linux-kernel - Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI

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Date:	Fri, 09 Oct 2015 15:37:29 +0800
From:	"Hillf Danton" <hillf.zj@...baba-inc.com>
To:	"'Nikolay Borisov'" <kernel@...p.com>
Cc:	"'linux-kernel'" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI

> >> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio)
> >>  			if (bio->bi_error)
> >>  				buffer_io_error(bh);
> >>  		} while ((bh = bh->b_this_page) != head);
> >> -		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> >>  		local_irq_restore(flags);
> >
> > What if it takes 100ms to unlock after IRQ restored?
> 
> I'm not sure I understand in what direction you are going? Care to
> elaborate?
> 
Your change introduces extra time cost the lock waiter has to pay in
the case that irq happens before the lock is released.

> >> +		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> >>  		if (!under_io) {
> >>  #ifdef CONFIG_EXT4_FS_ENCRYPTION
> >>  			if (ctx)
> >> --
> >> 2.5.0
> >

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