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Subject: Re: [PATCH 1/3] Increase lockdep limits: MAX_STACK_TRACE_ENTRIES
From: Catalin Marinas <catalin.marinas@arm.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Dave Jones <davej@redhat.com>, Joao Correia <joaomiguelcorreia@gmail.com>,
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       =?ISO-8859-1?Q?Am=E9rico?= Wang <xiyou.wangcong@gmail.com>,
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Organization: ARM Ltd
Date: Thu, 09 Jul 2009 10:06:56 +0100
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On Wed, 2009-07-08 at 20:36 +0200, Peter Zijlstra wrote:
> As a side node, I see that each and every trace ends with a -1 entry:
> 
> ...
> [ 1194.412158]    [<c01f7990>] do_mount+0x3c0/0x7c0
> [ 1194.412158]    [<c01f7e14>] sys_mount+0x84/0xb0
> [ 1194.412158]    [<c01221b1>] syscall_call+0x7/0xb
> [ 1194.412158]    [<ffffffff>] 0xffffffff
> 
> Which seems to come from:
> 
> void save_stack_trace(struct stack_trace *trace)
> {
>         dump_trace(current, NULL, NULL, 0, &save_stack_ops, trace);
>         if (trace->nr_entries < trace->max_entries)
>                 trace->entries[trace->nr_entries++] = ULONG_MAX;
> }
> EXPORT_SYMBOL_GPL(save_stack_trace);
> 
> commit 006e84ee3a54e393ec6bef2a9bc891dc5bde2843 seems involved,..

The reason for this is that if there are no more traces to show, it
inserts -1. In this case, it cannot trace beyond the system call. If the
stack trace is truncated because of the maximum number of trace entries
it can show, you won't get a -1.

Before the commit above, it was always inserting -1 even if the trace
was longer than the maximum number of entries.

-- 
Catalin

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