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Cmpxchg is used to implement adding new entry to the list, deleting
all entries from the list, deleting first entry of the list and some
other operations.
Because this is a single list, so the tail can not be accessed in O(1).
If there are multiple producers and multiple consumers, llist_add can
be used in producers and llist_del_all can be used in consumers. They
can work simultaneously without lock. But llist_del_first can not be
used here. Because llist_del_first depends on list->first->next does
not changed if list->first is not changed during its operation, but
llist_del_first, llist_add, llist_add (or llist_del_all, llist_add,
llist_add) sequence in another consumer may violate that.
If there are multiple producers and one consumer, llist_add can be
used in producers and llist_del_all or llist_del_first can be used in
the consumer.
This can be summarized as follow:
| add | del_first | del_all
add | - | - | -
del_first | | L | L
del_all | | | -
Where "-" stands for no lock is needed, while "L" stands for lock is
needed.
The list entries deleted via llist_del_all can be traversed with
traversing function such as llist_for_each etc. But the list entries
can not be traversed safely before deleted from the list. The order
of deleted entries is from the newest to the oldest added one. If you
want to traverse from the oldest to the newest, you must reverse the
order by yourself before traversing.
The basic atomic operation of this list is cmpxchg on long. On
architectures that don't have NMI-safe cmpxchg implementation, the
list can NOT be used in NMI handler. So code uses the list in NMI
handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
Signed-off-by: Huang Ying <ying.huang@intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>
130 lines
3.8 KiB
C
130 lines
3.8 KiB
C
/*
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* Lock-less NULL terminated single linked list
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*
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* The basic atomic operation of this list is cmpxchg on long. On
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* architectures that don't have NMI-safe cmpxchg implementation, the
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* list can NOT be used in NMI handler. So code uses the list in NMI
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* handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
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*
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* Copyright 2010,2011 Intel Corp.
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* Author: Huang Ying <ying.huang@intel.com>
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License version
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* 2 as published by the Free Software Foundation;
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*/
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/interrupt.h>
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#include <linux/llist.h>
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#include <asm/system.h>
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/**
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* llist_add - add a new entry
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* @new: new entry to be added
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* @head: the head for your lock-less list
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*/
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void llist_add(struct llist_node *new, struct llist_head *head)
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{
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struct llist_node *entry, *old_entry;
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#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
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BUG_ON(in_nmi());
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#endif
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entry = head->first;
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do {
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old_entry = entry;
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new->next = entry;
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, new)) != old_entry);
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}
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EXPORT_SYMBOL_GPL(llist_add);
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/**
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* llist_add_batch - add several linked entries in batch
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* @new_first: first entry in batch to be added
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* @new_last: last entry in batch to be added
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* @head: the head for your lock-less list
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*/
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void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
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struct llist_head *head)
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{
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struct llist_node *entry, *old_entry;
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#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
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BUG_ON(in_nmi());
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#endif
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entry = head->first;
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do {
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old_entry = entry;
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new_last->next = entry;
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, new_first)) != old_entry);
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}
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EXPORT_SYMBOL_GPL(llist_add_batch);
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/**
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* llist_del_first - delete the first entry of lock-less list
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* @head: the head for your lock-less list
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*
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* If list is empty, return NULL, otherwise, return the first entry
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* deleted, this is the newest added one.
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*
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* Only one llist_del_first user can be used simultaneously with
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* multiple llist_add users without lock. Because otherwise
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* llist_del_first, llist_add, llist_add (or llist_del_all, llist_add,
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* llist_add) sequence in another user may change @head->first->next,
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* but keep @head->first. If multiple consumers are needed, please
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* use llist_del_all or use lock between consumers.
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*/
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struct llist_node *llist_del_first(struct llist_head *head)
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{
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struct llist_node *entry, *old_entry, *next;
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#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
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BUG_ON(in_nmi());
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#endif
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entry = head->first;
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do {
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if (entry == NULL)
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return NULL;
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old_entry = entry;
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next = entry->next;
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cpu_relax();
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} while ((entry = cmpxchg(&head->first, old_entry, next)) != old_entry);
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return entry;
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}
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EXPORT_SYMBOL_GPL(llist_del_first);
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/**
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* llist_del_all - delete all entries from lock-less list
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* @head: the head of lock-less list to delete all entries
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*
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* If list is empty, return NULL, otherwise, delete all entries and
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* return the pointer to the first entry. The order of entries
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* deleted is from the newest to the oldest added one.
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*/
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struct llist_node *llist_del_all(struct llist_head *head)
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{
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#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
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BUG_ON(in_nmi());
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#endif
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return xchg(&head->first, NULL);
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}
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EXPORT_SYMBOL_GPL(llist_del_all);
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