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From: Al Viro <viro@ZenIV.linux.org.uk>
To: hejianet <hejianet@gmail.com>
Cc: Feifei Xu <xufeifei@linux.vnet.ibm.com>,
	linux-fsdevel@vger.kernel.org, xuhilar@gmail.com,
	boqun.feng@gmail.com
Subject: Re: [Bug] fs/dcache.c: NULL pointer dereference on dentry_string_cmp
Date: Thu, 21 Jul 2016 06:57:07 +0100	[thread overview]
Message-ID: <20160721055707.GL2356@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20160721041857.GK2356@ZenIV.linux.org.uk>

On Thu, Jul 21, 2016 at 05:18:57AM +0100, Al Viro wrote:
> 	Hash insertion does smp_store_release().  Hash chain traversal -
> smp_read_barrier_depends().  On ppc the former is lwsync, while the latter
> is no-op, so it boils down to
> 	store dentry->d_name.name
> 	lwsync
> 	store mangled address of dentry into hash chain
> vs.
> 	fetch mangled address of dentry
> 	demangle it
> 	fetch dentry->d_name.name
> which should be enough - lwsync paired with address dependency gives the
> ordering.  IOW, it's not about the barriers in __d_alloc(), it's those in
> hlist_bl_add_head_rcu() and hlist_bl_for_each_entry_rcu().
> 
> 	And it couldn't be a missing barrier anyway - crash dump shows that
> sucker with NULL ->d_name.name.

FWIW, originally I thought it might be a missing barrier; Paul McKenney
had pointed to barriers in RCU lists primitives.  And crashdump is pretty
much conclusive - broken barriers or not, having a store in some thread
*not* seen by crashdump writer is hard to believe...

  reply	other threads:[~2016-07-21  5:57 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-14 13:25 [Bug] fs/dcache.c: NULL pointer dereference on dentry_string_cmp Feifei Xu
2016-07-20  5:59 ` Al Viro
2016-07-21  3:20   ` hejianet
2016-07-21  4:18     ` Al Viro
2016-07-21  5:57       ` Al Viro [this message]
2016-07-21  6:40       ` Feifei Xu
2016-07-21  7:42         ` Feifei Xu
2016-07-21  6:30   ` Feifei Xu

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