Re: [dm-devel] [PATCH] dm-io: Prevent the danging point of the sync io callback function

[Mikulas Patocka]

On Fri, 27 Jun 2014, Mikulas Patocka wrote:

> 
> 
> On Fri, 27 Jun 2014, Minfei Huang wrote:
> 
> > BUG: unable to handle kernel NULL pointer dereference at 0000000000000046
> > IP: [<ffffffffa0009cef>] dec_count+0x5f/0x80 [dm_mod]
> > PGD 0
> > Oops: 0000 [#1] SMP
> > last sysfs file: /sys/devices/pci0000:00/0000:00:02.2/0000:02:00.0/host0/scsi_host/host0/proc_name
> > 
> > Pid: 2708, comm: kcopyd Tainted: G        W  --------------- H  2.6.32-279.19.5.el6.ucloud.x86_64 #1 Dell Inc. PowerEdge R720xd/0DCWD1
> > RIP: 0010:[<ffffffffa0009cef>]  [<ffffffffa0009cef>] dec_count+0x5f/0x80 [dm_mod]
> > RSP: 0018:ffff880100603c30  EFLAGS: 00010246
> > RAX: 0000000000000046 RBX: ffff8817968a5c30 RCX: 0000000000000000
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8817968a5c00
> > RBP: ffff880100603c50 R08: 0000000000000000 R09: 0000000000000000
> > R10: ffff880caa594cc0 R11: 0000000000000000 R12: ffff8817968a5c80
> > R13: ffffffff81013963 R14: 0000000000001000 R15: 0000000000000000
> > FS:  0000000000000000(0000) GS:ffff880100600000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
> > CR2: 0000000000000046 CR3: 000000020c309000 CR4: 00000000001426e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > Process kcopyd (pid: 2708, threadinfo ffff88180cd26000, task ffff881841c9aa80)
> > Stack:
> >  ffff880100603c40 ffff880aa8b32300 0000000000000000 ffff8817968a5c00
> > <d> ffff880100603c80 ffffffffa000a12a 0000000000000000 ffff880aa8b32300
> > <d> 0000000000000000 ffff880caa594cc0 ffff880100603c90 ffffffff811bcf6d
> > Call Trace:
> >  <IRQ>
> >  [<ffffffffa000a12a>] endio+0x4a/0x70 [dm_mod]
> >  [<ffffffff811bcf6d>] bio_endio+0x1d/0x40
> >  [<ffffffff81260beb>] req_bio_endio+0x9b/0xe0
> >  [<ffffffff81263114>] blk_update_request+0x104/0x500
> >  [<ffffffff81263331>] ? blk_update_request+0x321/0x500
> >  [<ffffffff81263537>] blk_update_bidi_request+0x27/0xa0
> >  [<ffffffff8126419f>] blk_end_bidi_request+0x2f/0x80
> >  [<ffffffff81264240>] blk_end_request+0x10/0x20
> >  [<ffffffff81375c6f>] scsi_io_completion+0xaf/0x6c0
> >  [<ffffffff8136cb92>] scsi_finish_command+0xc2/0x130
> >  [<ffffffff813763e5>] scsi_softirq_done+0x145/0x170
> >  [<ffffffff812698ed>] blk_done_softirq+0x8d/0xa0
> >  [<ffffffff81074c5f>] __do_softirq+0xdf/0x210
> >  [<ffffffff8100c2cc>] call_softirq+0x1c/0x30
> >  [<ffffffff8100df9d>] do_softirq+0xad/0xe0
> >  [<ffffffff81074995>] irq_exit+0x95/0xa0
> >  [<ffffffff81510515>] do_IRQ+0x75/0xf0
> >  [<ffffffff8100ba53>] ret_from_intr+0x0/0x16
> > 
> > The value of rdi register(0xffff8817968a5c00) is the io pointer,
> > If the sync io, the address of io point must be alloc from stack.
> > SO
> > crash> struct thread_info ffff8817968a4000
> > struct thread_info {
> >   task = 0xffff88180cd9a580,
> >   exec_domain = 0xffffffff81a98ac0,
> >  ...
> > }
> > 
> > crash> struct task_struct 0xffff88180cd9a580
> > struct task_struct {
> >   state = 2,
> >   stack = 0xffff8817968a4000,
> >  ...
> > }
> > 
> > It shows value exactly when use the value of io address.
> > 
> > The io address in callback function will become the danging point,
> > cause by the thread of sync io wakes up by other threads
> > and return to relieve the io address,
> > 
> > Signed-off-by: Minfei Huang <huangminfei@xxxxxxxxx>
> > ---
> >  drivers/md/dm-io.c |   19 +++++++++++++++----
> >  1 files changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
> > index 3842ac7..f992913 100644
> > --- a/drivers/md/dm-io.c
> > +++ b/drivers/md/dm-io.c
> > @@ -38,6 +38,7 @@ struct io {
> >  	void *context;
> >  	void *vma_invalidate_address;
> >  	unsigned long vma_invalidate_size;
> > +	atomic_t wakeup;
> >  } __attribute__((aligned(DM_IO_MAX_REGIONS)));
> >  
> >  static struct kmem_cache *_dm_io_cache;
> > @@ -121,10 +122,16 @@ static void dec_count(struct io *io, unsigned int region, int error)
> >  			invalidate_kernel_vmap_range(io->vma_invalidate_address,
> >  						     io->vma_invalidate_size);
> >  
> > -		if (io->sleeper)
> > -			wake_up_process(io->sleeper);
> > +		if (io->sleeper) {
> > +			struct task_struct *sleeper = io->sleeper;
> >  
> > -		else {
> > +			atomic_set(&io->wakeup, 1);
> 
> The problem here is that the processor may reorder the read of io->sleeper 
> with atomic_set(&io->wakeup, 1); (performing atomic_set first and "sleeper 
> = io->sleeper" afterwards) exposing the same race condition.
> 
> You need to use memory barriers to avoid reordering, but I think the 
> solution with the completion is better (the completion takes care of 
> barriers automatically).
> 
> Mikulas

Also - there is another race - after atomic_set(&io->wakeup, 1), the 
target process may terminate, so wake_up_process(sleeper) operates on 
non-existing process. You need to declare a special wait queue or use 
wait_on_atomic_t+wake_up_atomic_t (that uses uses pre-initialized hash of 
wait queues) to avoid that race.

But as I said - the completion approach is better. It doesn't suffer from 
these problems.

Mikulas

> > +/*
> > + * The thread may be waked up by other threads,
> > + * if then the sync io point will become the dangling pointer
> > + */
> > +			wake_up_process(sleeper);
> > +		} else {
> >  			unsigned long r = io->error_bits;
> >  			io_notify_fn fn = io->callback;
> >  			void *context = io->context;
> > @@ -401,12 +408,14 @@ static int sync_io(struct dm_io_client *client, unsigned int num_regions,
> >  	io->vma_invalidate_address = dp->vma_invalidate_address;
> >  	io->vma_invalidate_size = dp->vma_invalidate_size;
> >  
> > +	atomic_set(&io->wakeup, 0);
> > +
> >  	dispatch_io(rw, num_regions, where, dp, io, 1);
> >  
> >  	while (1) {
> >  		set_current_state(TASK_UNINTERRUPTIBLE);
> >  
> > -		if (!atomic_read(&io->count))
> > +		if (atomic_read(&io->wakeup))
> >  			break;
> >  
> >  		io_schedule();
> > @@ -442,6 +451,8 @@ static int async_io(struct dm_io_client *client, unsigned int num_regions,
> >  	io->vma_invalidate_address = dp->vma_invalidate_address;
> >  	io->vma_invalidate_size = dp->vma_invalidate_size;
> >  
> > +	atomic_set(&io->wakeup, 0);
> > +
> >  	dispatch_io(rw, num_regions, where, dp, io, 0);
> >  	return 0;
> >  }
> > -- 
> > 1.7.1
> > 
> > 
> > --
> > dm-devel mailing list
> > dm-devel@xxxxxxxxxx
> > https://www.redhat.com/mailman/listinfo/dm-devel
> > 
> 
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