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Subjectshould list poisoning only kick in after selecting CONFIG_DEBUG_LIST?

a bit confused about list poisoning. there's this from
<linux/list.h>:

#ifndef CONFIG_DEBUG_LIST
static inline void list_del(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
entry->next = LIST_POISON1;
entry->prev = LIST_POISON2;
}
#else
extern void list_del(struct list_head *entry);
#endif

which seems a bit weird since, even if you *don't* select
CONFIG_DEBUG_LIST, the deleted node's pointers are poisoned. why?
that's certainly being done in the debug version of list_del() in
lib/list_debug.c, but why is it also being done above? for what
purpose?

and if i don't select that debugging option, should *anything* be
checking for those poison values?

$ grep -r -A2 LIST_POISON drivers
drivers/usb/host/xhci-hub.c: if (cmd->cmd_list.next != LIST_POISON1)
drivers/usb/host/xhci-hub.c- list_del(&cmd->cmd_list);
drivers/usb/host/xhci-hub.c- spin_unlock_irqrestore(&xhci->lock, flags);
--
drivers/usb/host/xhci.c: if (reset_device_cmd->cmd_list.next != LIST_POISON1)
drivers/usb/host/xhci.c- list_del(&reset_device_cmd->cmd_list);
drivers/usb/host/xhci.c- spin_unlock_irqrestore(&xhci->lock, flags);
$

that looks just plain odd. why would normal driver code that is not
dependent on CONFIG_DEBUG_LIST be checking for poison values? and if
it finds them, why is that not treated as that something has gone
significantly wrong, rather than quietly glossing over it?

rday

--

========================================================================
Robert P. J. Day Waterloo, Ontario, CANADA
http://crashcourse.ca

Twitter: http://twitter.com/rpjday
LinkedIn: http://ca.linkedin.com/in/rpjday
========================================================================


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