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net: fix races in page->_count manipulation

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This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.

The only case it is valid is when page->_count is 0

Fixes: 540eb7bf0b ("net: Update alloc frag to reduce get/put page usage and recycle pages")
Signed-off-by: Eric Dumaze <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2014-10-10 04:48:18 -07:00 committed by David S. Miller
parent 98226208c8
commit 4c450583d9
1 changed files with 18 additions and 7 deletions
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23
net/core/skbuff.c
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@ -360,18 +360,29 @@ refill:
goto end;
}
nc->frag.size = PAGE_SIZE << order;
recycle:
atomic_set(&nc->frag.page->_count, NETDEV_PAGECNT_MAX_BIAS);
/* Even if we own the page, we do not use atomic_set().
* This would break get_page_unless_zero() users.
*/
atomic_add(NETDEV_PAGECNT_MAX_BIAS - 1,
&nc->frag.page->_count);
nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
nc->frag.offset = 0;
}
if (nc->frag.offset + fragsz > nc->frag.size) {
/* avoid unnecessary locked operations if possible */
if ((atomic_read(&nc->frag.page->_count) == nc->pagecnt_bias) ||
atomic_sub_and_test(nc->pagecnt_bias, &nc->frag.page->_count))
goto recycle;
if (atomic_read(&nc->frag.page->_count) != nc->pagecnt_bias) {
if (!atomic_sub_and_test(nc->pagecnt_bias,
&nc->frag.page->_count))
goto refill;
/* OK, page count is 0, we can safely set it */
atomic_set(&nc->frag.page->_count,
NETDEV_PAGECNT_MAX_BIAS);
} else {
atomic_add(NETDEV_PAGECNT_MAX_BIAS - nc->pagecnt_bias,
&nc->frag.page->_count);
}
nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
nc->frag.offset = 0;
}
data = page_address(nc->frag.page) + nc->frag.offset;