Re: bit fields && data tearing
From: Peter Hurley
Date: Sun Jul 13 2014 - 09:25:22 EST
On 07/12/2014 07:34 PM, Benjamin Herrenschmidt wrote:
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.
I'm not sure I understand your point here, Ben.
Suppose that two different spinlocks are used independently to
protect r-m-w access to adjacent data. In Oleg's example,
suppose spinlock 1 is used for access to the bitfield and
spinlock 2 is used for access to freeze_stop.
What would prevent an accidental write to freeze_stop from the
kt_1 thread?
Regards,
Peter Hurley
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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