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SubjectRe: [RFC PATCH] scsi: fix oops in scsi_uninit_cmd()
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On Tue, 2019-02-19 at 15:27 +0800, Jason Yan wrote:
> If we remove the scsi disk when running io with fio, oops occured with
> the following condition.
>
> [scsi_eh_0] [fio]
> scsi_end_request
> ->blk_update_request
> ->end_bio(io returned to userspace)
> close
> ->sd_release
> ->scsi_disk_put
> ->scsi_disk_release
> ->disk->private_data = NULL;
>
> ->scsi_mq_uninit_cmd
> ->scsi_uninit_cmd
> ->scsi_cmd_to_driver
> ->drv is NULL, Oops
>
> There is a small window between blk_update_request() and
> scsi_mq_uninit_cmd() that scsi disk may have been released. This will
> cause a oops like below:
>
> Unable to handle kernel NULL pointer dereference at virtual address
> 0000000000000000
> s/sync.c:67, func=xfer, error=In[11347.116050] Mem abort info:
> put/output error
> [11347.121598] ESR = 0x96000006
> [11347.126200] Exception class = DABT (current EL), IL = 32 bits
> [11347.132117] SET = 0, FnV = 0
> [11347.135170] EA = 0, S1PTW = 0
> [11347.138308] Data abort info:
> [11347.141186] ISV = 0, ISS = 0x00000006
> [11347.145019] CM = 0, WnR = 0
> [11347.147977] user pgtable: 4k pages, 48-bit VAs, pgdp =
> 00000000a67aece2
> [11347.154591] [0000000000000000] pgd=0000002f90774003,
> pud=0000002fab098003, pmd=0000000000000000
> [11347.163304] Internal error: Oops: 96000006 [#1] PREEMPT SMP
> [11347.168870] Modules linked in: hisi_sas_v3_hw hisi_sas_main libsas
> [11347.175044] CPU: 56 PID: 4294 Comm: scsi_eh_2 Not tainted
> 4.19.0-g8052059-dirty #2
> [11347.182600] Hardware name: Huawei D06/D06, BIOS Hisilicon D06 UEFI
> RC0 - B601 (V6.01) 11/08/2018
> [11347.191370] pstate: a0c00009 (NzCv daif +PAN +UAO)
> [11347.196155] pc : scsi_uninit_cmd+0x24/0x3c
> [11347.200240] lr : scsi_mq_uninit_cmd+0x1c/0x30
> [11347.204583] sp : ffff000024dabb60
> [11347.207884] x29: ffff000024dabb60 x28: ffff000024dabd38
> [11347.213184] x27: ffff000000f5b3a8 x26: ffff7df3b0181600
> [11347.218484] x25: 0000000000000000 x24: ffff803bc5d36778
> [11347.223783] x23: 000000000000000a x22: 0000000000000000
> [11347.229082] x21: ffff803bc7397000 x20: ffff802f9148e530
> [11347.234381] x19: ffff802f9148e530 x18: ffff7e0000000000
> [11347.239679] x17: 0000000000000000 x16: 0000002f9e37d000
> [11347.244979] x15: ffff7e0000000000 x14: 3863206336203839
> [11347.250278] x13: 2036302030302038 x12: a46fac3d0d363d00
> [11347.255578] x11: ffffffffffffffff x10: a46fac3d0d363d00
> [11347.260877] x9 : 0000000040040000 x8 : 000000000000eb4b
> [11347.266177] x7 : ffff000009771000 x6 : 0000000000210d00
> [11347.271476] x5 : ffff803bc9f50000 x4 : 0000000000000000
> [11347.276775] x3 : ffff802fb02b4380 x2 : ffff802f9148e400
> [11347.282075] x1 : 0000000000000000 x0 : ffff802f9148e530
> [11347.287375] Process scsi_eh_2 (pid: 4294, stack limit =
> 0x000000007d2257f8)
> [11347.294323] Call trace:
> Jobs: 6 (f=6): [R[RRR1XXX1XRR3] 47.296758] scsi_uninit_cmd+0x24/0x3c
> [22.7% done] [1516MB/0KB/0KB /s] [754/0/0 iops] [eta 08m:39s]
> [11347.308390] scsi_mq_uninit_cmd+0x1c/0x30
> [11347.312387] scsi_end_request+0x7c/0x1b8
> [11347.316297] scsi_io_completion+0x464/0x668
> [11347.320467] scsi_finish_command+0xbc/0x160
> [11347.324636] scsi_eh_flush_done_q+0x10c/0x170
> [11347.328990] sas_scsi_recover_host+0x84c/0xa98 [libsas]
> [11347.334202] scsi_error_handler+0x140/0x5b0
> [11347.338374] kthread+0x100/0x12c
> [11347.341590] ret_from_fork+0x10/0x18
> [11347.345153] Code: 71000c3f 540000e9 f9404c41 f941f421 (f9400021)
> [11347.351234] ---[ end trace f496aacdaa1dcc51 ]---
>
> To fix this, get a refcount of scsi_disk in sd_init_command() to ensure
> it will not be released before sd_uninit_command().
>
> Signed-off-by: Jason Yan <yanaijie@huawei.com>
> ---
> drivers/scsi/sd.c | 46 +++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 35 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 5464d467e23e..6bdb8fbb570f 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -1249,42 +1249,64 @@ static blk_status_t sd_setup_read_write_cmnd(struct scsi_cmnd *SCpnt)
> static blk_status_t sd_init_command(struct scsi_cmnd *cmd)
> {
> struct request *rq = cmd->request;
> + struct scsi_disk *sdkp = NULL;
> + blk_status_t ret;
>
> switch (req_op(rq)) {
> case REQ_OP_DISCARD:
> switch (scsi_disk(rq->rq_disk)->provisioning_mode) {
> case SD_LBP_UNMAP:
> - return sd_setup_unmap_cmnd(cmd);
> + ret = sd_setup_unmap_cmnd(cmd);
> + break;
> case SD_LBP_WS16:
> - return sd_setup_write_same16_cmnd(cmd, true);
> + ret = sd_setup_write_same16_cmnd(cmd, true);
> + break;
> case SD_LBP_WS10:
> - return sd_setup_write_same10_cmnd(cmd, true);
> + ret = sd_setup_write_same10_cmnd(cmd, true);
> + break;
> case SD_LBP_ZERO:
> - return sd_setup_write_same10_cmnd(cmd, false);
> + ret = sd_setup_write_same10_cmnd(cmd, false);
> + break;
> default:
> - return BLK_STS_TARGET;
> + ret = BLK_STS_TARGET;
> + break;
> }
> + break;
> case REQ_OP_WRITE_ZEROES:
> - return sd_setup_write_zeroes_cmnd(cmd);
> + ret = sd_setup_write_zeroes_cmnd(cmd);
> + break;
> case REQ_OP_WRITE_SAME:
> - return sd_setup_write_same_cmnd(cmd);
> + ret = sd_setup_write_same_cmnd(cmd);
> + break;
> case REQ_OP_FLUSH:
> - return sd_setup_flush_cmnd(cmd);
> + ret = sd_setup_flush_cmnd(cmd);
> + break;
> case REQ_OP_READ:
> case REQ_OP_WRITE:
> - return sd_setup_read_write_cmnd(cmd);
> + ret = sd_setup_read_write_cmnd(cmd);
> + break;
> case REQ_OP_ZONE_RESET:
> - return sd_zbc_setup_reset_cmnd(cmd);
> + ret = sd_zbc_setup_reset_cmnd(cmd);
> + break;
> default:
> WARN_ON_ONCE(1);
> - return BLK_STS_NOTSUPP;
> + ret = BLK_STS_NOTSUPP;
> + break;
> }
> +
> + if (!ret) {
> + sdkp = scsi_disk(rq->rq_disk);
> + get_device(&sdkp->dev);
> + }
> +
> + return ret;
> }
>
> static void sd_uninit_command(struct scsi_cmnd *SCpnt)
> {
> struct request *rq = SCpnt->request;
> u8 *cmnd;
> + struct scsi_disk *sdkp = NULL;
>
> if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
> mempool_free(rq->special_vec.bv_page, sd_page_pool);
> @@ -1295,6 +1317,8 @@ static void sd_uninit_command(struct scsi_cmnd *SCpnt)
> SCpnt->cmd_len = 0;
> mempool_free(cmnd, sd_cdb_pool);
> }
> + sdkp = scsi_disk(rq->rq_disk);
> + put_device(&sdkp->dev);
> }

Hi Jens and Christoph,

My interpretation of the above bug report and patch is that this is a
regression in the SCSI sd driver due to the switch from the legacy block
layer to scsi-mq. The above patch introduces two atomic operations in the
hot path and hence would introduce a performance regression. I think this
can be avoided by making sure that sd_uninit_command() gets called before
the request tag is freed. What changes would be required to make the block
layer core call sd_uninit_command() before the request tag is freed? Would
introducing prep_rq_fn and unprep_rq_fn callbacks in struct blk_mq_ops and
making sure that the SCSI core sets these callback function pointers
appropriately be sufficient? Would such a change allow to simplify the NVMe
initiator driver? Are there any alternatives to this approach that are more
elegant?

Thanks,

Bart.

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