linux-kernel - Re: Mysterious CFQ crash and RCU

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Message-ID: <1307270324.25014.14.camel@t41.thuisdomein>
Date:	Sun, 05 Jun 2011 12:38:43 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Mysterious CFQ crash and RCU

On Sun, 2011-06-05 at 10:39 +0200, Paul Bolle wrote:
> 0) I'd guess not, as the last thing I tried before simply ripping
> io_context.ioc_data out, was:
> 
>         spin_lock_irqsave(&ioc->lock, flags);
>         rcu_read_lock();
>         ioc_data = rcu_dereference(ioc->ioc_data);
>         rcu_read_unlock();
>         if (ioc_data == cic)
>                 rcu_assign_pointer(ioc->ioc_data, NULL);
>         spin_unlock_irqrestore(&ioc->lock, flags);

0) Strike that. It does seem to fix it. I must have done something silly
while doing yet another test of this stuff (perhaps by forgetting to
remove an old, crashing version of the cfq_iosched module before
installing a new version and switching back to cfq).

1) Jens, I suggest I'll send in a small series of patches, along these
lines:
- cleanup: make two functions static in block/cfq-iosched.c
- cleanup: rename io_context.cic_list -> io_context.cic_hlist
           rename io_context.ioc_data -> io_context.last_cic
           change io_context.last_cic from void* to cfq_io_context* 
           rename cfq_io_context.cic_list -> cfq_io_context.cic_node
- cleanup: rename *ret -> *ioc in two functions
- locking: make sure io_context.last_cic is accessed under
           io_context.lock and using the required RCU voodoo

2) Any objections beforehand?


Paul Bolle

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