linux-kernel - Re: [BUG] kgdb/ftrace - sleeping in invalid context

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Date:   Thu, 17 Nov 2016 14:06:15 -0600
From:   Jason Wessel <jason.wessel@...driver.com>
To:     Brian Norris <briannorris@...omium.org>
CC:     <kgdb-bugreport@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [BUG] kgdb/ftrace - sleeping in invalid context

On 11/17/2016 01:16 PM, Brian Norris wrote:
> Hi,
>
> I've been poking around KGDB, and I noticed that the KDB 'ftdump'
> command (to dump ftrace logs) produces warnings like this:
>
> (gdb) monitor ftdump
> Dumping ftrace buffer:
> BUG: sleeping function called from invalid context at mm/slab.h:359
> in_atomic(): 1, irqs_disabled(): 128, pid: 116, name: irq/65-chromeos
> CPU: 4 PID: 116 Comm: irq/65-chromeos Not tainted 4.4.21 #575
> Call trace:
> [<ffffffc0002087dc>] dump_backtrace+0x0/0x160
> [<ffffffc00020895c>] show_stack+0x20/0x28
> [<ffffffc0004bdc28>] dump_stack+0x90/0xb0
> [<ffffffc0002486d8>] ___might_sleep+0x140/0x14c
> [<ffffffc000248764>] __might_sleep+0x80/0x90
> [<ffffffc00034ace0>] kmem_cache_alloc_trace+0x5c/0x238
> [<ffffffc0002d3ab8>] ring_buffer_read_prepare+0x4c/0xa4
> [<ffffffc0002f3214>] kdb_ftdump+0x200/0x3e4
> [<ffffffc0002c6244>] kdb_parse+0x548/0x628
> [<ffffffc0002c17a8>] gdb_serial_stub+0x89c/0xaac
> [<ffffffc0002bf954>] kgdb_cpu_enter+0x1e0/0x5b0
> [<ffffffc0002c002c>] kgdb_handle_exception+0x1a0/0x1e4
> [<ffffffc000213004>] kgdb_compiled_brk_fn+0x30/0x3c
> [<ffffffc000201a18>] brk_handler+0x9c/0xb0
> [<ffffffc0002004d4>] do_debug_exception+0x60/0xe0
> Exception stack(0xffffffc0ecffb820 to 0xffffffc0ecffb950)
> b820: ffffffc0010a6000 0000008000000000 ffffffc0ecffba10 ffffffc0002bf020
> b840: 00000000600001c5 0000000000000000 ffffffc0ecffb870 0000000000018160
> b860: 0000000000000003 00000000000000c3 ffffffc000be15b9 ffffffc000be7053
> b880: 0000000000000005 ffffffc0010a6c48 ffffffc0ecffb930 ffffffc00026f8e8
> b8a0: ffffffc000c362ca ffffffc00108c000 ffffffc00026f880 0000000000000001
> b8c0: 0000000000000007 ffffffc0ecfb6600 0000000000000001 cb88537fdc8ba615
> b8e0: ffffffc0011b6430 0000000000000001 0000000000000000 ffffffc0011b6438
> b900: 0000000000000000 0000000000000000 0000000000000006 ffffffc0010cc1c0
> b920: ffffffc00043eed8 7f7f7f7fffffffff 681f39616369ff46 7f7f7f7f7f7f7f7f
> b940: 0101010101010101 0000000000000008
> [<ffffffc0002035d4>] el1_dbg+0x18/0x74
> [<ffffffc0002bf0c0>] sysrq_handle_dbg+0x54/0x5c
> [<ffffffc00054e5dc>] __handle_sysrq+0xa4/0x15c
> [<ffffffc00054e7ec>] sysrq_filter+0x11c/0x348
> [<ffffffc00069abb8>] input_to_handler+0x60/0x100
> [<ffffffc00069c6d0>] input_pass_values.part.2+0x78/0x144
> [<ffffffc00069e408>] input_handle_event+0x280/0x4d4
> [<ffffffc00069e6cc>] input_event+0x70/0x8c
> [...]
>
> It looks like (almost?) all KDB code gets run in an exception context,
> so I don't see how sleeping allocations (such as those in
> ring_buffer_read_prepare()) are supposed to be able to work if they
> don't immediately find enough memory.
>
> I can't think of a great simple fix for this, other than borrowing the
> hack from kdb_private.h:
>
> #define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
>
> AFAICT, the necessary allocations are all pretty small actually, and so
> GFP_ATOMIC may not be a problem, even in the non-KDB ring buffer case.
>
> Thoughts? I figured I'd ask questions before blindly sending patches, as
> I'm not very familiar with this code.

In the past some buffers had be pre-allocated outside of the exception context in order to properly dump the ftrace code. Perhaps these patches were lost or the interface changed slightly over time.  Certainly we should never perform allocations while in the exception context.

Cheers,
Jason.