Hi, do you still observe this behaviour? Which type of EC2 instances were affected? I tried to reproduce with kernel/tools from Dec 15 and did not manage to crash the machine.
Jakub On Sun, Dec 4, 2016 at 5:38 PM, Dimitry Andric <d...@freebsd.org> wrote: > On 04 Dec 2016, at 10:52, Hans Petter Selasky <h...@selasky.org> wrote: > > > > On 12/04/16 01:04, Colin Percival wrote: > >> Starting with r309124 (when clang/llvm 3.9.0 was imported) I'm seeing > EC2 > >> instances panic on boot with a division-by-zero error; the code in > question > >> is in blkfront.c, printing out the size of disks: > >> > >>> device_printf(dev, "%juMB <%s> at %s", > >>> (uintmax_t) sectors / (1048576 / sector_size), > >>> device_get_desc(dev), > >>> xenbus_get_node(dev)); > >> > >> My first thought was that 'sector_size' must be either zero or very > large... > >> but no, when I add printf("sector_size = %ju\n", > (uintmax_t)sector_size), it's > >> entirely normal. What's more, adding that printf makes the > division-by-zero > >> panic go away. > >> > >> I'd think I was just hallucinating, but earlier today I heard that a > similarly > >> "impossible" panic had been observed in the NFS client code when > compiled with > >> clang/llvm 3.9.0. > >> > >> So... is anyone else seeing unexpected panics or other odd behaviour > starting > >> after clang/llvm 3.9.0 was imported? > >> > > > > Hi, > > > > Can you look at the code with "objdump -Dx --source" and see what is > going on there? Might it be the "sector" variable is shadowed? > > I don't see anything in the generated code for the call that can cause > this, except for sector_size really being zero, or the result of > 1048576/sector_size being zero. > > On i386, you get this: > > .loc 1 1349 19 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:19 > movl -56(%ebp), %ecx # -56(%rbp) = sectors > .Ltmp1148: > #DEBUG_VALUE: xbd_connect:sectors <- %ECX > .loc 1 1349 38 is_stmt 0 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:38 > movl $1048576, %eax # imm = 0x100000 > xorl %edx, %edx > divl -52(%ebp) # -52(%ebp) = sector_size > movl %eax, %edi > .loc 1 1349 27 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:27 > xorl %edx, %edx > movl %ecx, %eax > divl %edi > movl %eax, -32(%ebp) # 4-byte Spill > > On amd64, it looks pretty similar: > > .loc 1 1349 19 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:19 > movq -112(%rbp), %rcx # -112(%rbp) = sectors > .Ltmp1128: > #DEBUG_VALUE: xbd_connect:sectors <- %RCX > .loc 1 1349 38 is_stmt 0 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:38 > movl $1048576, %eax # imm = 0x100000 > xorl %edx, %edx > divq -88(%rbp) # -88(%rbp) = sector_size > movq %rax, %rsi > .loc 1 1349 27 # /usr/src/sys/dev/xen/blkfront/ > blkfront.c:1349:27 > xorl %edx, %edx > movq %rcx, %rax > divq %rsi > movq %rax, %r15 > > Colin, does it panic for you in the first or the second div? > > -Dimitry > > _______________________________________________ firstname.lastname@example.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"