linux-kernel - RCU: non-atomic assignment to long/pointer variables in gcc

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Date:	Tue, 15 Jan 2013 14:30:32 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: RCU: non-atomic assignment to long/pointer variables in gcc

Documentation/atomic_ops.txt (182dd4b277177e8465ad11cd9f85f282946b5578)
says that pointers, longs, ints, and chars are stored and loaded atomically.

But GCC actually may split assignment to 'long' variable into two instructions.
see example in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981

GCC also splits assignments to 'volatile' variables and this is actually a bug in gcc.

volatile unsigned long y;

y = 0x100000001ul;

   400728:	c7 05 66 06 20 00 01 	movl   $0x1,0x200666(%rip)        # 600d98 <y>
   40072f:	00 00 00
   400732:	c7 05 60 06 20 00 01 	movl   $0x1,0x200660(%rip)        # 600d9c <y+0x4>
   400739:	00 00 00

fortunately for y = 0; it generates this:

   40071d:	48 c7 05 70 06 20 00 	movq   $0x0,0x200670(%rip)        # 600d98 <y>
   400724:	00 00 00 00

Thus NULL is safe, but constant ERR_PTR may be dangerous.

Probably rcu_assign_pointer() should use ACCESS_ONCE() around lvalue, because
splitting assignment for non-volatile variable seems like completely valid,
but this may help only after fixing that bug in GCC.
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