linux-kernel - Re: [PATCH] semaphore: use raw_spin_lock_irq instead of raw_spin_lock

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Message-ID: <20180621140259.GA8492@gmail.com>
Date:   Thu, 21 Jun 2018 16:02:59 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] semaphore: use raw_spin_lock_irq instead of
 raw_spin_lock_irqsave


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Mikulas Patocka <mpatocka@...hat.com> wrote:
> 
> > The sleeping functions down, down_interruptible, down_killable and
> > down_timeout can't be called with interrupts disabled, so we don't have to
> > save and restore interrupt flag.
> > 
> > This patch avoids the costly pushf and popf instructions on the semaphore
> > path.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> > 
> > ---
> >  kernel/locking/semaphore.c |   21 ++++++++-------------
> >  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> I've applied this to the locking tree, I suspect we can do this on the condition 
> that it doesn't explode in early boot code (which has irqs disabled) and doesn't 
> generate early boot lockdep splats either.

Hm, this blew up pretty quick on a pretty regular x86-64 PC white-box, during 
early bootup:

  PANIC: early exception 0x08 IP 246:10 error ffffffff811537b2 cr2 0xffff88000240cff8

and I think it's due to your patch - verifying that now.

Thanks,

	Ingo