From: Joerg Vehlow <lkml@jv-coder.de>
To: peterz@infradead.org
Cc: Steven Rostedt <rostedt@goodmis.org>,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Huang Ying <ying.huang@intel.com>,
linux-kernel@vger.kernel.org,
Joerg Vehlow <joerg.vehlow@aox-tech.de>
Subject: Re: [BUG RT] dump-capture kernel not executed for panic in interrupt context
Date: Tue, 8 Sep 2020 07:48:22 +0200 [thread overview]
Message-ID: <5600c9f8-2c9d-7776-161a-5f5c1be62c10@jv-coder.de> (raw)
In-Reply-To: <20200907162338.GN1362448@hirez.programming.kicks-ass.net>
Hi Peter
On 9/7/2020 6:23 PM, peterz@infradead.org wrote:
>> According to the original comment in __crash_kexec, the mutex was used to
>> prevent a sys_kexec_load, while crash_kexec is executed. Your proposed patch
>> does not lock the mutex in crash_kexec.
> Sure, but any mutex taker will (spin) wait for panic_cpu==CPU_INVALID.
> And if the mutex is already held, we'll not run __crash_kexec() just
> like the trylock() would do today.
Yes you are right, it should work.
>> This does not cover the original use
>> case anymore. The only thing that is protected now are two panicing cores at
>> the same time.
> I'm not following. AFAICT it does exactly what the old code did.
> Although maybe I didn't replace all kexec_mutex users, I now see that
> thing isn't static.
Same thing here.
>
>> Actually, this implementation feels even more hacky to me....
> It's more minimal ;-) It's simpler in that it only provides the required
> semantics (as I understand them) and does not attempt to implement a
> more general trylock() like primitive that isn't needed.
Here I cannot agree with you. There is a second trylock in kernel_kexec,
that cannot
be protected using the panic_cpu, but it actually could still use
mutex_trylock and check
the panic_cpu. This should work I guess:
int kexec_trylock(void) {
if (!mutex_trylock(&kexec_mutex)) {
return 0;
}
smp_mb();
if (panic_cpu != PANIC_CPU_INVALID) {
mutex_unlock(&kexec_mutex);
return 0;
}
return 1;
}
Or do I miss something now? All functions protected by mutex_lock cannot
be executed, after
kexec_trylock resturned 1. kexec_crash will execute up to
mutex_is_locked and then roll back.
The only thing that can go wrong now is: kexec_trylock executes up to
smb_mb. At the same time
kexec_crash executes mutex_is_locked, which returns false now and then
before panic_cpu is reset,
kexec_trylock executes the panic_cpu check, and returns. Now both
functions did not get the lock and
nothing is executed.
Does that sound right to you? If you have no further objections I will
post it here
Jörg
next prev parent reply other threads:[~2020-09-08 5:48 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-28 11:41 [BUG RT] dump-capture kernel not executed for panic in interrupt context Joerg Vehlow
2020-05-28 12:46 ` Steven Rostedt
2020-07-22 4:30 ` Joerg Vehlow
2020-07-22 20:51 ` Steven Rostedt
2020-07-27 23:36 ` Andrew Morton
2020-08-21 10:25 ` Joerg Vehlow
2020-08-21 15:08 ` Steven Rostedt
2020-08-21 20:47 ` Andrew Morton
2020-08-21 21:03 ` Steven Rostedt
2020-08-22 12:32 ` peterz
2020-08-22 23:49 ` Steven Rostedt
2020-09-07 11:41 ` peterz
2020-09-07 12:49 ` Valentin Schneider
2020-09-14 19:00 ` Steven Rostedt
2020-09-07 10:51 ` Joerg Vehlow
2020-09-07 11:46 ` peterz
2020-09-07 12:03 ` Joerg Vehlow
2020-09-07 16:23 ` peterz
2020-09-08 5:48 ` Joerg Vehlow [this message]
2020-09-09 5:46 ` Joerg Vehlow
2020-09-11 22:48 ` Eric W. Biederman
2020-09-14 6:03 ` Joerg Vehlow
2020-09-14 16:46 ` Eric W. Biederman
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