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Date:   Tue, 19 Nov 2019 11:44:48 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Alex Shi <alex.shi@linux.alibaba.com>, cgroups@vger.kernel.org,
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Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock
Message-ID: <20191119164448.GA396644@cmpxchg.org>
References: <1574166203-151975-1-git-send-email-alex.shi@linux.alibaba.com>
 <1574166203-151975-4-git-send-email-alex.shi@linux.alibaba.com>
 <20191119155704.GP20752@bombadil.infradead.org>
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On Tue, Nov 19, 2019 at 07:57:04AM -0800, Matthew Wilcox wrote:
> On Tue, Nov 19, 2019 at 08:23:17PM +0800, Alex Shi wrote:
> > +static inline struct lruvec *lock_page_lruvec_irqsave(struct page *page,
> > +				struct pglist_data *pgdat, unsigned long *flags)
> > +{
> > +	struct lruvec *lruvec = mem_cgroup_page_lruvec(page, pgdat);
> > +
> > +	spin_lock_irqsave(&lruvec->lru_lock, *flags);
> > +
> > +	return lruvec;
> > +}
> 
> This should be a macro, not a function.  You basically can't do this;
> spin_lock_irqsave needs to write to a variable which can then be passed
> to spin_unlock_irqrestore().  What you're doing here will dereference the
> pointer in _this_ function, but won't propagate the modified value back to
> the caller.  I suppose you could do something like this ...

This works because spin_lock_irqsave and local_irq_save() are
macros. It boils down to '*flags = arch_local_irq_save()' in this
function, and therefor does the right thing.

We exploit that in quite a few places:

$ git grep 'spin_lock_irqsave(.*\*flags' | wc -l
39