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e8c82c2e23
An XFS workload showed up a bug in the lockless pagecache patch. Basically it
would go into an "infinite" loop, although it would sometimes be able to break
out of the loop! The reason is a missing compiler barrier in the "increment
reference count unless it was zero" case of the lockless pagecache protocol in
the gang lookup functions.
This would cause the compiler to use a cached value of struct page pointer to
retry the operation with, rather than reload it. So the page might have been
removed from pagecache and freed (refcount==0) but the lookup would not correctly
notice the page is no longer in pagecache, and keep attempting to increment the
refcount and failing, until the page gets reallocated for something else. This
isn't a data corruption because the condition will be detected if the page has
been reallocated. However it can result in a lockup.
Linus points out that ACCESS_ONCE is also required in that pointer load, even
if it's absence is not causing a bug on our particular build. The most general
way to solve this is just to put an rcu_dereference in radix_tree_deref_slot.
Assembly of find_get_pages,
before:
.L220:
movq (%rbx), %rax #* ivtmp.1162, tmp82
movq (%rax), %rdi #, prephitmp.1149
.L218:
testb $1, %dil #, prephitmp.1149
jne .L217 #,
testq %rdi, %rdi # prephitmp.1149
je .L203 #,
cmpq $-1, %rdi #, prephitmp.1149
je .L217 #,
movl 8(%rdi), %esi # <variable>._count.counter, c
testl %esi, %esi # c
je .L218 #,
after:
.L212:
movq (%rbx), %rax #* ivtmp.1109, tmp81
movq (%rax), %rdi #, ret
testb $1, %dil #, ret
jne .L211 #,
testq %rdi, %rdi # ret
je .L197 #,
cmpq $-1, %rdi #, ret
je .L211 #,
movl 8(%rdi), %esi # <variable>._count.counter, c
testl %esi, %esi # c
je .L212 #,
(notice the obvious infinite loop in the first example, if page->count remains 0)
Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
194 lines
6.7 KiB
C
194 lines
6.7 KiB
C
/*
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* Copyright (C) 2001 Momchil Velikov
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* Portions Copyright (C) 2001 Christoph Hellwig
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* Copyright (C) 2006 Nick Piggin
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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 as
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* published by the Free Software Foundation; either version 2, or (at
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* your option) any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* 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., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#ifndef _LINUX_RADIX_TREE_H
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#define _LINUX_RADIX_TREE_H
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#include <linux/preempt.h>
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#include <linux/types.h>
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#include <linux/kernel.h>
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#include <linux/rcupdate.h>
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/*
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* An indirect pointer (root->rnode pointing to a radix_tree_node, rather
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* than a data item) is signalled by the low bit set in the root->rnode
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* pointer.
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*
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* In this case root->height is > 0, but the indirect pointer tests are
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* needed for RCU lookups (because root->height is unreliable). The only
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* time callers need worry about this is when doing a lookup_slot under
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* RCU.
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*/
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#define RADIX_TREE_INDIRECT_PTR 1
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#define RADIX_TREE_RETRY ((void *)-1UL)
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static inline void *radix_tree_ptr_to_indirect(void *ptr)
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{
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return (void *)((unsigned long)ptr | RADIX_TREE_INDIRECT_PTR);
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}
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static inline void *radix_tree_indirect_to_ptr(void *ptr)
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{
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return (void *)((unsigned long)ptr & ~RADIX_TREE_INDIRECT_PTR);
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}
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static inline int radix_tree_is_indirect_ptr(void *ptr)
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{
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return (int)((unsigned long)ptr & RADIX_TREE_INDIRECT_PTR);
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}
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/*** radix-tree API starts here ***/
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#define RADIX_TREE_MAX_TAGS 2
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/* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
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struct radix_tree_root {
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unsigned int height;
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gfp_t gfp_mask;
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struct radix_tree_node *rnode;
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};
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#define RADIX_TREE_INIT(mask) { \
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.height = 0, \
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.gfp_mask = (mask), \
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.rnode = NULL, \
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}
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#define RADIX_TREE(name, mask) \
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struct radix_tree_root name = RADIX_TREE_INIT(mask)
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#define INIT_RADIX_TREE(root, mask) \
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do { \
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(root)->height = 0; \
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(root)->gfp_mask = (mask); \
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(root)->rnode = NULL; \
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} while (0)
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/**
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* Radix-tree synchronization
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*
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* The radix-tree API requires that users provide all synchronisation (with
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* specific exceptions, noted below).
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*
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* Synchronization of access to the data items being stored in the tree, and
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* management of their lifetimes must be completely managed by API users.
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*
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* For API usage, in general,
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* - any function _modifying_ the tree or tags (inserting or deleting
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* items, setting or clearing tags) must exclude other modifications, and
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* exclude any functions reading the tree.
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* - any function _reading_ the tree or tags (looking up items or tags,
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* gang lookups) must exclude modifications to the tree, but may occur
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* concurrently with other readers.
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*
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* The notable exceptions to this rule are the following functions:
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* radix_tree_lookup
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* radix_tree_lookup_slot
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* radix_tree_tag_get
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* radix_tree_gang_lookup
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* radix_tree_gang_lookup_slot
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* radix_tree_gang_lookup_tag
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* radix_tree_gang_lookup_tag_slot
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* radix_tree_tagged
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*
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* The first 7 functions are able to be called locklessly, using RCU. The
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* caller must ensure calls to these functions are made within rcu_read_lock()
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* regions. Other readers (lock-free or otherwise) and modifications may be
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* running concurrently.
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*
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* It is still required that the caller manage the synchronization and lifetimes
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* of the items. So if RCU lock-free lookups are used, typically this would mean
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* that the items have their own locks, or are amenable to lock-free access; and
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* that the items are freed by RCU (or only freed after having been deleted from
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* the radix tree *and* a synchronize_rcu() grace period).
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*
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* (Note, rcu_assign_pointer and rcu_dereference are not needed to control
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* access to data items when inserting into or looking up from the radix tree)
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*
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* radix_tree_tagged is able to be called without locking or RCU.
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*/
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/**
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* radix_tree_deref_slot - dereference a slot
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* @pslot: pointer to slot, returned by radix_tree_lookup_slot
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* Returns: item that was stored in that slot with any direct pointer flag
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* removed.
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*
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* For use with radix_tree_lookup_slot(). Caller must hold tree at least read
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* locked across slot lookup and dereference. More likely, will be used with
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* radix_tree_replace_slot(), as well, so caller will hold tree write locked.
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*/
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static inline void *radix_tree_deref_slot(void **pslot)
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{
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void *ret = rcu_dereference(*pslot);
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if (unlikely(radix_tree_is_indirect_ptr(ret)))
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ret = RADIX_TREE_RETRY;
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return ret;
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}
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/**
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* radix_tree_replace_slot - replace item in a slot
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* @pslot: pointer to slot, returned by radix_tree_lookup_slot
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* @item: new item to store in the slot.
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*
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* For use with radix_tree_lookup_slot(). Caller must hold tree write locked
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* across slot lookup and replacement.
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*/
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static inline void radix_tree_replace_slot(void **pslot, void *item)
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{
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BUG_ON(radix_tree_is_indirect_ptr(item));
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rcu_assign_pointer(*pslot, item);
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}
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int radix_tree_insert(struct radix_tree_root *, unsigned long, void *);
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void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
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void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
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void *radix_tree_delete(struct radix_tree_root *, unsigned long);
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unsigned int
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radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
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unsigned long first_index, unsigned int max_items);
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unsigned int
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radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
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unsigned long first_index, unsigned int max_items);
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unsigned long radix_tree_next_hole(struct radix_tree_root *root,
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unsigned long index, unsigned long max_scan);
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int radix_tree_preload(gfp_t gfp_mask);
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void radix_tree_init(void);
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void *radix_tree_tag_set(struct radix_tree_root *root,
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unsigned long index, unsigned int tag);
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void *radix_tree_tag_clear(struct radix_tree_root *root,
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unsigned long index, unsigned int tag);
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int radix_tree_tag_get(struct radix_tree_root *root,
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unsigned long index, unsigned int tag);
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unsigned int
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radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
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unsigned long first_index, unsigned int max_items,
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unsigned int tag);
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unsigned int
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radix_tree_gang_lookup_tag_slot(struct radix_tree_root *root, void ***results,
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unsigned long first_index, unsigned int max_items,
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unsigned int tag);
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int radix_tree_tagged(struct radix_tree_root *root, unsigned int tag);
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static inline void radix_tree_preload_end(void)
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{
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preempt_enable();
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}
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#endif /* _LINUX_RADIX_TREE_H */
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