bit fields && data tearing
Benjamin Herrenschmidt
benh at kernel.crashing.org
Sun Jul 13 09:34:42 EST 2014
On Sat, 2014-07-12 at 22:51 +0200, Oleg Nesterov wrote:
> OK, looks like this is compiler bug,
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080
>
> Thanks to Dan who informed me privately.
So yes, there's is this compiler bug which means a bitfield
access can cause a r-m-w access to a neighbouring field but
in general, I would be weary of bitfields anyway since accessing
them isn't going to be atomic anyway... it's too easy to get things
wrong and in most cases the benefit is yet to be demonstrated.
In your example, I don't see the point of the bitfield.
Cheers,
Ben.
> On 07/12, Oleg Nesterov wrote:
> >
> > Hello,
> >
> > I am not sure I should ask here, but since Documentation/memory-barriers.txt
> > mentions load/store tearing perhaps my question is not completely off-topic...
> >
> > I am fighting with mysterious RHEL bug, it can be reproduced on ppc and s390
> > but not on x86. Finally I seem to understand the problem, and I even wrote the
> > stupid kernel module to ensure, see it below at the end.
> >
> > It triggers the problem immediately, kt_2() sees the wrong value in freeze_stop.
> > (If I turn ->freeze_stop int "long", the problem goes away).
> >
> > So the question is: is this gcc bug or the code below is buggy?
> >
> > If it is buggy, then probably memory-barriers.txt could mention that you should
> > be carefull with bit fields, even ACCESS_ONCE() obviously can't help.
> >
> > Or this just discloses my ignorance and you need at least aligned(long) after a
> > bit field to be thread-safe ? I thought that compiler should take care and add
> > the necessary alignment if (say) CPU can't update a single byte/uint.
> >
> > gcc version 4.4.7 20120313 (Red Hat 4.4.7-9) (GCC). Asm:
> >
> > 0000000000000000 <.kt_2>:
> > 0: 7c 08 02 a6 mflr r0
> > 4: fb 81 ff e0 std r28,-32(r1)
> > 8: fb a1 ff e8 std r29,-24(r1)
> > c: fb c1 ff f0 std r30,-16(r1)
> > 10: fb e1 ff f8 std r31,-8(r1)
> > 14: eb c2 00 00 ld r30,0(r2)
> > 18: f8 01 00 10 std r0,16(r1)
> > 1c: f8 21 ff 71 stdu r1,-144(r1)
> > 20: 7c 7d 1b 78 mr r29,r3
> > 24: 3b e0 00 00 li r31,0
> > 28: 78 3c 04 64 rldicr r28,r1,0,49
> > 2c: 3b 9c 00 80 addi r28,r28,128
> > 30: 48 00 00 2c b 5c <.kt_2+0x5c>
> > 34: 60 00 00 00 nop
> > 38: 60 00 00 00 nop
> > 3c: 60 00 00 00 nop
> > 40: 93 fd 00 04 stw r31,4(r29)
> > 44: e8 9d 00 06 lwa r4,4(r29)
> > 48: 7f 84 f8 00 cmpw cr7,r4,r31
> > 4c: 40 de 00 4c bne- cr7,98 <.kt_2+0x98>
> > 50: e8 1c 00 00 ld r0,0(r28)
> > 54: 78 09 f7 e3 rldicl. r9,r0,62,63
> > 58: 40 c2 00 54 bne- ac <.kt_2+0xac>
> > 5c: 48 00 00 01 bl 5c <.kt_2+0x5c>
> > 60: 60 00 00 00 nop
> > 64: 3b ff 00 01 addi r31,r31,1
> > 68: 2f a3 00 00 cmpdi cr7,r3,0
> > 6c: 7f ff 07 b4 extsw r31,r31
> > 70: 41 9e ff d0 beq+ cr7,40 <.kt_2+0x40>
> > 74: 38 21 00 90 addi r1,r1,144
> > 78: 38 60 00 00 li r3,0
> > 7c: e8 01 00 10 ld r0,16(r1)
> > 80: eb 81 ff e0 ld r28,-32(r1)
> > 84: eb a1 ff e8 ld r29,-24(r1)
> > 88: eb c1 ff f0 ld r30,-16(r1)
> > 8c: eb e1 ff f8 ld r31,-8(r1)
> > 90: 7c 08 03 a6 mtlr r0
> > 94: 4e 80 00 20 blr
> > 98: e8 7e 80 28 ld r3,-32728(r30)
> > 9c: 7f e5 fb 78 mr r5,r31
> > a0: 48 00 00 01 bl a0 <.kt_2+0xa0>
> > a4: 60 00 00 00 nop
> > a8: 4b ff ff a8 b 50 <.kt_2+0x50>
> > ac: 48 00 00 01 bl ac <.kt_2+0xac>
> > b0: 60 00 00 00 nop
> > b4: 4b ff ff a8 b 5c <.kt_2+0x5c>
> > b8: 60 00 00 00 nop
> > bc: 60 00 00 00 nop
> >
> > 00000000000000c0 <.kt_1>:
> > c0: 7c 08 02 a6 mflr r0
> > c4: fb 81 ff e0 std r28,-32(r1)
> > c8: fb a1 ff e8 std r29,-24(r1)
> > cc: fb c1 ff f0 std r30,-16(r1)
> > d0: fb e1 ff f8 std r31,-8(r1)
> > d4: eb c2 00 00 ld r30,0(r2)
> > d8: f8 01 00 10 std r0,16(r1)
> > dc: f8 21 ff 71 stdu r1,-144(r1)
> > e0: 7c 7d 1b 78 mr r29,r3
> > e4: 3b e0 00 00 li r31,0
> > e8: 78 3c 04 64 rldicr r28,r1,0,49
> > ec: 3b 9c 00 80 addi r28,r28,128
> > f0: 48 00 00 38 b 128 <.kt_1+0x68>
> > f4: 60 00 00 00 nop
> > f8: 60 00 00 00 nop
> > fc: 60 00 00 00 nop
> > 100: e8 1d 00 00 ld r0,0(r29)
> > 104: 79 20 e8 0e rldimi r0,r9,61,0
> > 108: f8 1d 00 00 std r0,0(r29)
> > 10c: 80 1d 00 00 lwz r0,0(r29)
> > 110: 54 00 1f 7e rlwinm r0,r0,3,29,31
> > 114: 7f 80 f8 00 cmpw cr7,r0,r31
> > 118: 40 de 00 6c bne- cr7,184 <.kt_1+0xc4>
> > 11c: e8 1c 00 00 ld r0,0(r28)
> > 120: 78 09 f7 e3 rldicl. r9,r0,62,63
> > 124: 40 c2 00 70 bne- 194 <.kt_1+0xd4>
> > 128: 48 00 00 01 bl 128 <.kt_1+0x68>
> > 12c: 60 00 00 00 nop
> > 130: 3b ff 00 01 addi r31,r31,1
> > 134: 2f a3 00 00 cmpdi cr7,r3,0
> > 138: 7f ff 07 b4 extsw r31,r31
> > 13c: 2f 1f 00 07 cmpwi cr6,r31,7
> > 140: 7b e9 07 60 clrldi r9,r31,61
> > 144: 40 9e 00 1c bne- cr7,160 <.kt_1+0xa0>
> > 148: 40 9a ff b8 bne+ cr6,100 <.kt_1+0x40>
> > 14c: 39 20 00 00 li r9,0
> > 150: 3b e0 00 00 li r31,0
> > 154: 4b ff ff ac b 100 <.kt_1+0x40>
> > 158: 60 00 00 00 nop
> > 15c: 60 00 00 00 nop
> > 160: 38 21 00 90 addi r1,r1,144
> > 164: 38 60 00 00 li r3,0
> > 168: e8 01 00 10 ld r0,16(r1)
> > 16c: eb 81 ff e0 ld r28,-32(r1)
> > 170: eb a1 ff e8 ld r29,-24(r1)
> > 174: eb c1 ff f0 ld r30,-16(r1)
> > 178: eb e1 ff f8 ld r31,-8(r1)
> > 17c: 7c 08 03 a6 mtlr r0
> > 180: 4e 80 00 20 blr
> > 184: e8 7e 80 30 ld r3,-32720(r30)
> > 188: 48 00 00 01 bl 188 <.kt_1+0xc8>
> > 18c: 60 00 00 00 nop
> > 190: 4b ff ff 8c b 11c <.kt_1+0x5c>
> > 194: 48 00 00 01 bl 194 <.kt_1+0xd4>
> > 198: 60 00 00 00 nop
> > 19c: 4b ff ff 8c b 128 <.kt_1+0x68>
> >
> > Unfortunately it tells me nothing, I do not know ppc.
> >
> > Oleg.
> >
> > --------------------------------------------------------------------------------
> > #include <linux/module.h>
> > #include <linux/kernel.h>
> > #include <linux/kthread.h>
> >
> > struct utrace {
> > unsigned int resume:3;
> > int freeze_stop;
> > };
> >
> > static int kt_1(void *arg)
> > {
> > struct utrace *u = arg;
> > int r = 0;
> >
> > while (!kthread_should_stop()) {
> > if (++r == 7)
> > r = 0;
> >
> > u->resume = r;
> > barrier();
> > if (u->resume != r)
> > printk(KERN_CRIT "BUG! bitfield\n");
> >
> > if (need_resched())
> > schedule();
> > }
> >
> > return 0;
> > }
> >
> > static int kt_2(void *arg)
> > {
> > struct utrace *u = arg;
> > int f = 0;
> >
> > while (!kthread_should_stop()) {
> > u->freeze_stop = ++f;
> > barrier();
> > if (u->freeze_stop != f)
> > printk(KERN_CRIT "BUG! freeze_stop %d != %d\n", u->freeze_stop, f);
> >
> > if (need_resched())
> > schedule();
> > }
> >
> > return 0;
> > }
> >
> > static struct task_struct *t_1, *t_2;
> >
> > static struct utrace utrace;
> >
> > static int __init mod_init(void)
> > {
> > WARN_ON(IS_ERR(t_1 = kthread_run(kt_1, &utrace, "kt_1")));
> > WARN_ON(IS_ERR(t_2 = kthread_run(kt_2, &utrace, "kt_2")));
> >
> > return 0;
> > }
> >
> > static void __exit mod_exit(void)
> > {
> > kthread_stop(t_1);
> > kthread_stop(t_2);
> > }
> >
> > MODULE_LICENSE("GPL");
> > module_init(mod_init);
> > module_exit(mod_exit);
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