[PATCH 8/9] PCI: hotplug: Embed hotplug_slot

[Sebastian Ott]

On Sun, 19 Aug 2018, Lukas Wunner wrote:
> When the PCI hotplug core and its first user, cpqphp, were introduced in
> February 2002 with historic commit a8a2069f432c, cpqphp allocated a slot
> struct for its internal use plus a hotplug_slot struct to be registered
> with the hotplug core and linked the two with pointers:
> https://git.kernel.org/tglx/history/c/a8a2069f432c
> 
> Nowadays, the predominant pattern in the tree is to embed ("subclass")
> such structures in one another and cast to the containing struct with
> container_of().  But it wasn't until July 2002 that container_of() was
> introduced with historic commit ec4f214232cf:
> https://git.kernel.org/tglx/history/c/ec4f214232cf
> 
> pnv_php, introduced in 2016, did the right thing and embedded struct
> hotplug_slot in its internal struct pnv_php_slot, but all other drivers
> cargo-culted cpqphp's design and linked separate structs with pointers.
> 
> Embedding structs is preferrable to linking them with pointers because
> it requires fewer allocations, thereby reducing overhead and simplifying
> error paths.  Casting an embedded struct to the containing struct
> becomes a cheap subtraction rather than a dereference.  And having fewer
> pointers reduces the risk of them pointing nowhere either accidentally
> or due to an attack.
> 
> Convert all drivers to embed struct hotplug_slot in their internal slot
> struct.  The "private" pointer in struct hotplug_slot thereby becomes
> unused, so drop it.
> 
> Signed-off-by: Lukas Wunner <lukas at wunner.de>
> Cc: Rafael J. Wysocki <rjw at rjwysocki.net>
> Cc: Len Brown <lenb at kernel.org>
> Cc: Scott Murray <scott at spiteful.org>
> Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> Cc: Paul Mackerras <paulus at samba.org>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Cc: Gavin Shan <gwshan at linux.vnet.ibm.com>
> Cc: Sebastian Ott <sebott at linux.vnet.ibm.com>
> Cc: Gerald Schaefer <gerald.schaefer at de.ibm.com>
> Cc: Corentin Chary <corentin.chary at gmail.com>
> Cc: Darren Hart <dvhart at infradead.org>
> Cc: Andy Shevchenko <andy at infradead.org>

for s390_pci_hpc.c:
Acked-by: Sebastian Ott <sebott at linux.ibm.com>