On 10/04/2012 09:25 PM, Mans Rullgard wrote: > On 5 October 2012 02:56, Rob Herring <robherri...@gmail.com> wrote: >> On 10/04/2012 08:26 PM, Mans Rullgard wrote: >>> On 5 October 2012 01:58, Michael Hope <michael.h...@linaro.org> wrote: >>>> On 5 October 2012 12:10, Rob Herring <robherri...@gmail.com> wrote: >>>>> I've been scratching my head with a "scheduling while atomic" bug I >>>>> started seeing on 3.6. I can easily reproduce this problem when doing a >>>>> wget on my system. It ultimately seems to be a combination of factors. >>>>> The "scheduling while atomic" bug is triggered in do_alignment which >>>>> gets triggered by this code in net/ipv4/af_inet.c, line 1356: >>>>> >>>>> id = ntohl(*(__be32 *)&iph->id); >>>>> flush = (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (id ^ IP_DF)); >>>>> id >>= 16; >>>>> >>>>> This code compiles into this using "gcc version 4.6.3 (Ubuntu/Linaro >>>>> 4.6.3-1ubuntu5)": >>>>> >>>>> c02ac020: e8920840 ldm r2, {r6, fp} >>>>> c02ac024: e6bfbf3b rev fp, fp >>>>> c02ac028: e6bf6f36 rev r6, r6 >>>>> c02ac02c: e22bc901 eor ip, fp, #16384 ; 0x4000 >>>>> c02ac030: e0266008 eor r6, r6, r8 >>>>> c02ac034: e18c6006 orr r6, ip, r6 >>>>> >>>>> which generates alignment faults on the ldm. These are silent until this >>>>> commit is applied: >>>> >>>> Hi Rob. I assume that iph is something like: >>>> >>>> struct foo { >>>> u32 x; >>>> char id[8]; >>>> }; >>>> >>>> struct foo *iph; >>>> >>>> GCC merged the two adjacent loads of x and id into one ldm. This is >>>> an ARM specific optimisation done in load_multiple_sequence() and >>>> enabled with -fpeephole2. >>>> >>>> I think the assembly is correct - GCC knows that iph is aligned and >>>> knows the offsets of both x and id. Happy to be corrected if I'm >>>> wrong, but I think the assembly is valid given the C code. >>> >>> The struct looks like this: >>> >>> struct iphdr { >>> #if defined(__LITTLE_ENDIAN_BITFIELD) >>> __u8 ihl:4, >>> version:4; >>> #elif defined (__BIG_ENDIAN_BITFIELD) >>> __u8 version:4, >>> ihl:4; >>> #else >>> #error "Please fix <asm/byteorder.h>" >>> #endif >>> __u8 tos; >>> __be16 tot_len; >>> __be16 id; >>> __be16 frag_off; >>> __u8 ttl; >>> __u8 protocol; >>> __sum16 check; >>> __be32 saddr; >>> __be32 daddr; >>> /*The options start here. */ >>> }; >>> >>> In a normal build (there's some magic for special checkers) __be32 is a >>> plain >>> __u32 so the struct should be at least 4-byte aligned. If somehow it is >>> not, >>> that is the real bug. >> >> This struct is the IP header, so a struct ptr is just set to the >> beginning of the received data. Since ethernet headers are 14 bytes, >> often the IP header is not aligned unless the NIC can place the frame at >> a 2 byte offset (which is something I need to investigate). So this >> function cannot make any assumptions about the alignment. Does the ABI >> define structs have some minimum alignment? Does the struct need to be >> declared as packed or something? > > The ABI defines the alignment of structs as the maximum alignment of its > members. Since this struct contains 32-bit members, the alignment for the > whole struct becomes 32 bits as well. Declaring it as packed tells gcc it > might be unaligned (in addition to removing any holes within).
Unfortunately, declaring the struct or __be32* cast as packed have no effect. I still get an ldm emitted. Rob _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain