linux-kernel - Re: [PATCH] xen: Enable interrupts when calling _cond

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Message-ID: <87h7zmpr1k.fsf@nanos.tec.linutronix.de>
Date:   Wed, 19 Feb 2020 15:53:11 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jürgen Groß <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] xen: Enable interrupts when calling _cond_resched()

Jürgen Groß <jgross@...e.com> writes:

> On 19.02.20 12:01, Thomas Gleixner wrote:
>> xen_maybe_preempt_hcall() is called from the exception entry point
>> xen_do_hypervisor_callback with interrupts disabled.
>> 
>> _cond_resched() evades the might_sleep() check in cond_resched() which
>> would have caught that and schedule_debug() unfortunately lacks a check
>> for irqs_disabled().
>> 
>> Enable interrupts around the call and use cond_resched() to catch future
>> issues.
>> 
>> Fixes: fdfd811ddde3 ("x86/xen: allow privcmd hypercalls to be preempted")
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> ---
>>   drivers/xen/preempt.c |    4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> --- a/drivers/xen/preempt.c
>> +++ b/drivers/xen/preempt.c
>> @@ -33,8 +33,10 @@ asmlinkage __visible void xen_maybe_pree
>>   		 * cpu.
>>   		 */
>>   		__this_cpu_write(xen_in_preemptible_hcall, false);
>> -		_cond_resched();
>> +		local_irq_enable();
>> +		cond_resched();
>>   		__this_cpu_write(xen_in_preemptible_hcall, true);
>> +		local_irq_disable();
>
> Could you please put the call of local_irq_disable() directly after the
> cond_resched() call to make the result symmetric regarding writing of
> xen_in_preemptible_hcall and irq enable/disable?

Darn. Of course does this need to be there. Stupid me, let me resend.