Re: [PATCH 12/12] scsi_transport_sas: fix delete vs scan race

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Sun, Apr 22, 2012 at 3:38 AM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 2012-04-13 at 16:37 -0700, Dan Williams wrote:
>> The following crash results from cases where the end_device has been
>> removed before scsi_sysfs_add_sdev has had a chance to run.
>>
>>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
>>  IP: [<ffffffff8115e100>] sysfs_create_dir+0x32/0xb6
>>  ...
>>  Call Trace:
>>   [<ffffffff8125e4a8>] kobject_add_internal+0x120/0x1e3
>>   [<ffffffff81075149>] ? trace_hardirqs_on+0xd/0xf
>>   [<ffffffff8125e641>] kobject_add_varg+0x41/0x50
>>   [<ffffffff8125e70b>] kobject_add+0x64/0x66
>>   [<ffffffff8131122b>] device_add+0x12d/0x63a
>>   [<ffffffff814b65ea>] ? _raw_spin_unlock_irqrestore+0x47/0x56
>>   [<ffffffff8107de15>] ? module_refcount+0x89/0xa0
>>   [<ffffffff8132f348>] scsi_sysfs_add_sdev+0x4e/0x28a
>>   [<ffffffff8132dcbb>] do_scan_async+0x9c/0x145
>>
>> ...teach sas_rphy_remove to wait for async scanning to quiesce before
>> removing the end_device.  It seems this is a more general problem [1],
>> but this patch only addresses sas transport.
>>
>> [1]: 23edb6e [SCSI] mpt2sas: Do not set sas_device->starget to NULL from
>> the slave_destroy callback when all the LUNS have been deleted
>>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> ---
>>  drivers/scsi/scsi_transport_sas.c |    6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/scsi_transport_sas.c b/drivers/scsi/scsi_transport_sas.c
>> index f7565fc..47abb90 100644
>> --- a/drivers/scsi/scsi_transport_sas.c
>> +++ b/drivers/scsi/scsi_transport_sas.c
>> @@ -33,8 +33,9 @@
>>  #include <linux/bsg.h>
>>
>>  #include <scsi/scsi.h>
>> -#include <scsi/scsi_device.h>
>>  #include <scsi/scsi_host.h>
>> +#include <scsi/scsi_scan.h>
>> +#include <scsi/scsi_device.h>
>>  #include <scsi/scsi_transport.h>
>>  #include <scsi/scsi_transport_sas.h>
>>
>> @@ -1667,6 +1668,9 @@ sas_rphy_remove(struct sas_rphy *rphy)
>>  {
>>       struct device *dev = &rphy->dev;
>>
>> +     /* prevent device_del() while child device_add() may be in-flight */
>> +     scsi_complete_async_scans();
>> +
>>       switch (rphy->identify.device_type) {
>
> This doesn't really fix the problem, it merely narrows the window (we
> still crash in the much shorter window if another async scan starts
> after you check for completion).

Oh, I was under the impression that async scanning was only the
initial scan and everything was sync thereafter since
scsi_finish_async_scan() clears the host ->async_scan flag?

> Isn't the fix that will prevent all of
> this to hold the scan mutex across scsi_remove_device() ... in which
> case it should probably be part of scsi_remove_device()?

I thought along these lines initially, but in this case we're crashing
because the sas rphy is removed before the starget is added, so
scsi_remove_device() is out of the picture.

--
Dan
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