linux-kernel - Re: dcache shrink list corruption?

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Message-ID: <20140501143434.GA18016@ZenIV.linux.org.uk>
Date:	Thu, 1 May 2014 15:34:35 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: dcache shrink list corruption?

On Thu, May 01, 2014 at 11:42:52AM +0200, Miklos Szeredi wrote:
> Two points about latest version (dentry_kill-2):
> 
> - Doing anything with dentry->d_parent in case of DCACHE_DENTRY_KILLED looks
>   seriously wrong.  Parent has been dealt with, at that point, by the other
>   caller, no?

In both branches, actually - we should bugger off earlier *and* return NULL
in that case.  Nice catch.

Hmm... I see why it failed to blow up on that.  It *did* trigger, all right -
udev is stepping into that right on boot.  The thing is, check should be
	if ((int)dentry->d_lockref.count > 0)
since the damn thing is unsigned int.  IOW, they did go through handover
and shrink_dentry_list() proceeded to lose them.  And with that braino
fixed, it steps into the extra dput crap just fine.

OK, fixed and pushed (both branches).

> - "bool foo = flag & FLAG" looks suspicious.  Is this guaranteed not to
>   overflow?

What do you mean, overflow?  It's not a 1-bit unsigned int; conversion to
_Bool is different (which is the only reason why it's more than mere
syntax sugar).  See C99 6.3.2.1 ("When any scalar value is converted
to _Bool, the result is 0 if the value compares equal to 0; otherwise,
the result is 1").

That, BTW, is also the reason why _Bool bitfields exist - try
struct {
	_Bool a:1;
	unsigned b:1;
} x;
x.a = 2;
x.b = 2;
if (x.a)
	printf("A");
if (x.b)
	printf("B");
and see what it does.  The first test triggers, same as if (2) would.
The second does not, since conversion to unsigned integer type other
than _Bool gives the value in range of that type comparable to original
modulo (maximal representable + 1).  And 2 modulo 2 is 0...
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