Re: [PATCH v2] virtio_blk: implement init_hctx MQ operation

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Hi Max,

On 17.09.2024 00:06, Max Gurtovoy wrote:
>
> On 12/09/2024 9:46, Marek Szyprowski wrote:
>> Dear All,
>>
>> On 08.08.2024 00:41, Max Gurtovoy wrote:
>>> Set the driver data of the hardware context (hctx) to point directly to
>>> the virtio block queue. This cleanup improves code readability and
>>> reduces the number of dereferences in the fast path.
>>>
>>> Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
>>> Signed-off-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
>>> ---
>>>    drivers/block/virtio_blk.c | 42 
>>> ++++++++++++++++++++------------------
>>>    1 file changed, 22 insertions(+), 20 deletions(-)
>> This patch landed in recent linux-next as commit 8d04556131c1
>> ("virtio_blk: implement init_hctx MQ operation"). In my tests I found
>> that it introduces a regression in system suspend/resume operation. From
>> time to time system crashes during suspend/resume cycle. Reverting this
>> patch on top of next-20240911 fixes this problem.
>
> Could you please provide a detailed explanation of the system 
> suspend/resume operation and the specific testing methodology employed?

In my tests I just call the 'rtcwake -s10 -mmem' command many times in a 
loop. I use standard Debian image under QEMU/ARM64. Nothing really special.

>
> The occurrence of a kernel panic from this commit is unexpected, given 
> that it primarily involves pointer reassignment without altering the 
> lifecycle of vblk/vqs.
>
> In the virtqueue_add_split function, which pointer is becoming null 
> and causing the issue? A detailed analysis would be helpful.
>
> The report indicates that the crash occurs sporadically rather than 
> consistently.
>
> is it possible that this is a race condition introduced by a different 
> commit? How can we rule out this possibility?
This is the commit pointed by bisecting between v6.11-rc1 and 
next-20240911. The problem is reproducible, it just need a few calls to 
the rtcwake command.
>
> Prior to applying this commit, what were the test results? 
> Specifically, out of 100 test runs, how many passed successfully?

All 100 were successful, see https://pastebin.com/3yETvXK9 (kernel is 
compiled from 6d17035a7402, which is a parent of $subject in linux-next).

>
> After applying this commit, what are the updated test results? Again, 
> out of 100 test runs, how many passed successfully?

Usually it freezes or panics after the second try, see 
https://pastebin.com/u5n9K1Dz (kernel compiled from 8d04556131c1, which 
is $subject in linux-next).

>
>>
>> I've even managed to catch a kernel panic log of this problem on QEMU's
>> ARM64 'virt' machine:
>>
>> root@target:~# time rtcwake -s10 -mmem
>> rtcwake: wakeup from "mem" using /dev/rtc0 at Thu Sep 12 07:11:52 2024
>> Unable to handle kernel NULL pointer dereference at virtual address
>> 0000000000000090
>> Mem abort info:
>>     ESR = 0x0000000096000046
>>     EC = 0x25: DABT (current EL), IL = 32 bits
>>     SET = 0, FnV = 0
>>     EA = 0, S1PTW = 0
>>     FSC = 0x06: level 2 translation fault
>> Data abort info:
>>     ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000
>>     CM = 0, WnR = 1, TnD = 0, TagAccess = 0
>>     GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>> user pgtable: 4k pages, 48-bit VAs, pgdp=0000000046bbb000
>> ...
>> Internal error: Oops: 0000000096000046 [#1] PREEMPT SMP
>> Modules linked in: bluetooth ecdh_generic ecc rfkill ipv6
>> CPU: 0 UID: 0 PID: 9 Comm: kworker/0:0H Not tainted 6.11.0-rc6+ #9024
>> Hardware name: linux,dummy-virt (DT)
>> Workqueue: kblockd blk_mq_requeue_work
>> pstate: 800000c5 (Nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> pc : virtqueue_add_split+0x458/0x63c
>> lr : virtqueue_add_split+0x1d0/0x63c
>> ...
>> Call trace:
>>    virtqueue_add_split+0x458/0x63c
>>    virtqueue_add_sgs+0xc4/0xec
>>    virtblk_add_req+0x8c/0xf4
>>    virtio_queue_rq+0x6c/0x1bc
>>    blk_mq_dispatch_rq_list+0x21c/0x714
>>    __blk_mq_sched_dispatch_requests+0xb4/0x58c
>>    blk_mq_sched_dispatch_requests+0x30/0x6c
>>    blk_mq_run_hw_queue+0x14c/0x40c
>>    blk_mq_run_hw_queues+0x64/0x124
>>    blk_mq_requeue_work+0x188/0x1bc
>>    process_one_work+0x20c/0x608
>>    worker_thread+0x238/0x370
>>    kthread+0x124/0x128
>>    ret_from_fork+0x10/0x20
>> Code: f9404282 79401c21 b9004a81 f94047e1 (f8206841)
>> ---[ end trace 0000000000000000 ]---
>> note: kworker/0:0H[9] exited with irqs disabled
>> note: kworker/0:0H[9] exited with preempt_count 1
>>
>>
>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>> index 2351f411fa46..35a7a586f6f5 100644
>>> --- a/drivers/block/virtio_blk.c
>>> +++ b/drivers/block/virtio_blk.c
>>> @@ -129,14 +129,6 @@ static inline blk_status_t virtblk_result(u8 
>>> status)
>>>        }
>>>    }
>>>    -static inline struct virtio_blk_vq *get_virtio_blk_vq(struct 
>>> blk_mq_hw_ctx *hctx)
>>> -{
>>> -    struct virtio_blk *vblk = hctx->queue->queuedata;
>>> -    struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
>>> -
>>> -    return vq;
>>> -}
>>> -
>>>    static int virtblk_add_req(struct virtqueue *vq, struct 
>>> virtblk_req *vbr)
>>>    {
>>>        struct scatterlist out_hdr, in_hdr, *sgs[3];
>>> @@ -377,8 +369,7 @@ static void virtblk_done(struct virtqueue *vq)
>>>       static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
>>>    {
>>> -    struct virtio_blk *vblk = hctx->queue->queuedata;
>>> -    struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
>>> +    struct virtio_blk_vq *vq = hctx->driver_data;
>>>        bool kick;
>>>           spin_lock_irq(&vq->lock);
>>> @@ -428,10 +419,10 @@ static blk_status_t virtio_queue_rq(struct 
>>> blk_mq_hw_ctx *hctx,
>>>                   const struct blk_mq_queue_data *bd)
>>>    {
>>>        struct virtio_blk *vblk = hctx->queue->queuedata;
>>> +    struct virtio_blk_vq *vq = hctx->driver_data;
>>>        struct request *req = bd->rq;
>>>        struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
>>>        unsigned long flags;
>>> -    int qid = hctx->queue_num;
>>>        bool notify = false;
>>>        blk_status_t status;
>>>        int err;
>>> @@ -440,26 +431,26 @@ static blk_status_t virtio_queue_rq(struct 
>>> blk_mq_hw_ctx *hctx,
>>>        if (unlikely(status))
>>>            return status;
>>>    -    spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
>>> -    err = virtblk_add_req(vblk->vqs[qid].vq, vbr);
>>> +    spin_lock_irqsave(&vq->lock, flags);
>>> +    err = virtblk_add_req(vq->vq, vbr);
>>>        if (err) {
>>> -        virtqueue_kick(vblk->vqs[qid].vq);
>>> +        virtqueue_kick(vq->vq);
>>>            /* Don't stop the queue if -ENOMEM: we may have failed to
>>>             * bounce the buffer due to global resource outage.
>>>             */
>>>            if (err == -ENOSPC)
>>>                blk_mq_stop_hw_queue(hctx);
>>> -        spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
>>> +        spin_unlock_irqrestore(&vq->lock, flags);
>>>            virtblk_unmap_data(req, vbr);
>>>            return virtblk_fail_to_queue(req, err);
>>>        }
>>>    -    if (bd->last && virtqueue_kick_prepare(vblk->vqs[qid].vq))
>>> +    if (bd->last && virtqueue_kick_prepare(vq->vq))
>>>            notify = true;
>>> -    spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
>>> +    spin_unlock_irqrestore(&vq->lock, flags);
>>>           if (notify)
>>> -        virtqueue_notify(vblk->vqs[qid].vq);
>>> +        virtqueue_notify(vq->vq);
>>>        return BLK_STS_OK;
>>>    }
>>>    @@ -504,7 +495,7 @@ static void virtio_queue_rqs(struct request 
>>> **rqlist)
>>>        struct request *requeue_list = NULL;
>>>           rq_list_for_each_safe(rqlist, req, next) {
>>> -        struct virtio_blk_vq *vq = get_virtio_blk_vq(req->mq_hctx);
>>> +        struct virtio_blk_vq *vq = req->mq_hctx->driver_data;
>>>            bool kick;
>>>               if (!virtblk_prep_rq_batch(req)) {
>>> @@ -1164,6 +1155,16 @@ static const struct attribute_group 
>>> *virtblk_attr_groups[] = {
>>>        NULL,
>>>    };
>>>    +static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void 
>>> *data,
>>> +        unsigned int hctx_idx)
>>> +{
>>> +    struct virtio_blk *vblk = data;
>>> +    struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
>>> +
>>> +    hctx->driver_data = vq;
>>> +    return 0;
>>> +}
>>> +
>>>    static void virtblk_map_queues(struct blk_mq_tag_set *set)
>>>    {
>>>        struct virtio_blk *vblk = set->driver_data;
>>> @@ -1205,7 +1206,7 @@ static void virtblk_complete_batch(struct 
>>> io_comp_batch *iob)
>>>    static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct 
>>> io_comp_batch *iob)
>>>    {
>>>        struct virtio_blk *vblk = hctx->queue->queuedata;
>>> -    struct virtio_blk_vq *vq = get_virtio_blk_vq(hctx);
>>> +    struct virtio_blk_vq *vq = hctx->driver_data;
>>>        struct virtblk_req *vbr;
>>>        unsigned long flags;
>>>        unsigned int len;
>>> @@ -1236,6 +1237,7 @@ static const struct blk_mq_ops virtio_mq_ops = {
>>>        .queue_rqs    = virtio_queue_rqs,
>>>        .commit_rqs    = virtio_commit_rqs,
>>>        .complete    = virtblk_request_done,
>>> +    .init_hctx    = virtblk_init_hctx,
>>>        .map_queues    = virtblk_map_queues,
>>>        .poll        = virtblk_poll,
>>>    };
>> Best regards
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland





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