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7e934cf5ac
xas_for_each_marked() is using entry == NULL as a termination condition
of the iteration. When xas_for_each_marked() is used protected only by
RCU, this can however race with xas_store(xas, NULL) in the following
way:
TASK1 TASK2
page_cache_delete() find_get_pages_range_tag()
xas_for_each_marked()
xas_find_marked()
off = xas_find_chunk()
xas_store(&xas, NULL)
xas_init_marks(&xas);
...
rcu_assign_pointer(*slot, NULL);
entry = xa_entry(off);
And thus xas_for_each_marked() terminates prematurely possibly leading
to missed entries in the iteration (translating to missing writeback of
some pages or a similar problem).
If we find a NULL entry that has been marked, skip it (unless we're trying
to allocate an entry).
Reported-by: Jan Kara <jack@suse.cz>
CC: stable@vger.kernel.org
Fixes: ef8e5717db ("page cache: Convert delete_batch to XArray")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
87 lines
1.6 KiB
C
87 lines
1.6 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* iteration_check_2.c: Check that deleting a tagged entry doesn't cause
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* an RCU walker to finish early.
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* Copyright (c) 2020 Oracle
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* Author: Matthew Wilcox <willy@infradead.org>
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*/
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#include <pthread.h>
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#include "test.h"
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static volatile bool test_complete;
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static void *iterator(void *arg)
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{
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XA_STATE(xas, arg, 0);
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void *entry;
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rcu_register_thread();
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while (!test_complete) {
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xas_set(&xas, 0);
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rcu_read_lock();
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xas_for_each_marked(&xas, entry, ULONG_MAX, XA_MARK_0)
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;
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rcu_read_unlock();
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assert(xas.xa_index >= 100);
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}
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rcu_unregister_thread();
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return NULL;
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}
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static void *throbber(void *arg)
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{
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struct xarray *xa = arg;
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rcu_register_thread();
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while (!test_complete) {
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int i;
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for (i = 0; i < 100; i++) {
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xa_store(xa, i, xa_mk_value(i), GFP_KERNEL);
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xa_set_mark(xa, i, XA_MARK_0);
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}
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for (i = 0; i < 100; i++)
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xa_erase(xa, i);
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}
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rcu_unregister_thread();
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return NULL;
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}
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void iteration_test2(unsigned test_duration)
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{
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pthread_t threads[2];
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DEFINE_XARRAY(array);
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int i;
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printv(1, "Running iteration test 2 for %d seconds\n", test_duration);
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test_complete = false;
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xa_store(&array, 100, xa_mk_value(100), GFP_KERNEL);
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xa_set_mark(&array, 100, XA_MARK_0);
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if (pthread_create(&threads[0], NULL, iterator, &array)) {
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perror("create iterator thread");
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exit(1);
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}
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if (pthread_create(&threads[1], NULL, throbber, &array)) {
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perror("create throbber thread");
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exit(1);
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}
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sleep(test_duration);
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test_complete = true;
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for (i = 0; i < 2; i++) {
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if (pthread_join(threads[i], NULL)) {
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perror("pthread_join");
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exit(1);
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}
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}
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xa_destroy(&array);
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}
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