Re: [PATCH] net: use raw_cpu ops in snmp stats bh

[David Miller]

From: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Date: Thu,  6 Mar 2014 15:55:36 +0300

> Commit a25982c15ae52 ("percpu: add preemption checks to __this_cpu
> ops") added preemption checks to __this_cpu ops, which are used in
> SNMP_INC_STATS_BH() and SNMP_ADD_STATS_BH(), resulting in following
> warnings:
> 
> BUG: using __this_cpu_add() in preemptible [00000000] code: ssh/4661
> caller is __this_cpu_preempt_check+0x2b/0x2d
> CPU: 3 PID: 4661 Comm: ssh Not tainted 3.14.0-rc5-next-20140306-dbg-dirty #162
> Hardware name: Acer             Aspire 5741G    /Aspire 5741G    , BIOS V1.20 02/08/2011
>  0000000000000000 ffff880094939be0 ffffffff813b8305 0000000000000003
>  ffff880094939c00 ffffffff8121221f ffff88015130a8c0 0000000000000000
>  ffff880094939c38 ffffffff8121226b 635f736968745f5f 29286464615f7570
> Call Trace:
>  [<ffffffff813b8305>] dump_stack+0x4e/0x7a
>  [<ffffffff8121221f>] check_preemption_disabled+0xce/0xdd
>  [<ffffffff8121226b>] __this_cpu_preempt_check+0x2b/0x2d
>  [<ffffffff81370b5a>] ? tcp_rearm_rto+0x9f/0xa1
>  [<ffffffff81373456>] tcp_event_new_data_sent+0x6d/0x91
>  [<ffffffff81374dc6>] tcp_write_xmit+0x3f1/0x935
>  [<ffffffff813754e4>] __tcp_push_pending_frames+0x28/0x82
>  [<ffffffff81368109>] tcp_push+0xcc/0x102
>  [<ffffffff8136a0c1>] tcp_sendmsg+0x8c5/0xb7a
>  [<ffffffff81207d59>] ? string.isra.6+0x3b/0x9f
>  [<ffffffff8138a10a>] inet_sendmsg+0x75/0xa0
>  [<ffffffff81320fb6>] sock_aio_write+0xe7/0xfe
>  [<ffffffff81207d59>] ? string.isra.6+0x3b/0x9f
>  [<ffffffff810e08ab>] do_sync_write+0x54/0x73
>  [<ffffffff810e0df7>] vfs_write+0xc6/0x17c
>  [<ffffffff810e1603>] SyS_write+0x44/0x78
>  [<ffffffff813be0a1>] tracesys+0xd4/0xd9
> 
> use raw_cpu_inc() and raw_cpu_add() instead of __this_cpu_inc()
> and __this_cpu_add().
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>

This is a bug in the TCP code, if this code path can happen outside of
software interrupts or BH protected sections, which it can as seen in
this trace, it must use the non-_BH-postfixed versions of the SNMP
counter bumps.
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