Re: System hangs after echo value > /sys/block/dm-0/queue/nr_requests

[Andrew Morton <akpm@osdl.org>]

Miquel van Smoorenburg <miquels@cistron.nl> wrote:
>
> If you echo a value (any value; for example the default 128) to
> /sys/block/dm-0/queue/nr_requests the shell you are in hangs.
> After about 5 seconds, the whole system hangs 100%.

hm, nice.  It does the same thing for /sys/block/md0/queue/nr-requests.

With CONFIG_DEBUG_SPINLOCK enabled we go BUG in __wake_up():

Program received signal SIGTRAP, Trace/breakpoint trap.
__wake_up (q=0xcf299e90, mode=3, nr_exclusive=1) at include/asm/spinlock.h:137
137                     BUG();
(gdb) bt
#0  __wake_up (q=0xcf299e90, mode=3, nr_exclusive=1) at include/asm/spinlock.h:137
#1  0xc02cc847 in queue_requests_store (q=0xcf299df8, page=0xe <Address 0xe out of bounds>, count=14)
    at drivers/block/ll_rw_blk.c:2843
#2  0xc02cc907 in queue_attr_store (kobj=0xcf299f68, attr=0xe, page=0xe <Address 0xe out of bounds>, 
    length=14) at drivers/block/ll_rw_blk.c:2892
#3  0xc01aa84f in flush_write_buffer (file=0xe, buffer=0xc0456100, count=14) at fs/sysfs/file.c:205
#4  0xc01aa8ac in sysfs_write_file (file=0xc0493550, buf=0xe <Address 0xe out of bounds>, count=2, 
    ppos=0xc851cf84) at fs/sysfs/file.c:233
#5  0xc016f458 in vfs_write (file=0xc0493550, buf=0x80b2d00 "128\n", count=4, pos=0xc851cf84)
    at fs/read_write.c:257
#6  0xc016f55e in sys_write (fd=14, buf=0xe <Address 0xe out of bounds>, count=14) at fs/read_write.c:293


Where queue_requests_store() does wake_up(&rl->wait[READ]);

It looks like nobody has called blk_init_queue() for this queue and the
waitqueue head is uninitialised.

No md was in use on this machine: it was simply enabled in kernel config.