Re: SCSI bug

From: John David Anglin <dave.anglin@bell.net>
To: Helge Deller <deller@gmx.de>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>,
	linux-parisc List <linux-parisc@vger.kernel.org>
Subject: Re: SCSI bug
Date: Tue, 23 Feb 2016 14:10:12 -0500	[thread overview]
Message-ID: <56CCAE94.3020109@bell.net> (raw)
In-Reply-To: <56CC9FA2.6020107@gmx.de>

On 2016-02-23 1:06 PM, Helge Deller wrote:
> On 23.02.2016 04:04, John David Anglin wrote:
>> On 2016-02-21, at 10:24 PM, John David Anglin wrote:
>>
>>> On 2016-02-21, at 7:53 PM, John David Anglin wrote:
>>>
>>>> Backtrace:
>>>> [<000000004046f4b0>] scsi_init_sgtable+0x70/0xb8
>>>> [<000000004046f564>] scsi_init_io+0x6c/0x220
>>>> [<000000000811c5c0>] sd_setup_read_write_cmnd+0x58/0x968 [sd_mod]
>>>> [<000000000811cf14>] sd_init_command+0x44/0x130 [sd_mod]
>>>> [<000000004046f81c>] scsi_setup_cmnd+0x104/0x1b0
>>>> [<000000004046fab8>] scsi_prep_fn+0x100/0x1a0
>>>> [<000000004035b9b0>] blk_peek_request+0x1b8/0x298
>>>> [<0000000040471028>] scsi_request_fn+0xf8/0xa90
>>>> [<0000000040357244>] __blk_run_queue+0x4c/0x70
>>>> [<00000000403802c4>] cfq_insert_request+0x2dc/0x580
>>>> [<0000000040356404>] __elv_add_request+0x1b4/0x300
>>>>
>>>> We have in blk-merge.c:
>>>>
>>>>                else {
>>>>                        /*
>>>>                         * If the driver previously mapped a shorter
>>>>                         * list, we could see a termination bit
>>>>                         * prematurely unless it fully inits the sg
>>>>                         * table on each mapping. We KNOW that there
>>>>                         * must be more entries here or the driver
>>>>                         * would be buggy, so force clear the
>>>>                         * termination bit to avoid doing a full
>>>>                         * sg_init_table() in drivers for each command.
>>>>                         */
>>>>                        if (sg_is_last (*sg))
>>>>                          printk ("__blk_segment_map_sg: clearing termination bi
>>>> t\n");
>>>>                        sg_unmark_end(*sg);
>>>>                        *sg = sg_next(*sg);
>>>>                        BUG_ON (!*sg);
>>>>                }
>>>>
>>>> The comment suggests there must be more entries...
>>> I'm thinking with the split the scsi driver needs to provide one or two extra entires in the sg list.
>>
>> With the attached patch, I'm able to boot 4.2.0-rc2+ on linux-block at commit
>> 54efd50bfd873e2dbf784e0b21a8027ba4299a3e.
>>
>> I didn't try to optimize the number of extra entries but I know one is not enough.
>>
>> I guess the puzzle is why the number of entries isn't calculated correctly in the first place.
>> Further, why does blk-merge believe that it's okay to go beyond the terminator?  Clearly,
>> the magic number isn't always set, etc.
>>
>> I added the WARN_ON so I'd know when we run off the end of the the list.
> Still fails to boot for me on c3000
> (although I think the patch is going into the right direction!):
>
> [   25.140000] cdrom: Uniform CD-ROM driver Revision: 3.20
> [   25.200000] sd 3:0:6:0: [sdb] Write cache: disabled, read cache: enabled, supports DPO and FUA
> [   25.304000] sd 3:0:5:0: [sda] Write cache: disabled, read cache: enabled, supports DPO and FUA
> [   25.436000]  sdb: sdb1 sdb2 sdb3 < sdb5 sdb6 >
> [   25.488000]  sda: sda1 sda2 sda3 < sda5 sda6 >
> [   25.560000] sd 3:0:6:0: [sdb] Attached SCSI disk
> [   25.636000] scsi_id(112): unaligned access to 0x00000000faad5009 at ip=0x000000004100390b
> [   25.752000] sd 3:0:5:0: [sda] Attached SCSI disk
> [   25.832000] scsi_id(113): unaligned access to 0x00000000fa90a009 at ip=0x000000004100390b
> [   25.972000] ------------[ cut here ]------------
> [   26.028000] WARNING: at /build/linux-4.4-neu/linux-4.4.2/block/blk-merge.c:466
This is this warning (not the one I added):

         /*
          * Something must have been wrong if the figured number of
          * segment is bigger than number of req's physical segments
          */
         WARN_ON(nsegs > rq->nr_phys_segments);

We need backtrace to see who called blk_rq_map_sg.  Think driver didn't 
provide enough segments.  Maybe
my "+8" addition should be moved.  I don't think this warning was actual 
failure point.

James mentioned "Apparently in parisc we don't set the max segment size, 
so we inherit 64k even in SCSI drivers."
Can this be changed?

> [   26.116000] random: nonblocking pool is initialized
> [   26.172000] Modules linked in: sr_mod cdrom sd_mod ata_generic ohci_pci ehci_pci ohci_hcd ehci_hcd pata_ns87415 libata sym53c8xx scsi_transport_spi usbcore scsi_modp
> [   26.368000] CPU: 0 PID: 65 Comm: systemd-udevd Not tainted 4.4.0-1-parisc64-smp #1 Debian 4.4.2-3
> [   26.476000] task: 00000000bbd70b08 ti: 00000000bbea8000 task.ti: 00000000bbea8000
> [   26.564000]
> [   26.584000]      YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI
> [   26.640000] PSW: 00001000000001001111111100001110 Not tainted
> [   26.708000] r00-03  000000ff0804ff0e 00000000409e8380 00000000404f18f4 00000000bbea91e0
> [   26.804000] r04-07  00000000409b2b80 0000000000000000 0000000000000000 000000000000001e

Dave

-- 
John David Anglin  dave.anglin@bell.net