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SubjectRe: [PATCH] usb: core: Don't wait for completion of urbs
On Tue, Oct 13, 2020 at 04:17:02PM +0530, Pratham Pratap wrote:
> Consider a case where host is trying to submit urbs to the
> connected device while holding the us->dev_mutex and due to
> some reason it is stuck while waiting for the completion of
> the urbs. Now the scsi error mechanism kicks in and it calls
> the device reset handler which is trying to acquire the same
> mutex causing a deadlock situation.
>
> Below is the call stack of the task which acquired the mutex
> (0xFFFFFFC660447460) and waiting for completion.
>
> B::v.f_/task_0xFFFFFFC6604DB280
> -000|__switch_to(prev = 0xFFFFFFC6604DB280, ?)
> -001|prepare_lock_switch(inline)
> -001|context_switch(inline)
> -001|__schedule(?)
> -002|schedule()
> -003|schedule_timeout(timeout = 9223372036854775807)
> -004|do_wait_for_common(x = 0xFFFFFFC660447570,
> action = 0xFFFFFF98ED5A7398, timeout = 9223372036854775807, ?)
> -005|spin_unlock_irq(inline)
> -005|__wait_for_common(inline)
> -005|wait_for_common(inline)
> -005|wait_for_completion(x = 0xFFFFFFC660447570)
> -006|sg_clean(inline)
> -006|usb_sg_wait()
> -007|atomic64_andnot(inline)
> -007|atomic_long_andnot(inline)
> -007|clear_bit(inline)
> -007|usb_stor_bulk_transfer_sglist(us = 0xFFFFFFC660447460,
> pipe = 3221291648, sg = 0xFFFFFFC65D6415D0, ?, length = 512,
> act_len = 0xFFFFFF801258BC90)

No need to line-wrap for stuff like this.



> -008|scsi_bufflen(inline)
> -008|usb_stor_bulk_srb(inline)
> -008|usb_stor_Bulk_transport(srb = 0xFFFFFFC65D641438,
> us = 0xFFFFFFC660447460)
> -009|test_bit(inline)
> -009|usb_stor_invoke_transport(srb = 0xFFFFFFC65D641438,
> us = 0xFFFFFFC660447460)
> -010|usb_stor_transparent_scsi_command(?, ?)
> -011|usb_stor_control_thread(__us = 0xFFFFFFC660447460) //us->dev_mutex
> -012|kthread(_create = 0xFFFFFFC6604C5E80)
> -013|ret_from_fork(asm)
> ---|end of frame
>
> Below is the call stack of the task which trying to acquire the same
> mutex(0xFFFFFFC660447460) in the error handling path.
>
> B::v.f_/task_0xFFFFFFC6609AA1C0
> -000|__switch_to(prev = 0xFFFFFFC6609AA1C0, ?)
> -001|prepare_lock_switch(inline)
> -001|context_switch(inline)
> -001|__schedule(?)
> -002|schedule()
> -003|schedule_preempt_disabled()
> -004|__mutex_lock_common(lock = 0xFFFFFFC660447460, state = 2, ?, ?, ?,
> ?, ?)
> -005|__mutex_lock_slowpath(?)
> -006|__cmpxchg_acq(inline)
> -006|__mutex_trylock_fast(inline)
> -006|mutex_lock(lock = 0xFFFFFFC660447460) //us->dev_mutex
> -007|device_reset(?)
> -008|scsi_try_bus_device_reset(inline)
> -008|scsi_eh_bus_device_reset(inline)
> -008|scsi_eh_ready_devs(shost = 0xFFFFFFC660446C80,
> work_q = 0xFFFFFF80191C3DE8, done_q = 0xFFFFFF80191C3DD8)
> -009|scsi_error_handler(data = 0xFFFFFFC660446C80)
> -010|kthread(_create = 0xFFFFFFC66042C080)
> -011|ret_from_fork(asm)
> ---|end of frame
>
> Fix this by adding 5 seconds timeout while waiting for completion.
>
> Fixes: 3e35bf39e (USB: fix codingstyle issues in drivers/usb/core/message.c)

Please read the documentation for how to properly add a Fixes: line
(hint, your sha1 isn't big enough.)

And does this really "fix" a commit that chnaged the coding style? I
doubt that...

> Cc: stable@vger.kernel.org
> Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
> ---
> drivers/usb/core/message.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
> index ae1de9c..b1e839c 100644
> --- a/drivers/usb/core/message.c
> +++ b/drivers/usb/core/message.c
> @@ -515,15 +515,13 @@ EXPORT_SYMBOL_GPL(usb_sg_init);
> */
> void usb_sg_wait(struct usb_sg_request *io)
> {
> - int i;
> + int i, retval;
> int entries = io->entries;
>
> /* queue the urbs. */
> spin_lock_irq(&io->lock);
> i = 0;
> while (i < entries && !io->status) {
> - int retval;
> -
> io->urbs[i]->dev = io->dev;
> spin_unlock_irq(&io->lock);
>
> @@ -569,7 +567,13 @@ void usb_sg_wait(struct usb_sg_request *io)
> * So could the submit loop above ... but it's easier to
> * solve neither problem than to solve both!
> */
> - wait_for_completion(&io->complete);
> + retval = wait_for_completion_timeout(&io->complete,
> + msecs_to_jiffies(5000));

Where did you pick 5 seconds from? Are you sure that will work
properly? What about devices with very long i/o stalls when data is
being flushed out, are you sure this will not trigger there?

> + if (retval == 0) {
> + dev_err(&io->dev->dev, "%s, timed out while waiting for io_complete\n",
> + __func__);
> + usb_sg_cancel(io);

So this is cancelled, but how does userspace know the error happened and
it was a timeout?

thanks,

greg k-h

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