Folks,
I had a crash of a qemu guest in tracked_request_begin.
The testcase was a guest with ramdisk/kernel that reboots in a
loop. (about 10 times per second) with a single null-co disk
attached. No idea how to reproduce this, seems to be a lucky hit.
(gdb) bt
#0 0x00000000101db5ba in tracked_request_begin (req=req@entry=0x3ff90f1bdc0,
bs=bs@entry=0x42a39190, offset=offset@entry=0, bytes=bytes@entry=4096,
type=type@entry=BDRV_TRACKED_READ)
at /home/cborntra/REPOS/qemu/block/io.c:390
#1 0x00000000101de91e in bdrv_co_do_preadv (bs=0x42a39190, offset=0,
bytes=4096, qiov=0x3ff7400cbd8, flags=<optimized out>, flags@entry=(unknown: 0))
at /home/cborntra/REPOS/qemu/block/io.c:1001
#2 0x00000000101dfc3e in bdrv_co_do_readv (flags=(unknown: 0), qiov=<optimized
out>, nb_sectors=<optimized out>, sector_num=<optimized out>, bs=<optimized
out>)
at /home/cborntra/REPOS/qemu/block/io.c:1024
#3 bdrv_co_do_rw (opaque=0x3ff7400e370) at
/home/cborntra/REPOS/qemu/block/io.c:2173
#4 0x000000001022d8f6 in coroutine_trampoline (i0=<optimized out>,
i1=-1946150928) at /home/cborntra/REPOS/qemu/util/coroutine-ucontext.c:79
#5 0x000003ff95ed150a in __makecontext_ret () from /lib64/libc.so.6
looking at the code we are at
QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
which translates to
if (((req)->list.le_next = (&bs->tracked_requests)->lh_first) != NULL)
(&bs->tracked_requests)->lh_first->list.le_prev = &(req)->list.le_next;
(&bs->tracked_requests)->lh_first = (req);
(req)->list.le_prev = &(&bs->tracked_requests)->lh_first;
gdb says, that (&bs->tracked_requests)->lh_first) is zero in the corefile
(gdb) print /x bs->tracked_requests
$6 = {lh_first = 0x0}
Now looking at the code I am asking myself if this can happen in parallel
to another code that touches tracked_requests, because gcc seems to read
&bs->tracked_requests)->lh_first twice (first to check the value, then
to use it as pointer)
388 qemu_co_queue_init(&req->wait_queue);
0x00000000101db594 <+76>: la %r2,72(%r13)
0x00000000101db598 <+80>: brasl %r14,0x1022cdc0 <qemu_co_queue_init>
389
390 QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
0x00000000101db59e <+86>: lg %r1,12744(%r12) # r1 =
(&bs->tracked_requests)->lh_first)
0x00000000101db5a4 <+92>: stg %r1,48(%r13) #
(req)->list.le_next = r1
0x00000000101db5aa <+98>: cgij %r1,0,8,0x101db5c0 ---+ # if r1==0 goto
0x00000000101db5b0 <+104>: lg %r1,12744(%r12) | # r1 =
(&bs->tracked_requests)->lh_first) (again!!)
0x00000000101db5b6 <+110>: la %r2,48(%r13) |
=> 0x00000000101db5ba <+114>: stg %r2,56(%r1) | # r1==0 bang
0x00000000101db5c0 <+120>: stg %r13,12744(%r12)<-----+
0x00000000101db5c6 <+126>: lay %r12,12744(%r12)
0x00000000101db5cc <+132>: stg %r12,56(%r13)
Christian
