Re: [PATCHv4 00/10] add on-demand device creation

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (05/06/15 14:25), Sergey Senozhatsky wrote:
> a-ha... found it:
> http://lkml.iu.edu/hypermail/linux/kernel/1505.0/04389.html
> 

well... that's weird. can you reproduce this easily?


we do

zram_add():		zram_remove():
-------------------------------------------
mutex_lock(idr);
idr_alloc()
...
->major = 252;
->first_minor = idr index;
add_disk();
mumutex_unlock(idr);
			mutex_lock(idr);
			...
			idr_remove(idr index);
			del_gendisk();
			put_disk();
			mutex_unlock(idr);


script attempted to create a new device with minor (idr index) 2, but there
was a kernfs node from already removed zram2: '/devices/virtual/bdi/252:2'

from your logs:
...
[ 98.756017] zram: Removed device: zram2
[ 98.757087] ------------[ cut here ]------------
...

locked zram_index_mutex, removed zram2, unlocked zram_index_mutex,
locked zram_index_mutex, attempted to create zram2: zram2 sysfs already exist.


hm... need to think. zram hot/remove is serialized.




a separate issue:


/*
 * FIXME: error handling
 */
void add_disk(struct gendisk *disk)    does not handle any errors at all nor it
reports any errors back to caller:

 612         /* Register BDI before referencing it from bdev */
 613         bdi = &disk->queue->backing_dev_info;
 614         bdi_register_dev(bdi, disk_devt(disk));
 615 
 616         blk_register_region(disk_devt(disk), disk->minors, NULL,
 617                             exact_match, exact_lock, disk);
 618         register_disk(disk);
 619         blk_register_queue(disk);
 620 
 621         /*
 622          * Take an extra ref on queue which will be put on disk_release()
 623          * so that it sticks around as long as @disk is there.
 624          */
 625         WARN_ON_ONCE(!blk_get_queue(disk->queue));
 626 
 627         retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
 628                                    "bdi");
 629         WARN_ON(retval);


if bdi_register_dev()->device_add() fails (for example), then sysfs_create_link()
is just "expected" to cause:


[ 98.819810] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
[ 98.820591] IP: [<ffffffff8126da50>] sysfs_do_create_link_sd.isra.2+0x40/0xd0
[ 98.821290] PGD 61669067 PUD 61553067 PMD 0
[ 98.821709] Oops: 0000 [#1] SMP
[..]
[ 98.823663] Call Trace:
[ 98.823663] [<ffffffff8135931b>] ? disk_part_iter_next+0x2b/0x280
[ 98.823663] [<ffffffff8126db05>] sysfs_create_link+0x25/0x50
[ 98.823663] [<ffffffff81359fc2>] add_disk+0x252/0x4e0


or:

[ 98.823663] BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:21
[ 98.823663] in_atomic(): 1, irqs_disabled(): 1, pid: 2952, name: cat
[ 98.823663] INFO: lockdep is turned off.
[ 98.823663] irq event stamp: 3392
[ 98.823663] hardirqs last enabled at (3391): [<ffffffff81644133>] __mutex_unlock_slowpath+0xb3/0x180
[ 98.823663] hardirqs last disabled at (3392): [<ffffffff81648d33>] error_sti+0x5/0x6
[ 98.823663] softirqs last enabled at (0): [<ffffffff8105c6a9>] copy_process.part.35+0x4d9/0x1ce0
[ 98.823663] softirqs last disabled at (0): [< (null)>] (null)
[ 98.823663] CPU: 5 PID: 2952 Comm: cat Tainted: G D W 4.1.0-rc2-next-20150505+ #1260
[ 98.823663] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[ 98.823663] ffff88005d40f8c8 ffff88005d40f8c8 ffffffff8163d8d8 0000000000000000
[ 98.823663] 0000000000000000 ffff88005d40f8f8 ffffffff8108c18e ffffffff8108c005
[ 98.823663] ffffffff819ec8b3 0000000000000015 0000000000000000 ffff88005d40f928
[ 98.823663] Call Trace:
[ 98.823663] [<ffffffff8163d8d8>] dump_stack+0x4c/0x65
[ 98.823663] [<ffffffff8108c18e>] ___might_sleep+0x18e/0x250
[ 98.823663] [<ffffffff8108c005>] ? ___might_sleep+0x5/0x250
[ 98.823663] [<ffffffff8108c29d>] __might_sleep+0x4d/0x90
[ 98.823663] [<ffffffff8108c255>] ? __might_sleep+0x5/0x90
[ 98.823663] [<ffffffff81644494>] down_read+0x24/0x70
[ 98.823663] [<ffffffff81644475>] ? down_read+0x5/0x70
[ 98.823663] [<ffffffff810722a4>] ? exit_signals+0x24/0x130
[ 98.823663] [<ffffffff810722a4>] exit_signals+0x24/0x130
[ 98.823663] [<ffffffff81072285>] ? exit_signals+0x5/0x130
[ 98.823663] [<ffffffff810606f8>] ? do_exit+0xb8/0xbc0
[ 98.823663] [<ffffffff810606f8>] do_exit+0xb8/0xbc0
[ 98.823663] [<ffffffff81060645>] ? do_exit+0x5/0xbc0
[ 98.823663] [<ffffffff810c4a79>] ? kmsg_dump+0x119/0x1a0
[ 98.823663] [<ffffffff810c4985>] ? kmsg_dump+0x25/0x1a0
[ 98.823663] [<ffffffff8100665e>] oops_end+0x8e/0xd0
[ 98.823663] [<ffffffff810065d5>] ? oops_end+0x5/0xd0
[ 98.823663] [<ffffffff81637f65>] no_context+0x2d9/0x33e
[ 98.823663] [<ffffffff81637c91>] ? no_context+0x5/0x33e
[ 98.823663] [<ffffffff81638042>] __bad_area_nosemaphore+0x78/0x1d1
[ 98.823663] [<ffffffff816381a0>] ? bad_area_nosemaphore+0x5/0x15
[ 98.823663] [<ffffffff816381ae>] bad_area_nosemaphore+0x13/0x15
[ 98.823663] [<ffffffff8104c4de>] __do_page_fault+0x9e/0x490
[ 98.823663] [<ffffffff8104c445>] ? __do_page_fault+0x5/0x490
[ 98.823663] [<ffffffff8104c8dc>] do_page_fault+0xc/0x10
[ 98.823663] [<ffffffff81648b62>] page_fault+0x22/0x30
[ 98.823663] [<ffffffff8126da50>] ? sysfs_do_create_link_sd.isra.2+0x40/0xd0
[ 98.823663] [<ffffffff8126da50>] ? sysfs_do_create_link_sd.isra.2+0x40/0xd0
[ 98.823663] [<ffffffff8135931b>] ? disk_part_iter_next+0x2b/0x280
[ 98.823663] [<ffffffff8126db05>] sysfs_create_link+0x25/0x50
[ 98.823663] [<ffffffff81359fc2>] add_disk+0x252/0x4e0


there are lots of places in add_disk() that can potentially fail. it's probably time
to "fix" it. including numerous add_disk() callers, that don't check for errors.

	-ss