linux-kernel - Re: [PATCH 05/26] cxl/core: Simplify cxl_dpa_set

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Message-ID: <6638572a2afd_258421294e0@iweiny-mobl.notmuch>
Date: Sun, 5 May 2024 21:06:02 -0700
From: Ira Weiny <ira.weiny@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	Alison Schofield <alison.schofield@...el.com>
CC: Dave Jiang <dave.jiang@...el.com>, Fan Ni <fan.ni@...sung.com>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, Navneet Singh
	<navneet.singh@...el.com>, Dan Williams <dan.j.williams@...el.com>, Davidlohr
 Bueso <dave@...olabs.net>, Vishal Verma <vishal.l.verma@...el.com>,
	<linux-btrfs@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/26] cxl/core: Simplify cxl_dpa_set_mode()

Dan Williams wrote:
> Ira Weiny wrote:
> > Alison Schofield wrote:
> > > On Sun, Mar 24, 2024 at 04:18:08PM -0700, Ira Weiny wrote:
> > > > cxl_dpa_set_mode() checks the mode for validity two times, once outside
> > > > of the DPA RW semaphore and again within.
> > > 
> > > Not true.
> > 
> > Sorry for not being clear.  It does check the mode 2x but not for
> > validity.  I'll clarify.
> > 
> > > It only checks mode once before the lock. It checks for
> > > capacity after the lock. If it didn't check mode before the lock,
> > > then unsupported modes would fall through.
> > 
> > But we can check the mode 1 time and either check the size or fail.
> > 
> > > 
> > > > The function is not in a critical path.
> > > 
> > > Implying what here?  OK to check twice (even though it wasn't)
> > > or OK to expand scope of locking.
> > 
> > Implying that checking the mode outside the lock is not required.
> 
> The @mode check outside the lock is there to taking the lock when not
> necessary because the passed in mode is already bogus.

Sorry I meant to say 'is required'.

> 
> The lock is about making sure the write of cxled->mode relative to the
> state of the dpa partitions is an atomic check-and-set.
> 
> So this makes the function unconditionally take the lock when it might
> be bogus to do so. The value of reorganizing this is questionable.

Why would it be bogus?  I don't see that.  Regardless I dropped the patch as it
is not worth spending more time on.  There are bigger issues to resolve with
this series.

Ira