linux-kernel - Re: [PATCH] libnvdimm: rework region badblocks clearing

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Date:   Mon, 1 May 2017 08:52:22 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kani, Toshimitsu" <toshi.kani@....com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Verma, Vishal L" <vishal.l.verma@...el.com>
Subject: Re: [PATCH] libnvdimm: rework region badblocks clearing

On Mon, May 1, 2017 at 8:43 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Mon, May 1, 2017 at 8:34 AM, Kani, Toshimitsu <toshi.kani@....com> wrote:
>> On Sun, 2017-04-30 at 05:39 -0700, Dan Williams wrote:
>>> Toshi noticed that the new support for a region-level badblocks
>>> missed the case where errors are cleared due to BTT I/O.
>>>
>>> An initial attempt to fix this ran into a "sleeping while atomic"
>>> warning due to taking the nvdimm_bus_lock() in the BTT I/O path to
>>> satisfy the locking requirements of __nvdimm_bus_badblocks_clear().
>>> However, that lock is not needed since we are not acting any data
>>> that is subject to change due to a change of state of the bus /
>>> region. The badblocks instance has its own internal lock to handle
>>> mutations of the error list.
>>>
>>> So, to make it clear that we are just acting on region devices and
>>> don't need the lock rename __nvdimm_bus_badblocks_clear() to
>>> nvdimm_clear_badblocks_regions(). Eliminate the lock and consolidate
>>> all routines in drivers/nvdimm/bus.c. Also, make some cleanups to
>>> remove unnecessary casts, make the calling convention of
>>> nvdimm_clear_badblocks_regions() clearer by replacing struct resource
>>> with the minimal struct clear_badblocks_context, and use the
>>> DEVICE_ATTR macro.
>>
>> Hi Dan,
>>
>> I was testing the change with CONFIG_DEBUG_ATOMIC_SLEEP set this time,
>> and hit the following BUG with BTT.  This is a separate issue (not
>> introduced by this patch), but it shows that we have an issue with the
>> DSM call path as well.
>
> Ah, great find, thanks! We don't see this in the unit tests because
> the nfit_test infrastructure takes no sleeping actions in its
> simulated DSM path. Outside of converting btt to use sleeping locks
> I'm not sure I see a path forward. I wonder how bad the performance
> impact of that would be? Perhaps with opportunistic spinning it won't
> be so bad, but I don't see another choice.

It's worse than that. Part of the performance optimization of BTT I/O
was to avoid locking altogether when we could rely on a BTT lane
percpu, so that would also need to be removed.