linux-kernel - Re: [PATCH] arm: kgdb: Don't try to stop the machine when setting breakpoints

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Message-ID: <20150825222626.GK19120@codeaurora.org>
Date:	Tue, 25 Aug 2015 15:26:26 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Douglas Anderson <dianders@...omium.org>
Cc:	Kees Cook <keescook@...omium.org>, Nicolas Pitre <nico@...aro.org>,
	Aapo Vienamo <avienamo@...dia.com>, linux@....linux.org.uk,
	masami.hiramatsu.pt@...achi.com, wangnan0@...wei.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm: kgdb: Don't try to stop the machine when setting
 breakpoints

On 08/25, Douglas Anderson wrote:
> In (23a4e40 arm: kgdb: Handle read-only text / modules) we moved to
> using patch_text() to set breakpoints so that we could handle the case
> when we had CONFIG_DEBUG_RODATA.  That patch used patch_text().
> Unfortunately, patch_text() assumes that we're not in atomic context
> when it runs since it needs to grab a mutex and also wait for other
> CPUs to stop (which it does with a completion).
> 
> This would result in a stack crawl if you had
> CONFIG_DEBUG_ATOMIC_SLEEP and tried to set a breakpoint in kgdb.  The
> crawl looked something like:
> 
>  BUG: scheduling while atomic: swapper/0/0/0x00010007
>  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.2.0-rc7-00133-geb63b34 #1073
>  Hardware name: Rockchip (Device Tree)
>   (unwind_backtrace) from [<c00133d4>] (show_stack+0x20/0x24)
>   (show_stack) from [<c05400e8>] (dump_stack+0x84/0xb8)
>   (dump_stack) from [<c004913c>] (__schedule_bug+0x54/0x6c)
>   (__schedule_bug) from [<c054065c>] (__schedule+0x80/0x668)
>   (__schedule) from [<c0540cfc>] (schedule+0xb8/0xd4)
>   (schedule) from [<c0543a3c>] (schedule_timeout+0x2c/0x234)
>   (schedule_timeout) from [<c05417c0>] (wait_for_common+0xf4/0x188)
>   (wait_for_common) from [<c0541874>] (wait_for_completion+0x20/0x24)
>   (wait_for_completion) from [<c00a0104>] (__stop_cpus+0x58/0x70)
>   (__stop_cpus) from [<c00a0580>] (stop_cpus+0x3c/0x54)
>   (stop_cpus) from [<c00a06c4>] (__stop_machine+0xcc/0xe8)
>   (__stop_machine) from [<c00a0714>] (stop_machine+0x34/0x44)
>   (stop_machine) from [<c00173e8>] (patch_text+0x28/0x34)
>   (patch_text) from [<c001733c>] (kgdb_arch_set_breakpoint+0x40/0x4c)
>   (kgdb_arch_set_breakpoint) from [<c00a0d68>] (kgdb_validate_break_address+0x2c/0x60)
>   (kgdb_validate_break_address) from [<c00a0e90>] (dbg_set_sw_break+0x1c/0xdc)
>   (dbg_set_sw_break) from [<c00a2e88>] (gdb_serial_stub+0x9c4/0xba4)
>   (gdb_serial_stub) from [<c00a11cc>] (kgdb_cpu_enter+0x1f8/0x60c)
>   (kgdb_cpu_enter) from [<c00a18cc>] (kgdb_handle_exception+0x19c/0x1d0)
>   (kgdb_handle_exception) from [<c0016f7c>] (kgdb_compiled_brk_fn+0x30/0x3c)
>   (kgdb_compiled_brk_fn) from [<c00091a4>] (do_undefinstr+0x1a4/0x20c)
>   (do_undefinstr) from [<c001400c>] (__und_svc_finish+0x0/0x34)
> 
> It turns out that when we're in kgdb all the CPUs are stopped anyway
> so there's no reason we should be calling patch_text().  We can
> instead directly call __patch_text() which assumes that CPUs have
> already been stopped.
> 
> Fixes: 23a4e4050ba9 ("arm: kgdb: Handle read-only text / modules")
> Reported-by: Aapo Vienamo <avienamo@...dia.com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

Reviewed-by: Stephen Boyd <sboyd@...eaurora.org>

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