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RE: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

Started by河合英宏 / KAWAI，HIDEHIRO <hidehiro.kawai.ez@hitachi.com>
First post2015-12-02 13:00 +0100
Last post2015-12-03 13:30 +0100
Articles 6 — 2 participants

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  RE: [V5 PATCH 3/4] kexec: Fix race between panic() and  crash_kexec() called directly 河合英宏 / KAWAI，HIDEHIRO   <hidehiro.kawai.ez@hitachi.com> - 2015-12-02 13:00 +0100
    Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()  called directly Borislav Petkov <bp@alien8.de> - 2015-12-02 16:50 +0100
      RE: [V5 PATCH 3/4] kexec: Fix race between panic() and  crash_kexec() called directly 河合英宏 / KAWAI，HIDEHIRO   <hidehiro.kawai.ez@hitachi.com> - 2015-12-03 03:10 +0100
        Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()  called directly Borislav Petkov <bp@alien8.de> - 2015-12-03 10:40 +0100
          RE: [V5 PATCH 3/4] kexec: Fix race between panic() and  crash_kexec() called directly 河合英宏 / KAWAI，HIDEHIRO   <hidehiro.kawai.ez@hitachi.com> - 2015-12-03 12:30 +0100
            Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()  called directly Borislav Petkov <bp@alien8.de> - 2015-12-03 13:30 +0100

#1281701 — RE: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

From河合英宏 / KAWAI，HIDEHIRO <hidehiro.kawai.ez@hitachi.com>
Date2015-12-02 13:00 +0100
SubjectRE: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Message-ID<qBeeK-6GK-11@gated-at.bofh.it>
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#1281927 — Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

FromBorislav Petkov <bp@alien8.de>
Date2015-12-02 16:50 +0100
SubjectRe: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Message-ID<qBhPk-FC-35@gated-at.bofh.it>
In reply to#1281701
On Wed, Dec 02, 2015 at 11:57:38AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> We can do so, but I think resetting panic_cpu always would be
> simpler and safer.

Well, I think executing code needlessly *especially* at panic time is
not all that rosy either.

Besides something like this:

	static bool kexec_failed;

	...

        if (crash_kexec_post_notifiers && !kexec_failed)
		kexec_failed = __crash_kexec(NULL);

is as simple as it gets.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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#1282646

From河合英宏 / KAWAI，HIDEHIRO <hidehiro.kawai.ez@hitachi.com>
Date2015-12-03 03:10 +0100
Message-ID<qBrvk-749-9@gated-at.bofh.it>
In reply to#1281927
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#1282849 — Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

FromBorislav Petkov <bp@alien8.de>
Date2015-12-03 10:40 +0100
SubjectRe: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Message-ID<qBywO-38I-23@gated-at.bofh.it>
In reply to#1282646
On Thu, Dec 03, 2015 at 02:01:38AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > On Wed, Dec 02, 2015 at 11:57:38AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > > We can do so, but I think resetting panic_cpu always would be
> > > simpler and safer.
> 
> I'll state in detail.
> 
> When we call crash_kexec() without entering panic() and return from
> it, panic() should be called eventually.

Huh, the call chain is

panic->crash_kexec

Or do you mean, when crash_kexec() is not called by panic() but by some
of its other callers?

> But the code paths are a bit complicated and there are many
> implementations for each architecture. So one day, this assumption may
> be broken; the CPU doesn't call panic(). Or the CPU may fail to call
> panic() because we are already in insane state. It would be nervous,
> but allowing another CPU to process panic routines by resetting
> panic_cpu is safer approach.

My suggestion was to do this only on the panic path - not necessarily on
the others.

> Since this code is executed only once due to panic_cpu,
> I think introducing this logic is not much valuable.
> Also, current implementation is already quite simple:
> 
> panic()
> {
> ...
> 	__crash_kexec(NULL) {
> 		if (mutex_trylock(&kexec_mutex)) {
> 			if (kexec_crash_image) {
> 				/* don't return */
> 			}

I don't mean the kexec_crash_image case - I mean the opposite one:
!kexec_crash_image. And I think I know now what you're trying to tell
me: the first CPU which hits panic, will finish panic eventually and so
it will take down the machine.

Every other CPU which happens to enter panic in between the first CPU
and the machine being taken down, doesn't matter because, well, who
cares, we're panicking already.

Am I close?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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#1282960

From河合英宏 / KAWAI，HIDEHIRO <hidehiro.kawai.ez@hitachi.com>
Date2015-12-03 12:30 +0100
Message-ID<qBAfg-4hh-21@gated-at.bofh.it>
In reply to#1282849
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#1282994 — Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

FromBorislav Petkov <bp@alien8.de>
Date2015-12-03 13:30 +0100
SubjectRe: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Message-ID<qBBbl-4U8-27@gated-at.bofh.it>
In reply to#1282960
On Thu, Dec 03, 2015 at 11:29:21AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> I was arguing about the case of oops_end --> crash_kexec
> --> return from crash_kexec because of !kexec_crash_image -->
> panic.

Aha.

> In the case of panic --> __crash_kexec, __crash_kexec is called
> only once, so we don't need to check the return value of __crash_kexec
> as you suggested.  So I thought you stated about crash_kexec --> panic
> case.

No, I meant the other way around.

> I also mentioned !kexec_crash_image case...

I must've missed it.

> No.  The first CPU calls panic, and then it calls __crash_kexec.
> Because of !kexec_crash_image, it returns from __crash_kexec and
> continues to the panic procedure.  At the same time, another CPU
> tries to call panic(), but it doesn't run the panic procedure;
> panic_cpu prevents the second CPU from running it.
>
> This means __crash_kexec is called only once even if we don't
> check the return value of __crash_kexec.

I think we're on the same page, even if we express it differently - the
other CPUs entering panic() will loop in panic_smp_self_stop() so they
won't reach __crash_kexec().

> (Please note that crash_kexec can be called multiple times in the
> case of oops_end() --> crash_kexec().)

Right, and that was the case that was bugging me - calling into
crash_kexec() on multiple CPUs but it is a trylock and a pointer test -
I guess that's diminishingly small overhead to care.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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