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From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: keescook@chromium.org, jannh@google.com,
	linux-kernel@vger.kernel.org, vcaputo@pengaru.com,
	mingo@redhat.com, juri.lelli@redhat.com,
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	bristot@redhat.com, akpm@linux-foundation.org,
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	michael.weiss@aisec.fraunhofer.de, mhocko@suse.com,
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	davem@davemloft.net, chris@zankel.net
Subject: Re: [PATCH 6/7] arch: __get_wchan || STACKTRACE_SUPPORT
Date: Thu, 14 Oct 2021 19:03:07 +0100	[thread overview]
Message-ID: <20211014180307.GB39276@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <YWBLl0mMTGPE/7hM@hirez.programming.kicks-ass.net>

On Fri, Oct 08, 2021 at 03:45:59PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 08, 2021 at 01:40:52PM +0100, Mark Rutland wrote:
> > [Adding Josh, since there might be a concern here from a livepatch pov]
> > 
> 
> > > +static unsigned long __get_wchan(struct task_struct *p)
> > > +{
> > > +	unsigned long entry = 0;
> > > +
> > > +	stack_trace_save_tsk(p, &entry, 1, 0);
> > 
> > This assumes stack_trace_save_tsk() will skip sched functions, but I
> > don't think that's ever been a requirement? It's certinaly not
> > documented anywhere that I could find, and arm64 doesn't do so today,
> > and this patch causes wchan to just log `__switch_to` for everything.
> 
> Confused, arm64 has arch_stack_walk() and should thus use
> kernel/stacktrace.c's stack_trace_consume_entry_nosched.

Looking at this arm64's *current* get_wchan() unwinds once before
checking in_sched_functions(), so it skips __switch_to(). As of this
patch, we check in_sched_functions() first, which stops the unwind
immediately as __switch_to() isn't marked as __sched.

I think x86 gets away with this because switch_to() is asm, and that
tail-calls __switch_to() when returning.

Does switch_to() and below need to be marked __sched?

Thanks,
Mark.

  parent reply	other threads:[~2021-10-14 18:03 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-08 11:15 [PATCH 0/7] wchan: Fix wchan support Peter Zijlstra
2021-10-08 11:15 ` [PATCH 1/7] Revert "proc/wchan: use printk format instead of lookup_symbol_name()" Peter Zijlstra
2021-10-15  9:45   ` [tip: sched/core] " tip-bot2 for Kees Cook
2021-10-08 11:15 ` [PATCH 2/7] leaking_addresses: Always print a trailing newline Peter Zijlstra
2021-10-15  9:45   ` [tip: sched/core] " tip-bot2 for Kees Cook
2021-10-08 11:15 ` [PATCH 3/7] proc: Use task_is_running() for wchan in /proc/$pid/stat Peter Zijlstra
2021-10-15  9:45   ` [tip: sched/core] " tip-bot2 for Kees Cook
2021-10-08 11:15 ` [PATCH 4/7] x86: Fix get_wchan() to support the ORC unwinder Peter Zijlstra
2021-10-15  9:45   ` [tip: sched/core] " tip-bot2 for Qi Zheng
2021-10-08 11:15 ` [PATCH 5/7] sched: Add wrapper for get_wchan() to keep task blocked Peter Zijlstra
2021-10-08 11:26   ` Geert Uytterhoeven
2021-10-08 12:45   ` Mark Rutland
2021-10-14 10:46   ` Russell King (Oracle)
2021-10-15  9:45   ` [tip: sched/core] " tip-bot2 for Kees Cook
2021-10-08 11:15 ` [PATCH 6/7] arch: __get_wchan || STACKTRACE_SUPPORT Peter Zijlstra
2021-10-08 12:40   ` Mark Rutland
2021-10-08 13:45     ` Peter Zijlstra
2021-10-08 16:17       ` Josh Poimboeuf
2021-10-14 18:07         ` Mark Rutland
2021-10-14 18:41           ` Josh Poimboeuf
2021-10-14 18:03       ` Mark Rutland [this message]
2021-10-14 18:48         ` Josh Poimboeuf
2021-10-14 11:07   ` Russell King (Oracle)
2021-10-08 11:15 ` [PATCH 7/7] arch: Fix STACKTRACE_SUPPORT Peter Zijlstra
2021-10-08 12:52   ` Mark Rutland
2021-10-09  9:36   ` Guo Ren
2021-10-14 12:02 ` [PATCH 0/7] wchan: Fix wchan support Russell King (Oracle)
2021-10-14 13:38   ` Russell King (Oracle)
2021-10-14 19:51     ` Josh Poimboeuf

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