linux-kernel - Re: BUG: using __this_cpu_read() in preemptible code in trace_hardirqs

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Date:   Fri, 23 Oct 2020 10:18:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     syzbot <syzbot+53f8ce8bbc07924b6417@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: BUG: using __this_cpu_read() in preemptible code in
 trace_hardirqs_on

On Thu, Oct 22, 2020 at 04:32:47PM -0400, Steven Rostedt wrote:
> On Thu, 22 Oct 2020 12:30:28 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Subject: lockdep: Fix preemption WARN for spurious IRQ-enable
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Thu Oct 22 12:23:02 CEST 2020
> > 
> > It is valid (albeit uncommon) to call local_irq_enable() without first
> > having called local_irq_disable(). In this case we enter
> > lockdep_hardirqs_on*() with IRQs enabled and trip a preemption warning
> > for using __this_cpu_read().
> > 
> > Use this_cpu_read() instead to avoid the warning.
> 
> I was wondering why you were using __this_cpu_read() in the first place.

Well, because all other sites are actually with IRQs disabled :/ It's
just this spurious enable thing that's an exception.

> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Thanks!