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Date: Tue, 26 May 2009 17:55:27 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Peter Zijlstra <peterz@infradead.org>
cc: Heiko Carstens <heiko.carstens@de.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] trace: disable preemption before taking raw spinlocks
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On Tue, 26 May 2009, Peter Zijlstra wrote:

> On Tue, 2009-05-26 at 17:28 +0200, Heiko Carstens wrote:
> > From: Heiko Carstens <heiko.carstens@de.ibm.com>
> > 
> > s390 code uses smp_processor_id() in __raw_spin_lock() code which
> > reveals that a (raw) spinlock is taken without preemption disabled.
> > This can potentially deadlock.
> > 
> > To fix this explicitly disable and enable preemption.
> > 
> > BUG: using smp_processor_id() in preemptible [00000000] code: cat/2278
> > caller is trace_find_cmdline+0x40/0xfc
> > CPU: 0 Not tainted 2.6.30-rc7-dirty #39
> > Process cat (pid: 2278, task: 000000003faedb68, ksp: 000000003b33b988)
> > 000000003b33b988 000000003b33bae0 0000000000000002 0000000000000000 
> >        000000003b33bb80 000000003b33baf8 000000003b33baf8 00000000000175d6 
> >        0000000000000001 000000003b33b988 000000003f9b0000 000000000000000b 
> >        000000000000000c 000000003b33bb40 000000003b33bae0 0000000000000000 
> >        0000000000000000 00000000000175d6 000000003b33bae0 000000003b33bb28 
> > Call Trace:
> > ([<00000000000174b2>] show_trace+0x112/0x170)
> >  [<0000000000017582>] show_stack+0x72/0x100
> >  [<0000000000441538>] dump_stack+0xc8/0xd8
> >  [<000000000025c350>] debug_smp_processor_id+0x114/0x130
> >  [<00000000000bf0e4>] trace_find_cmdline+0x40/0xfc
> >  [<00000000000c35d4>] trace_print_context+0x58/0xac
> >  [<00000000000bb676>] print_trace_line+0x416/0x470
> >  [<00000000000bc8fe>] s_show+0x4e/0x428
> >  [<000000000013834e>] seq_read+0x36a/0x5d4
> >  [<0000000000112a78>] vfs_read+0xc8/0x174
> >  [<0000000000112c58>] SyS_read+0x74/0xc4
> >  [<000000000002c7ae>] sysc_noemu+0x10/0x16
> >  [<000002000012436c>] 0x2000012436c
> > 1 lock held by cat/2278:
> >  #0:  (&p->lock){+.+.+.}, at: [<0000000000138056>] seq_read+0x72/0x5d4
> 
> Shouldn't that be preempt_disable_notrace() and co to avoid tracer
> recursion?

Doesn't need to be. That function is done in the output (slow path), not 
during an actual trace.

-- Steve