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{
    "id": 812661,
    "url": "http://patchwork.ozlabs.org/api/patches/812661/?format=api",
    "web_url": "http://patchwork.ozlabs.org/project/netdev/patch/20170911233332.7594-4-xiyou.wangcong@gmail.com/",
    "project": {
        "id": 7,
        "url": "http://patchwork.ozlabs.org/api/projects/7/?format=api",
        "name": "Linux network development",
        "link_name": "netdev",
        "list_id": "netdev.vger.kernel.org",
        "list_email": "netdev@vger.kernel.org",
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    "msgid": "<20170911233332.7594-4-xiyou.wangcong@gmail.com>",
    "list_archive_url": null,
    "date": "2017-09-11T23:33:32",
    "name": "[net,v3,3/3] net_sched: carefully handle tcf_block_put()",
    "commit_ref": null,
    "pull_url": null,
    "state": "accepted",
    "archived": true,
    "hash": "5f0e9b3a3fca735a442b1494e94f5cd358b6d8d9",
    "submitter": {
        "id": 211,
        "url": "http://patchwork.ozlabs.org/api/people/211/?format=api",
        "name": "Cong Wang",
        "email": "xiyou.wangcong@gmail.com"
    },
    "delegate": {
        "id": 34,
        "url": "http://patchwork.ozlabs.org/api/users/34/?format=api",
        "username": "davem",
        "first_name": "David",
        "last_name": "Miller",
        "email": "davem@davemloft.net"
    },
    "mbox": "http://patchwork.ozlabs.org/project/netdev/patch/20170911233332.7594-4-xiyou.wangcong@gmail.com/mbox/",
    "series": [
        {
            "id": 2596,
            "url": "http://patchwork.ozlabs.org/api/series/2596/?format=api",
            "web_url": "http://patchwork.ozlabs.org/project/netdev/list/?series=2596",
            "date": "2017-09-11T23:33:29",
            "name": "net_sched: fix filter chain reference counting",
            "version": 3,
            "mbox": "http://patchwork.ozlabs.org/series/2596/mbox/"
        }
    ],
    "comments": "http://patchwork.ozlabs.org/api/patches/812661/comments/",
    "check": "pending",
    "checks": "http://patchwork.ozlabs.org/api/patches/812661/checks/",
    "tags": {},
    "related": [],
    "headers": {
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        "X-Received": "by 10.84.224.75 with SMTP id a11mr15133377plt.106.1505172833774; \n\tMon, 11 Sep 2017 16:33:53 -0700 (PDT)",
        "From": "Cong Wang <xiyou.wangcong@gmail.com>",
        "To": "netdev@vger.kernel.org",
        "Cc": "jiri@mellanox.com, jakub.kicinski@netronome.com, jhs@mojatatu.com,\n\tCong Wang <xiyou.wangcong@gmail.com>",
        "Subject": "[Patch net v3 3/3] net_sched: carefully handle tcf_block_put()",
        "Date": "Mon, 11 Sep 2017 16:33:32 -0700",
        "Message-Id": "<20170911233332.7594-4-xiyou.wangcong@gmail.com>",
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        "In-Reply-To": "<20170911233332.7594-1-xiyou.wangcong@gmail.com>",
        "References": "<20170911233332.7594-1-xiyou.wangcong@gmail.com>",
        "Sender": "netdev-owner@vger.kernel.org",
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        "List-ID": "<netdev.vger.kernel.org>",
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    },
    "content": "As pointed out by Jiri, there is still a race condition between\ntcf_block_put() and tcf_chain_destroy() in a RCU callback. There\nis no way to make it correct without proper locking or synchronization,\nbecause both operate on a shared list.\n\nLocking is hard, because the only lock we can pick here is a spinlock,\nhowever, in tc_dump_tfilter() we iterate this list with a sleeping\nfunction called (tcf_chain_dump()), which makes using a lock to protect\nchain_list almost impossible.\n\nJiri suggested the idea of holding a refcnt before flushing, this works\nbecause it guarantees us there would be no parallel tcf_chain_destroy()\nduring the loop, therefore the race condition is gone. But we have to\nbe very careful with proper synchronization with RCU callbacks.\n\nSuggested-by: Jiri Pirko <jiri@mellanox.com>\nCc: Jamal Hadi Salim <jhs@mojatatu.com>\nSigned-off-by: Cong Wang <xiyou.wangcong@gmail.com>\n---\n net/sched/cls_api.c | 24 ++++++++++++++++++------\n 1 file changed, 18 insertions(+), 6 deletions(-)",
    "diff": "diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c\nindex d29e79d98a69..0b2219adf520 100644\n--- a/net/sched/cls_api.c\n+++ b/net/sched/cls_api.c\n@@ -275,15 +275,27 @@ void tcf_block_put(struct tcf_block *block)\n \n \t/* XXX: Standalone actions are not allowed to jump to any chain, and\n \t * bound actions should be all removed after flushing. However,\n-\t * filters are destroyed in RCU callbacks, we have to flush and wait\n-\t * for them inside the loop, otherwise we race with RCU callbacks on\n-\t * this list.\n+\t * filters are destroyed in RCU callbacks, we have to hold the chains\n+\t * first, otherwise we would always race with RCU callbacks on this list\n+\t * without proper locking.\n \t */\n-\tlist_for_each_entry_safe(chain, tmp, &block->chain_list, list) {\n+\n+\t/* Wait for existing RCU callbacks to cool down. */\n+\trcu_barrier();\n+\n+\t/* Hold a refcnt for all chains, except 0, in case they are gone. */\n+\tlist_for_each_entry(chain, &block->chain_list, list)\n+\t\tif (chain->index)\n+\t\t\ttcf_chain_hold(chain);\n+\n+\t/* No race on the list, because no chain could be destroyed. */\n+\tlist_for_each_entry(chain, &block->chain_list, list)\n \t\ttcf_chain_flush(chain);\n-\t\trcu_barrier();\n-\t}\n \n+\t/* Wait for RCU callbacks to release the reference count. */\n+\trcu_barrier();\n+\n+\t/* At this point, all the chains should have refcnt == 1. */\n \tlist_for_each_entry_safe(chain, tmp, &block->chain_list, list)\n \t\ttcf_chain_put(chain);\n \tkfree(block);\n",
    "prefixes": [
        "net",
        "v3",
        "3/3"
    ]
}