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. -- Michael _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain