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Date:   Fri, 2 Oct 2020 12:35:17 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Kalle Valo <kvalo@codeaurora.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: [RFC] Status of orinoco_usb
Message-ID: <20201002103517.fhsi5gaepzbzo2s4@linutronix.de>
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I was trying to get rid of the in in_softirq() in ezusb_req_ctx_wait()
within the orinoco usb driver,
drivers/net/wireless/intersil/orinoco/orinoco_usb.c. A small snippet:

| static void ezusb_req_ctx_wait(struct ezusb_priv *upriv,
|                                struct request_context *ctx)
=E2=80=A6
|                 if (in_softirq()) {
|                         /* If we get called from a timer, timeout timers =
don't
|                          * get the chance to run themselves. So we make s=
ure
|                          * that we don't sleep for ever */
|                         int msecs =3D DEF_TIMEOUT * (1000 / HZ);
|=20
|                         while (!try_wait_for_completion(&ctx->done) && ms=
ecs--)
|                                 udelay(1000);
|                 } else {
|                         wait_for_completion(&ctx->done);
=E2=80=A6
| }

This is broken. The EHCI and XHCI HCD will complete the URB in
BH/tasklet. Should we ever get here in_softirq() then we will spin
here/wait here until the timeout passes because the tasklet won't be
able to run. OHCI/UHCI HCDs still complete in hard-IRQ so it would work
here.

Is it possible to end up here in softirq context or is this a relic?
Well I have no hardware but I see this:

  orinoco_set_monitor_channel() [I assume that this is fully preemtible]
  -> orinoco_lock() [this should point to ezusb_lock_irqsave() which
                     does spin_lock_bh(lock), so from here on
		     in_softirq() returns true]
  -> hw->ops->cmd_wait() [-> ezusb_docmd_wait()]
  -> ezusb_alloc_ctx() [ sets ctx->in_rid to EZUSB_RID_ACK/0x0710 ]
  -> ezusb_access_ltv()
     -> if (ctx->in_rid)
       -> ezusb_req_ctx_wait(upriv, ctx);
	 -> ctx->state should be EZUSB_CTX_REQ_COMPLETE so we end up in
	    the while loop above. So we udelay() 3 * 1000 * 1ms =3D 3sec.
	 -> Then ezusb_access_ltv() should return with an error due to
	    timeout.

This isn't limited to exotic features like monitor mode. orinoco_open()
does orinoco_lock() followed by orinoco_hw_program_rids() which in the
end invokes ezusb_write_ltv(,, EZUSB_RID_ACK) which is non-zero and also
would block (ezusb_xmit() would use 0 as the last argument so it won't
block).

I don't see how this driver can work on EHCI/XHCI HCD as of today.
The driver is an orphan since commit
   3a59babbee409 ("orinoco: update status in MAINTAINERS")

which is ten years ago. If I replace in_softirq() with a `may_sleep'
argument then it is still broken.
Should it be removed?

Sebastian