Messages in this thread | | | Date | Tue, 15 Jan 2013 17:07:50 +0400 | From | Konstantin Khlebnikov <> | Subject | Re: RCU: non-atomic assignment to long/pointer variables in gcc |
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Paul E. McKenney wrote: > 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 > > ------------------------------------------------------------------------ > > 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)
Seems like RCU_INIT_POINTER() need this too.
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