Re: [PATCH 0/2] tty: n_gsm: revert tx_mutex usage

Re: [PATCH 0/2] tty: n_gsm: revert tx_mutex usage

[Greg KH]

On Mon, Oct 24, 2022 at 01:30:23PM +0000, Starke, Daniel wrote:
> I have checked both patches. Those appear to be correct.
> Furthermore, I also recommend to bring in these reverts.

Can you send a "Reviewed-by:" tag for me to apply?  Hint, don't send it
in an HTML email so that lore.kernel.org will properly pick it up.

thanks,

greg k-h

Re: [PATCH v2] tty: n_gsm: avoid call of sleeping functions from atomic context

[Fedor Pchelkin]

On 05.10.2022 13:47, Daniel Starke wrote:
 > This patch breaks packet retransmission. Basically tx_lock and now 
tx_mutex
 > protects the transmission packet queue. This works fine as long as 
packets
 > are transmitted in a context that allows sleep. However, the 
retransmission
 > timer T2 is called from soft IRQ context and spans an additional atomic
 > context via control_lock within gsm_control_retransmit(). The call path
 > looks like this:
 > gsm_control_retransmit()
 >    spin_lock_irqsave(&gsm->control_lock, flags)
 >      gsm_control_transmit()
 >        gsm_data_queue()
 >          mutex_lock(&gsm->tx_mutex) // -> sleep in atomic context

As far as switching to tx_mutex turns out to have its own problems,
we suggest to revert it and to find another solution for the original
issue.

As it is described in commit 32dd59f ("tty: n_gsm: fix race condition in 
gsmld_write()"), the issue is that gsmld_write() may be used by the user 
directly and also by the n_gsm internal functions. But the proposed 
solution to add a spinlock around the low side tty write is not suitable 
since the tty write may sleep:

   gsmld_write(...)
    spin_lock_irqsave(&gsm->tx_lock, flags)
     tty->ops->write(...);
      con_write(...)
       do_con_write(...)
        console_lock()
         might_sleep() // -> bug

So let's consider alternative approaches to avoid the race condition.

We have found the only potential concurrency place:
gsm->tty->ops->write() in gsmld_output() and tty->ops->write() in
gsmld_write().

Is that right? Or there are some other cases?

On 05.10.2022 13:47, Daniel Starke wrote:
 > Long story short: The patch via mutex does not solve the issue. It is 
only
 > shifted to another function. I suggest splitting the TX lock into packet
 > queue lock and underlying tty write mutex.
 >
 > I would have implemented the patch if I had means to verify it.

Probably splitting the TX lock would be rather complex as there is
gsm_data_kick() which in this way has to be protected by packet queue
spinlock and at the same time it contains gsmld_output() (via
gsm_send_packet()) that would require mutex protection.

[PATCH 0/2] tty: n_gsm: revert tx_mutex usage

[Fedor Pchelkin]

As far as switching to tx_mutex turns out to have its own problems,
we suggest to revert it and to find another solution for the original
issue described in 902e02ea9385 ("tty: n_gsm: avoid call of sleeping
functions from atomic context").

Re: [PATCH 0/2] tty: n_gsm: revert tx_mutex usage

[Pavel Machek]

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Hi!

> As far as switching to tx_mutex turns out to have its own problems,
> we suggest to revert it and to find another solution for the original
> issue described in 902e02ea9385 ("tty: n_gsm: avoid call of sleeping
> functions from atomic context").

We have lived with locking problems for a long time, and we really
don't want data corruption.

Reviewed-by: Pavel Machek <pavel@denx.de>

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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