Date: Tue, 22 Mar 2005 10:32:01 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-01
Message-ID: <20050322093201.GA21945@elte.hu>
References: <20050319191658.GA5921@elte.hu> <20050320174508.GA3902@us.ibm.com> <20050321085332.GA7163@elte.hu> <20050321090122.GA8066@elte.hu> <20050321090622.GA8430@elte.hu> <20050322054345.GB1296@us.ibm.com> <20050322072413.GA6149@elte.hu> <20050322092331.GA21465@elte.hu>
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* Ingo Molnar <mingo@elte.hu> wrote:

> seems to be a true SMP race: when i boot with 1 CPU it doesnt trigger,
> the same kernel image and 2 CPUs triggers it on CPU#1. (CPU#0 is the
> boot CPU) Note that the timing of the crash is not deterministic
> (sometimes i get it during net startup, sometimes during ACPI
> startup), but it always crashes within rcu_advance_callbacks().
> 
> one difference between your tests and mine is that your kernel is
> doing _synchronize_kernel() from preempt-off sections (correct?),
> while my kernel with PREEMPT_RT does it on preemptable sections.

hm, another thing: i think call_rcu() needs to take the read-lock. Right
now it assumes that it has the data structure private, but that's only
statistically true on PREEMPT_RT: another CPU may have this CPU's RCU
control structure in use. So IRQs-off (or preempt-off) is not a
guarantee to have the data structure, the read lock has to be taken.

	Ingo
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