linux-kernel - Re: block: ioc->refcount accessed twice in put_io

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Message-ID: <BANLkTi=ma7WDo9=kKwZGwuhi=qUEB8EW5g@mail.gmail.com>
Date:	Mon, 11 Apr 2011 09:54:11 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Paul Bolle <pebolle@...cali.nl>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: block: ioc->refcount accessed twice in put_io_context()?

2011/4/10 Paul Bolle <pebolle@...cali.nl>:
> 0) Looking for clues to solve a problem I ran into, I noticed something
> odd in block/blk-ioc.c:put_io_context(). It seems it accesses the atomic
> variable ioc->refcount twice in a way which suggests things might race.
>
> 1) Code is more exact than words, so this (entirely untested) patch to
> solve this possible race might describe better what this is all about:
>
> @@ -33,12 +33,16 @@ static void cfq_dtor(struct io_context *ioc)
>  */
>  int put_io_context(struct io_context *ioc)
>  {
> +       int new;
> +
>        if (ioc == NULL)
>                return 1;
>
> -       BUG_ON(atomic_long_read(&ioc->refcount) == 0);
> +       new = atomic_long_dec_return(&ioc->refcount);
> +
> +       BUG_ON(new < 0);
>
> -       if (atomic_long_dec_and_test(&ioc->refcount)) {
> +       if (new == 0) {
>                rcu_read_lock();
>                cfq_dtor(ioc);
>                rcu_read_unlock();
>
so you hit this line?
BUG_ON(atomic_long_read(&ioc->refcount) == 0);
this suggests something else is already wrong, you should fix that.
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