Messages in this thread Patch in this message | | | Date | Tue, 15 Jan 2013 04:32:50 -0800 | From | "Paul E. McKenney" <> | Subject | Re: RCU: non-atomic assignment to long/pointer variables in gcc |
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On Tue, Jan 15, 2013 at 02:30:32PM +0400, Konstantin Khlebnikov wrote: > 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.
Good catch! I has queued the following patch.
Thanx, Paul
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rcu: Add ACCESS_ONCE() to rcu_assign_pointer()
GCC may split assignment to 'long' variable into two instructions:
volatile unsigned long y;
y = 0x100000001ul;
movl $0x1,0x200666(%rip) movl $0x1,0x200660(%rip)
This commit fixes this by applying ACCESS_ONCE() within __rcu_assign_pointer(), but note that some versions and architectures of GCC have a bug that defeats ACCESS_ONCE():
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981
I added a comment to this bug report asking that the bug be fixed for volatiles as well as atomics, citing a device driver storing a constant into a 64-bit device register as motivation.
Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 9ed2c9a..3435174 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -556,7 +556,7 @@ static inline void rcu_preempt_sleep_check(void) #define __rcu_assign_pointer(p, v, space) \ do { \ smp_wmb(); \ - (p) = (typeof(*v) __force space *)(v); \ + ACCESS_ONCE(p) = (typeof(*v) __force space *)(v); \ } while (0)
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