linux-kernel - Re: [PATCH v2 01/25] staging: lustre: libcfs: restore UMP handling

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Message-ID: <87h8m6ialz.fsf@notabene.neil.brown.name>
Date:   Thu, 14 Jun 2018 08:29:28 +1000
From:   NeilBrown <neilb@...e.com>
To:     James Simmons <jsimmons@...radead.org>
Cc:     Andreas Dilger <andreas.dilger@...el.com>,
        Oleg Drokin <oleg.drokin@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lustre Development List <lustre-devel@...ts.lustre.org>,
        Doug Oucharek <dougso@...com>,
        Amir Shehata <amir.shehata@...el.com>
Subject: Re: [PATCH v2 01/25] staging: lustre: libcfs: restore UMP handling

On Wed, Jun 13 2018, James Simmons wrote:

>> > With the cleanup of the libcfs SMP handling all UMP handling
>> > was removed. In the process now various NULL pointers and
>> > empty fields are return in the UMP case which causes lustre
>> > to crash hard. Restore the proper UMP handling so Lustre can
>> > properly function.
>> 
>> Can't we just get lustre to handle the NULL pointer?
>> Is most cases, the pointer is accessed through an accessor function, and
>> on !CONFIG_SMP, that can be a static inline that doesn't even look at
>> the pointer.
>
> Lots of NULL pointer checks for a structure allocated at libcfs module 
> start and only cleaned up at libcfs removal is not a clean approach.
> So I have thought about it and I have to ask why allocate a global
> struct cfs_cpu_table. It could be made static and fill it in which would
> avoid the whole NULL pointer issue. Plus for the UMP case why allocate
> a new cfs_cpu_table with cfs_cpt_table_alloc() which is exactly like
> the default UMP cfs_cpu_table. Instead we could just return the pointer
> to the static default cfs_cpt_tab every time. We still have the NULL
> ctb_cpumask field to deal with. Does that sound like a better solution
> to you? Doug what do you think?

I'm not convinced there will be lots of NULL pointer checks - maybe one
or two.  Most the accesses to the structure are inside helper
functions which are empty inlines in the UP build.

However I don't object to a static cfs_cpt_tab if that turns out to make
some code simpler.   I would want it to be clear up-front which code was
simplified so that an informed decision could be made.

Thanks,
NeilBrown


>  
>> I really think this is a step backwards.  If you can identify specific
>> problems caused by the current code, I'm sure we can fix them.
>> 
>> >
>> > Signed-off-by: James Simmons <uja.ornl@...oo.com>
>> > Signed-off-by: Amir Shehata <amir.shehata@...el.com>
>> > Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7734
>> 
>> This bug doesn't seem to mention this patch at all
>> 
>> > Reviewed-on: http://review.whamcloud.com/18916
>> 
>> Nor does this review.
>
> Yeah its mutated so much from what is in the Intel tree.
> I do believe it was the last patch to touch this.

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