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Subject: Re: [PATCH] Revert "seqlock: lockdep assert non-preemptibility on
 seqcount_t write"
To:     Greg KH <gregkh@linuxfoundation.org>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>
Cc:     bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        mingo@redhat.com, paulmck@kernel.org, peterz@infradead.org,
        rostedt@goodmis.org, tglx@linutronix.de, will@kernel.org
References: <20200810085954.GA1591892@kroah.com>
 <20200810095428.2602276-1-a.darwish@linutronix.de>
 <20200810100502.GA2406768@kroah.com>
From:   Guenter Roeck <linux@roeck-us.net>
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Message-ID: <bcf94ea2-05e6-2212-f732-b9a79a142f7e@roeck-us.net>
Date:   Mon, 10 Aug 2020 07:10:32 -0700
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On 8/10/20 3:05 AM, Greg KH wrote:
> On Mon, Aug 10, 2020 at 11:54:28AM +0200, Ahmed S. Darwish wrote:
>> This reverts commit 859247d39fb008ea812e8f0c398a58a20c12899e.
>>
>> Current implementation of lockdep_assert_preemption_disabled() uses
>> per-CPU variables, which was done to untangle the existing
>> seqlock.h<=>sched.h 'current->' task_struct circular dependency.
>>
>> Using per-CPU variables did not fully untangle the dependency for
>> various non-x86 architectures though, resulting in multiple broken
>> builds. For the affected architectures, raw_smp_processor_id() led
>> back to 'current->', thus having the original seqlock.h<=>sched.h
>> dependency in full-effect.
>>
>> For now, revert adding lockdep_assert_preemption_disabled() to
>> seqlock.h.
>>
>> Reported-by: Guenter Roeck <linux@roeck-us.net>
>> Link: https://lkml.kernel.org/r/20200808232122.GA176509@roeck-us.net
>> Link: https://lkml.kernel.org/r/20200810085954.GA1591892@kroah.com
>> References: Commit a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
>> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> Even after this, there are still some build errors on arm32, but I don't
> think they are due to this change:
> 
> 	ERROR: modpost: "__aeabi_uldivmod" [drivers/net/ethernet/sfc/sfc.ko] undefined!

This affects every 32 bit architecture.

> 	ERROR: modpost: "__bad_udelay" [drivers/net/ethernet/aquantia/atlantic/atlantic.ko] undefined!
> 

I don't think that is new. If anything, it is surprising that builds don't fail more
widely because of it. AFAICS it was introduced back in 2018 (a hot 50-ms delay loop
really isn't such a good idea).

Guenter