On 5 October 2012 01:58, Michael Hope michael.hope@linaro.org wrote:
On 5 October 2012 12:10, Rob Herring robherring2@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.