On 21.05.2013 17:44, Eric Dumazet wrote:
On Tue, 2013-05-21 at 05:09 -0700, Paul E. McKenney wrote:
-#define hlist_nulls_first_rcu(head) \
- (*((struct hlist_nulls_node __rcu __force **)&(head)->first))
+#define hlist_nulls_first_rcu(head) \
+ (*((struct hlist_nulls_node __rcu __force **) \
+ &((volatile typeof(*head) *)head)->first))
Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here?
More exactly we have :
#define list_entry_rcu(ptr, type, member) \
({typeof (*ptr) __rcu *__ptr = (typeof (*ptr) __rcu __force *)ptr; \
container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
})
#define list_for_each_entry_rcu(pos, head, member) \
for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
&pos->member != (head); \
pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
<< and >>
#define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)
\
for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));
\
(!is_a_nulls(pos)) &&
\
({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1;
}); \
pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))
We need to change hlist_nulls_for_each_entry_rcu() to use same construct,
so that the rcu_dereference_raw() is performed at the right place.
No.
This code has the same mistake: it is rcu_dereference_raw(head->first),
so there is nothing that prevents gcc to store the (head->first) value
in a register.
Let's see on assembler (3.4.46, net/ipv4/udp.c, udp4_lib_lookup2()):
(0): here we get head address from stack
(1): here we get head->first
(2): this is a place, where we jump (by mistake) in original version
from (3), (4) and (5). But in (4) we make same as in (1), so it's not a
problem. In patched version we always jump to (1).
1) original version:
0000000000002840 <udp4_lib_lookup2.isra.35>:
2840: 41 57 push %r15
2842: 41 89 d7 mov %edx,%r15d
2845: 41 56 push %r14
2847: 41 55 push %r13
2849: 41 54 push %r12
284b: 55 push %rbp
284c: 48 89 fd mov %rdi,%rbp
284f: 53 push %rbx
2850: 48 83 ec 28 sub $0x28,%rsp
(0) 2854: 48 8b 44 24 60 mov 0x60(%rsp),%rax
2859: 44 8b 64 24 68 mov 0x68(%rsp),%r12d
(1) 285e: 4c 8b 28 mov (%rax),%r13
(2) 2861: 31 ff xor %edi,%edi
2863: 41 f6 c5 01 test $0x1,%r13b
2867: 4d 89 ea mov %r13,%r10
286a: bb ff ff ff ff mov $0xffffffff,%ebx
286f: 74 32 je 28a3
<udp4_lib_lookup2.isra.35+0x63>
2871: e9 20 02 00 00 jmpq 2a96
<udp4_lib_lookup2.isra.35+0x256>
...
2940: 49 d1 ea shr %r10
2943: 4d 39 e2 cmp %r12,%r10
(3) 2946: 0f 85 15 ff ff ff jne 2861
<udp4_lib_lookup2.isra.35+0x21>
294c: 48 85 ff test %rdi,%rdi
294f: 74 27 je 2978
<udp4_lib_lookup2.isra.35+0x138>
2951: 41 89 db mov %ebx,%r11d
2954: 48 8d 5f 4c lea 0x4c(%rdi),%rbx
2958: 41 ba 02 00 00 00 mov $0x2,%r10d
295e: 66 90 xchg %ax,%ax
2960: 45 8d 6a 01 lea 0x1(%r10),%r13d
2964: 44 89 d0 mov %r10d,%eax
2967: f0 44 0f b1 2b lock cmpxchg %r13d,(%rbx)
296c: 41 39 c2 cmp %eax,%r10d
296f: 74 36 je 29a7
<udp4_lib_lookup2.isra.35+0x167>
2971: 85 c0 test %eax,%eax
2973: 41 89 c2 mov %eax,%r10d
2976: 75 e8 jne 2960
<udp4_lib_lookup2.isra.35+0x120>
2978: 31 ff xor %edi,%edi
297a: 48 83 c4 28 add $0x28,%rsp
297e: 48 89 f8 mov %rdi,%rax
2981: 5b pop %rbx
2982: 5d pop %rbp
2983: 41 5c pop %r12
2985: 41 5d pop %r13
2987: 41 5e pop %r14
2989: 41 5f pop %r15
298b: c3 retq
298c: 0f 1f 40 00 nopl 0x0(%rax)
2990: 4d 8b b2 58 02 00 00 mov 0x258(%r10),%r14
2997: 41 f6 46 6e 10 testb $0x10,0x6e(%r14)
299c: 0f 85 de fe ff ff jne 2880
<udp4_lib_lookup2.isra.35+0x40>
29a2: e9 17 ff ff ff jmpq 28be
<udp4_lib_lookup2.isra.35+0x7e>
29a7: 48 3b 6f 30 cmp 0x30(%rdi),%rbp
29ab: 74 43 je 29f0
<udp4_lib_lookup2.isra.35+0x1b0>
29ad: b8 ff ff ff ff mov $0xffffffff,%eax
29b2: 41 39 c3 cmp %eax,%r11d
29b5: 7e c3 jle 297a
<udp4_lib_lookup2.isra.35+0x13a>
29b7: 89 4c 24 10 mov %ecx,0x10(%rsp)
29bb: 89 74 24 18 mov %esi,0x18(%rsp)
29bf: 44 89 44 24 08 mov %r8d,0x8(%rsp)
29c4: 44 89 0c 24 mov %r9d,(%rsp)
29c8: e8 c3 dc ff ff callq 690 <sock_put>
29cd: 48 8b 44 24 60 mov 0x60(%rsp),%rax
29d2: 8b 4c 24 10 mov 0x10(%rsp),%ecx
29d6: 8b 74 24 18 mov 0x18(%rsp),%esi
29da: 44 8b 44 24 08 mov 0x8(%rsp),%r8d
29df: 44 8b 0c 24 mov (%rsp),%r9d
29e3: 4c 8b 28 mov (%rax),%r13
(4) 29e6: e9 76 fe ff ff jmpq 2861
<udp4_lib_lookup2.isra.35+0x21>
29eb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
29f0: 44 0f b7 57 0c movzwl 0xc(%rdi),%r10d
29f5: 66 41 83 fa 0a cmp $0xa,%r10w
29fa: 74 7f je 2a7b
<udp4_lib_lookup2.isra.35+0x23b>
29fc: 3b 4f 04 cmp 0x4(%rdi),%ecx
29ff: b8 ff ff ff ff mov $0xffffffff,%eax
2a04: 75 ac jne 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a06: 0f b7 9f 7a 02 00 00 movzwl 0x27a(%rdi),%ebx
2a0d: 41 39 d8 cmp %ebx,%r8d
2a10: 75 a0 jne 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a12: 8b 1f mov (%rdi),%ebx
2a14: 66 41 83 fa 02 cmp $0x2,%r10w
2a19: 41 0f 94 c2 sete %r10b
2a1d: 45 0f b6 d2 movzbl %r10b,%r10d
2a21: 85 db test %ebx,%ebx
2a23: 44 89 d0 mov %r10d,%eax
2a26: 74 0d je 2a35
<udp4_lib_lookup2.isra.35+0x1f5>
2a28: 39 de cmp %ebx,%esi
2a2a: b8 ff ff ff ff mov $0xffffffff,%eax
2a2f: 75 81 jne 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a31: 41 8d 42 02 lea 0x2(%r10),%eax
2a35: 44 0f b7 97 78 02 00 movzwl 0x278(%rdi),%r10d
2a3c: 00
2a3d: 66 45 85 d2 test %r10w,%r10w
2a41: 74 0d je 2a50
<udp4_lib_lookup2.isra.35+0x210>
2a43: 66 45 39 d7 cmp %r10w,%r15w
2a47: 0f 85 60 ff ff ff jne 29ad
<udp4_lib_lookup2.isra.35+0x16d>
2a4d: 83 c0 02 add $0x2,%eax
2a50: 44 8b 57 10 mov 0x10(%rdi),%r10d
2a54: 45 85 d2 test %r10d,%r10d
2a57: 0f 84 55 ff ff ff je 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a5d: 8d 58 02 lea 0x2(%rax),%ebx
2a60: 45 39 d1 cmp %r10d,%r9d
2a63: b8 ff ff ff ff mov $0xffffffff,%eax
2a68: 0f 44 c3 cmove %ebx,%eax
2a6b: e9 42 ff ff ff jmpq 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a70: 41 bb ff ff ff ff mov $0xffffffff,%r11d
2a76: e9 05 fe ff ff jmpq 2880
<udp4_lib_lookup2.isra.35+0x40>
2a7b: 48 8b 9f 70 02 00 00 mov 0x270(%rdi),%rbx
2a82: b8 ff ff ff ff mov $0xffffffff,%eax
2a87: f6 43 6e 10 testb $0x10,0x6e(%rbx)
2a8b: 0f 85 21 ff ff ff jne 29b2
<udp4_lib_lookup2.isra.35+0x172>
2a91: e9 66 ff ff ff jmpq 29fc
<udp4_lib_lookup2.isra.35+0x1bc>
2a96: 4c 89 e8 mov %r13,%rax
2a99: 48 d1 e8 shr %rax
2a9c: 4c 39 e0 cmp %r12,%rax
(5) 2a9f: 0f 85 bc fd ff ff jne 2861
<udp4_lib_lookup2.isra.35+0x21>
2aa5: e9 ce fe ff ff jmpq 2978
<udp4_lib_lookup2.isra.35+0x138>
2aaa: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
2) patched version:
0000000000002840 <udp4_lib_lookup2>:
2840: 41 57 push %r15
2842: 41 89 d7 mov %edx,%r15d
2845: 41 56 push %r14
2847: 41 55 push %r13
2849: 41 54 push %r12
284b: 55 push %rbp
284c: 48 89 fd mov %rdi,%rbp
284f: 53 push %rbx
2850: 48 83 ec 28 sub $0x28,%rsp
2854: 44 8b 6c 24 68 mov 0x68(%rsp),%r13d
(0) 2859: 4c 8b 64 24 60 mov 0x60(%rsp),%r12
(1) 285e: 4d 8b 14 24 mov (%r12),%r10
(2) 2862: 31 ff xor %edi,%edi
2864: bb ff ff ff ff mov $0xffffffff,%ebx
2869: 41 f6 c2 01 test $0x1,%r10b
286d: 74 2c je 289b <udp4_lib_lookup2+0x5b>
286f: e9 0a 02 00 00 jmpq 2a7e <udp4_lib_lookup2+0x23e>
...
2930: 49 d1 ea shr %r10
2933: 4d 39 d5 cmp %r10,%r13
(3) 2936: 0f 85 22 ff ff ff jne 285e <udp4_lib_lookup2+0x1e>
293c: 48 85 ff test %rdi,%rdi
293f: 74 27 je 2968 <udp4_lib_lookup2+0x128>
2941: 41 89 db mov %ebx,%r11d
2944: 48 8d 5f 4c lea 0x4c(%rdi),%rbx
2948: 41 ba 02 00 00 00 mov $0x2,%r10d
294e: 66 90 xchg %ax,%ax
2950: 45 8d 72 01 lea 0x1(%r10),%r14d
2954: 44 89 d0 mov %r10d,%eax
2957: f0 44 0f b1 33 lock cmpxchg %r14d,(%rbx)
295c: 41 39 c2 cmp %eax,%r10d
295f: 74 36 je 2997 <udp4_lib_lookup2+0x157>
2961: 85 c0 test %eax,%eax
2963: 41 89 c2 mov %eax,%r10d
2966: 75 e8 jne 2950 <udp4_lib_lookup2+0x110>
2968: 31 ff xor %edi,%edi
296a: 48 83 c4 28 add $0x28,%rsp
296e: 48 89 f8 mov %rdi,%rax
2971: 5b pop %rbx
2972: 5d pop %rbp
2973: 41 5c pop %r12
2975: 41 5d pop %r13
2977: 41 5e pop %r14
2979: 41 5f pop %r15
297b: c3 retq
297c: 0f 1f 40 00 nopl 0x0(%rax)
2980: 4d 8b b2 58 02 00 00 mov 0x258(%r10),%r14
2987: 41 f6 46 6e 10 testb $0x10,0x6e(%r14)
298c: 0f 85 e6 fe ff ff jne 2878 <udp4_lib_lookup2+0x38>
2992: e9 1f ff ff ff jmpq 28b6 <udp4_lib_lookup2+0x76>
2997: 48 3b 6f 30 cmp 0x30(%rdi),%rbp
299b: 74 3b je 29d8 <udp4_lib_lookup2+0x198>
299d: b8 ff ff ff ff mov $0xffffffff,%eax
29a2: 41 39 c3 cmp %eax,%r11d
29a5: 7e c3 jle 296a <udp4_lib_lookup2+0x12a>
29a7: 89 4c 24 10 mov %ecx,0x10(%rsp)
29ab: 89 74 24 18 mov %esi,0x18(%rsp)
29af: 44 89 44 24 08 mov %r8d,0x8(%rsp)
29b4: 44 89 0c 24 mov %r9d,(%rsp)
29b8: e8 d3 dc ff ff callq 690 <sock_put>
29bd: 8b 4c 24 10 mov 0x10(%rsp),%ecx
29c1: 8b 74 24 18 mov 0x18(%rsp),%esi
29c5: 44 8b 44 24 08 mov 0x8(%rsp),%r8d
29ca: 44 8b 0c 24 mov (%rsp),%r9d
(4) 29ce: e9 8b fe ff ff jmpq 285e <udp4_lib_lookup2+0x1e>
29d3: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
29d8: 44 0f b7 57 0c movzwl 0xc(%rdi),%r10d
29dd: 66 41 83 fa 0a cmp $0xa,%r10w
29e2: 74 7f je 2a63 <udp4_lib_lookup2+0x223>
29e4: 3b 4f 04 cmp 0x4(%rdi),%ecx
29e7: b8 ff ff ff ff mov $0xffffffff,%eax
29ec: 75 b4 jne 29a2 <udp4_lib_lookup2+0x162>
29ee: 0f b7 9f 7a 02 00 00 movzwl 0x27a(%rdi),%ebx
29f5: 41 39 d8 cmp %ebx,%r8d
29f8: 75 a8 jne 29a2 <udp4_lib_lookup2+0x162>
29fa: 8b 1f mov (%rdi),%ebx
29fc: 66 41 83 fa 02 cmp $0x2,%r10w
2a01: 41 0f 94 c2 sete %r10b
2a05: 45 0f b6 d2 movzbl %r10b,%r10d
2a09: 85 db test %ebx,%ebx
2a0b: 44 89 d0 mov %r10d,%eax
2a0e: 74 0d je 2a1d <udp4_lib_lookup2+0x1dd>
2a10: 39 de cmp %ebx,%esi
2a12: b8 ff ff ff ff mov $0xffffffff,%eax
2a17: 75 89 jne 29a2 <udp4_lib_lookup2+0x162>
2a19: 41 8d 42 02 lea 0x2(%r10),%eax
2a1d: 44 0f b7 97 78 02 00 movzwl 0x278(%rdi),%r10d
2a24: 00
2a25: 66 45 85 d2 test %r10w,%r10w
2a29: 74 0d je 2a38 <udp4_lib_lookup2+0x1f8>
2a2b: 66 45 39 d7 cmp %r10w,%r15w
2a2f: 0f 85 68 ff ff ff jne 299d <udp4_lib_lookup2+0x15d>
2a35: 83 c0 02 add $0x2,%eax
2a38: 44 8b 57 10 mov 0x10(%rdi),%r10d
2a3c: 45 85 d2 test %r10d,%r10d
2a3f: 0f 84 5d ff ff ff je 29a2 <udp4_lib_lookup2+0x162>
2a45: 8d 58 02 lea 0x2(%rax),%ebx
2a48: 45 39 d1 cmp %r10d,%r9d
2a4b: b8 ff ff ff ff mov $0xffffffff,%eax
2a50: 0f 44 c3 cmove %ebx,%eax
2a53: e9 4a ff ff ff jmpq 29a2 <udp4_lib_lookup2+0x162>
2a58: 41 bb ff ff ff ff mov $0xffffffff,%r11d
2a5e: e9 15 fe ff ff jmpq 2878 <udp4_lib_lookup2+0x38>
2a63: 48 8b 9f 70 02 00 00 mov 0x270(%rdi),%rbx
2a6a: b8 ff ff ff ff mov $0xffffffff,%eax
2a6f: f6 43 6e 10 testb $0x10,0x6e(%rbx)
2a73: 0f 85 29 ff ff ff jne 29a2 <udp4_lib_lookup2+0x162>
2a79: e9 66 ff ff ff jmpq 29e4 <udp4_lib_lookup2+0x1a4>
2a7e: 49 d1 ea shr %r10
2a81: 4d 39 d5 cmp %r10,%r13
(5) 2a84: 0f 85 d4 fd ff ff jne 285e <udp4_lib_lookup2+0x1e>
2a8a: e9 d9 fe ff ff jmpq 2968 <udp4_lib_lookup2+0x128>
2a8f: 90 nop
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