Messages in this thread Patch in this message | | | Date | Tue, 15 Jan 2013 08:17:59 -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 05:07:50PM +0400, Konstantin Khlebnikov wrote: > 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.
For the third use case, which is updating a pointer to reference data that has already been exposed to RCU readers, you are quite correct! I must confess that I had forgotten about that use case. Please see below for an updated patch.
And the gcc bug also now has a patch:
http://gcc.gnu.org/bugzilla/attachment.cgi?id=29169&action=diff
Thanx, Paul
------------------------------------------------------------------------
rcu: Add ACCESS_ONCE() to rcu_assign_pointer() and RCU_INIT_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. There is now a patch: http://gcc.gnu.org/bugzilla/attachment.cgi?id=29169&action=diff 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..4627abd 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) @@ -945,7 +945,7 @@ static inline notrace void rcu_read_unlock_sched_notrace(void) */ #define RCU_INIT_POINTER(p, v) \ do { \ - p = (typeof(*v) __force __rcu *)(v); \ + ACCESS_ONCE(p) = (typeof(*v) __force __rcu *)(v); \ } while (0) /**
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