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![Alexei Starovoitov](/assets/img/avatar_default.png)
when all map elements are pre-allocated one cpu can delete and reuse htab_elem
while another cpu is still walking the hlist. In such case the lookup may
miss the element. Convert hlist to hlist_nulls to avoid such scenario.
When bucket lock is taken there is no need to take such precautions,
so only convert map_lookup and map_get_next to nulls.
The race window is extremely small and only reproducible with explicit
udelay() inside lookup_nulls_elem_raw()
Similar to hlist add hlist_nulls_for_each_entry_safe() and
hlist_nulls_entry_safe() helpers.
Fixes: 6c90598174
("bpf: pre-allocate hash map elements")
Reported-by: Jonathan Perry <jonperry@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
174 lines
6.1 KiB
C
174 lines
6.1 KiB
C
#ifndef _LINUX_RCULIST_NULLS_H
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#define _LINUX_RCULIST_NULLS_H
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#ifdef __KERNEL__
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/*
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* RCU-protected list version
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*/
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#include <linux/list_nulls.h>
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#include <linux/rcupdate.h>
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/**
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* hlist_nulls_del_init_rcu - deletes entry from hash list with re-initialization
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* @n: the element to delete from the hash list.
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*
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* Note: hlist_nulls_unhashed() on the node return true after this. It is
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* useful for RCU based read lockfree traversal if the writer side
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* must know if the list entry is still hashed or already unhashed.
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*
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* In particular, it means that we can not poison the forward pointers
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* that may still be used for walking the hash list and we can only
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* zero the pprev pointer so list_unhashed() will return true after
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* this.
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*
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* The caller must take whatever precautions are necessary (such as
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* holding appropriate locks) to avoid racing with another
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* list-mutation primitive, such as hlist_nulls_add_head_rcu() or
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* hlist_nulls_del_rcu(), running on this same list. However, it is
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* perfectly legal to run concurrently with the _rcu list-traversal
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* primitives, such as hlist_nulls_for_each_entry_rcu().
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*/
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static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
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{
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if (!hlist_nulls_unhashed(n)) {
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__hlist_nulls_del(n);
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n->pprev = NULL;
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}
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}
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#define hlist_nulls_first_rcu(head) \
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(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
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#define hlist_nulls_next_rcu(node) \
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(*((struct hlist_nulls_node __rcu __force **)&(node)->next))
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/**
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* hlist_nulls_del_rcu - deletes entry from hash list without re-initialization
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* @n: the element to delete from the hash list.
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*
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* Note: hlist_nulls_unhashed() on entry does not return true after this,
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* the entry is in an undefined state. It is useful for RCU based
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* lockfree traversal.
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*
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* In particular, it means that we can not poison the forward
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* pointers that may still be used for walking the hash list.
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*
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* The caller must take whatever precautions are necessary
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* (such as holding appropriate locks) to avoid racing
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* with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
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* or hlist_nulls_del_rcu(), running on this same list.
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* However, it is perfectly legal to run concurrently with
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* the _rcu list-traversal primitives, such as
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* hlist_nulls_for_each_entry().
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*/
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static inline void hlist_nulls_del_rcu(struct hlist_nulls_node *n)
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{
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__hlist_nulls_del(n);
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n->pprev = LIST_POISON2;
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}
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/**
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* hlist_nulls_add_head_rcu
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* @n: the element to add to the hash list.
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* @h: the list to add to.
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*
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* Description:
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* Adds the specified element to the specified hlist_nulls,
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* while permitting racing traversals.
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*
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* The caller must take whatever precautions are necessary
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* (such as holding appropriate locks) to avoid racing
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* with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
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* or hlist_nulls_del_rcu(), running on this same list.
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* However, it is perfectly legal to run concurrently with
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* the _rcu list-traversal primitives, such as
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* hlist_nulls_for_each_entry_rcu(), used to prevent memory-consistency
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* problems on Alpha CPUs. Regardless of the type of CPU, the
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* list-traversal primitive must be guarded by rcu_read_lock().
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*/
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static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
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struct hlist_nulls_head *h)
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{
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struct hlist_nulls_node *first = h->first;
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n->next = first;
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n->pprev = &h->first;
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rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
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if (!is_a_nulls(first))
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first->pprev = &n->next;
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}
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/**
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* hlist_nulls_add_tail_rcu
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* @n: the element to add to the hash list.
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* @h: the list to add to.
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*
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* Description:
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* Adds the specified element to the end of the specified hlist_nulls,
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* while permitting racing traversals. NOTE: tail insertion requires
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* list traversal.
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*
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* The caller must take whatever precautions are necessary
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* (such as holding appropriate locks) to avoid racing
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* with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
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* or hlist_nulls_del_rcu(), running on this same list.
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* However, it is perfectly legal to run concurrently with
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* the _rcu list-traversal primitives, such as
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* hlist_nulls_for_each_entry_rcu(), used to prevent memory-consistency
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* problems on Alpha CPUs. Regardless of the type of CPU, the
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* list-traversal primitive must be guarded by rcu_read_lock().
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*/
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static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
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struct hlist_nulls_head *h)
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{
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struct hlist_nulls_node *i, *last = NULL;
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for (i = hlist_nulls_first_rcu(h); !is_a_nulls(i);
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i = hlist_nulls_next_rcu(i))
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last = i;
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if (last) {
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n->next = last->next;
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n->pprev = &last->next;
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rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
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} else {
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hlist_nulls_add_head_rcu(n, h);
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}
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}
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/**
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* hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
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* @tpos: the type * to use as a loop cursor.
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* @pos: the &struct hlist_nulls_node to use as a loop cursor.
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* @head: the head for your list.
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* @member: the name of the hlist_nulls_node within the struct.
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*
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* The barrier() is needed to make sure compiler doesn't cache first element [1],
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* as this loop can be restarted [2]
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* [1] Documentation/atomic_ops.txt around line 114
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* [2] Documentation/RCU/rculist_nulls.txt around line 146
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*/
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#define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \
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for (({barrier();}), \
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pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \
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(!is_a_nulls(pos)) && \
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({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
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pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))
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/**
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* hlist_nulls_for_each_entry_safe -
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* iterate over list of given type safe against removal of list entry
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* @tpos: the type * to use as a loop cursor.
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* @pos: the &struct hlist_nulls_node to use as a loop cursor.
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* @head: the head for your list.
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* @member: the name of the hlist_nulls_node within the struct.
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*/
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#define hlist_nulls_for_each_entry_safe(tpos, pos, head, member) \
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for (({barrier();}), \
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pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \
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(!is_a_nulls(pos)) && \
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({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); \
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pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)); 1; });)
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#endif
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#endif
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