linux-kernel - [PATCH 0/3] rculist: Fix list_entry_rcu to read ptr with, rcu_dereference

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Date:	Tue, 24 Mar 2015 11:31:33 +0100
From:	Patrick Marlier <patrick.marlier@...il.com>
To:	linux-kernel@...r.kernel.org
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: [PATCH 0/3] rculist: Fix list_entry_rcu to read ptr with, rcu_dereference_raw

Hi Paul and Josh,

While running some benchmarks with rcu, I noticed that rculist had a 
performance issue compared to the non-rcu list.
Having a closer look at the rlulist, I found that there is a bad usage 
of rcu_dereference_raw in list_entry_rcu().
Indeed, "typeof (*ptr) *__ptr = ptr;" reads ptr and 
"rcu_dereference_raw(__ptr)" forces read on the stack variable __ptr.
However, ptr should be read with rcu_dereference_raw not the __ptr.

This was introduced some years ago with commit 67bdbffd (2010-02-25).
I am proposing to revert this part of the patch but probably this can be 
fixed differently.

Fixing this showed 2 problems in the usage of list_entry_rcu.
Since it touches other parts of kernel, I guess it requires ack from 
other maintainers.

Thanks.

PS: I will Cc the others maintainers when somebody confirms the problem.

Signed-off-by: Patrick Marlier <patrick.marlier@...il.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Arnd Bergmann <arnd@...db.de>
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