linux-kernel - RE: Question about the barrier() in hlist_nulls_for_each_entry

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Message-ID: <9905e2f8e28246929be7b77b78c07fb4@AcuMS.aculab.com>
Date:   Thu, 3 Aug 2023 14:39:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'paulmck@...nel.org'" <paulmck@...nel.org>,
        Alan Huang <mmpgouride@...il.com>
CC:     Joel Fernandes <joel@...lfernandes.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>
Subject: RE: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

From: Paul E. McKenney
> Sent: 03 August 2023 14:54
....
> > > If both are READ_ONCE(), you should not need the barrier().  Unless there
> > > is some other code not shown in your example that requires it, that is.
> >
> > And unless the compiler has a bug. :)
> >
> > So, the barrier() in hlist_nulls_for_each_entry_rcu() is a workaround for a compiler bug.
> 
> Fair enough!!!  ;-)

Except that it is likely that the compiler bug is avoided by the
implementation of READ_ONCE() rather than ACCESS_ONCE().

Also the code that looped forever (UDP receive socket lookup)
no longer has the retry - which is a different bug.
If a socket rehash hits the lookup then an erroneous ICMP
'port unreachable' is sent rather than doing a rescan.

	David
 

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