critical section

On Mon, Jan 28, 2019 at 08:23:18AM -0800, Paul E. McKenney wrote:
>On Mon, Jan 28, 2019 at 10:39:05AM -0500, Sasha Levin wrote:
>> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>>
>> [ Upstream commit 0607ba8403c4cdb253f8c5200ecf654dfb7790cc ]
>>
>> Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from
>> both process and interrupt context"), it has been permissible
>> to use SRCU read-side critical sections in interrupt context.
>> This allows __call_srcu() to use SRCU read-side critical sections to
>> prevent a new SRCU grace period from ending before the call to either
>> srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing
>> SRCU grace-period counter overflow during that time.
>>
>> Note that this does not permit removal of the counter-wrap checks in
>> srcu_gp_end().  These check are necessary to handle the case where
>> a given CPU does not interact at all with SRCU for an extended time
>> period.
>>
>> This commit therefore adds an SRCU read-side critical section to
>> __call_srcu() in order to prevent grace period counter wrap during
>> the funnel-locking process.
>>
>> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>
>Hello, Sasha,
>
>I recommend -against- backporting this one.  It is a theoretical bug
>that requires rather long preemption, so the risk of backporting likely
>greatly exceeeds the risk of the bug actually happening, especially on
>64-bit systems.

I'll drop it, thanks Paul!

--
Thanks,
Sasha