linux-kernel - Re: xarray reserve/release?

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Date:   Tue, 19 Feb 2019 20:46:27 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: xarray reserve/release?

On Tue, Feb 19, 2019 at 05:26:09PM -0800, Matthew Wilcox wrote:
> On Tue, Feb 19, 2019 at 04:53:49PM -0700, Jason Gunthorpe wrote:
> > Hey Matt,
> > 
> > Did you intend that xa_release doesn't work on allocating arrays:
> 
> That surprises me.  I'll take a look in the morning.

I think the issue is that this:

static inline void xa_release(struct xarray *xa, unsigned long index)
{
	xa_cmpxchg(xa, index, NULL, NULL, 0);

relies on the NULL actually being xas_store(NULL), but cmpxchg
transforms it into xas_store(XA_ZERO_ENTRY) when allocating..

So xa_reserve(), xa_release() and xa_cmpxchg() all do the same thing
for allocating arrays.

Perhaps this:

void __xa_release(struct xarray *xa, unsigned long index)
{
	XA_STATE(xas, xa, index);
	void *curr;

	curr = xas_load(&xas);
	if (curr == XA_ZERO_ENTRY)
		xas_store(&xas, NULL);
}

?

Also, I wonder if xa_reserve() is better written as as

       xa_cmpxchg(xa, index, NULL, XA_ZERO_ENTRY)

Bit clearer what is going on..

Jason