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Subject: preempt in kprobe
From: gmail <zhaoxiaoqiang007@gmail.com>
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Date: Mon, 2 Mar 2015 09:44:36 +0800
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        srostedt@redhat.com
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To: masami <masami.hiramatsu.pt@hitachi.com>
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Hi, masami:

        I have read your post at https://lkml.org/lkml/2011/7/1/185 , which remove the  preempt disable code in kprobe.
Does it safe on arm(32) ? In my scenario I (have to) write some sleep code which may cause reschedule in the probe handler.  Then I got schedule bug report like below:

<4>[  369.000535] [<c0110e54>] (unwind_backtrace+0x0/0x158) from [<c0ad9724>] (dump_stack+0x20/0x24)
<4>[  369.000609] [<c0ad9724>] (dump_stack+0x20/0x24) from [<c0ada664>] (__schedule_bug+0x58/0x64)
<4>[  369.000654] [<c0ada664>] (__schedule_bug+0x58/0x64) from [<c0aec4e0>] (__schedule+0x764/0x8e0)
<4>[  369.000730] [<c0aec4e0>] (__schedule+0x764/0x8e0) from [<c0aec7ac>] (schedule+0x40/0x80)
<4>[  369.000806] [<c0aec7ac>] (schedule+0x40/0x80) from [<c0ae9bac>] (schedule_timeout+0x230/0x3dc)
<4>[  369.000884] [<c0ae9bac>] (schedule_timeout+0x230/0x3dc) from [<c0aebbdc>] (wait_for_common+0x100/0x184)
<4>[  369.000929] [<c0aebbdc>] (wait_for_common+0x100/0x184) from [<c0aebc84>] (wait_for_completion+0x24/0x28)
<4>[  369.001007] [<c0aebc84>] (wait_for_completion+0x24/0x28) from [<c01c1a14>] (call_usermodehelper_exec+0x150/0x168)
<4>[  369.001083] [<c01c1a14>] (call_usermodehelper_exec+0x150/0x168) from [<bf0005b4>] (log_to_user.constprop.1+0x11c/0x16

     The bug was triggered by call_usermodehelper with UMH_WAIT_PROC argument , then the schedule function will read the preempt_count. It should be 1, but here it is 2.
Is it legal to sleep in kprobe handler? Is there is way to walkaroud this problem?

        Thanks !!