于 2014年07月21日 23:38, Greg KH 写道:
> On Mon, Jul 21, 2014 at 08:47:16PM +0800, pp wrote:
>> As reuse the cdev may cause panic. After we unregister the tty device, we
>> may use tty_hangup() o
>> other similar function to send a signal(SIGHUP) to process which has opend
>> our device. But that
>> not succeed if the process couldn't get the signal. for example, a process
>> forked
>> but his parent quited never get SIGHUP.
>>
>> Here is our scence.
>> tty driver register its device and init the cdevs, then process "A" open one
>> cdev.
>> tty driver unregister its device and cdev_del the cdevs, call tty_hangup to
>> (S)send signal SIGHUP to process A.
>> But that step(S) fails.
>
> How can that fail? What driver does this fail for?
hi, Greg
Thanks for your nice comments. :)
It's gsm driver that want to unregister/register tty device. We are
working on our intel mobile phone,
When the phone goes into airplane-mode, the modem will disconnect from system,
then gsmld_close() -> gsmld_detach_gsm() -> tty_unregister_device().
When the phone leaves airplane-mode, the modem will connect to system, then
gsmld_open() -> gsmld_attach_gsm() -> tty_register_device()
In this way how gsm driver works.
It seems very normal and can work well. :)
But there is always something bad for us to deal with.
If a process(A, its name) opens the /dev/gsmttyXX, and the process(A) is, for
example, running with command "A &".
The process(A) is not able to receive the signal SIGHUP from __tty_hangup() ->
tty_signal_session_leader().
There are several reasons that can stop process(A) from receiving signal
SIGHUP.
another example, B is running, and he makes a fork(), A is the child of B, then
B quit, leave A running.
in such scenario, A is not able to receive signal SIGHUP, either.
Anyway, we cannot guarantee process(A) will close /dev/gsmttyXX in time. That
means we don't know when we can reuse the tty_driver->cdevs[XX].
one second, one minute? We don't know. We just don't trust user space. :)
>
>> tty driver register its device and (D)init the cdevs again.
>
> What driver does this with an "old" device, it should have created a new
> one, otherwise, as you have pointed out, it's a bug.
>
I can't agree more with you. we should not use "old" device.
gsm driver(gsm_tty_driver) call tty_register_device() to export gsmttyXX to
userspace.
but tty core code manages cdevs by itself. cdevs is not directly used by any
other tty drivers. Adding a check in n_gsm.c before call tty_register_device()
seems
breaking a rule and making things complex in my opinion.
I also find a fact that most tty drivers only call tty_register_device() in
probe(), and call tty_unregister_device() in remove().
Q1:
Is this a rule, too? :)
I think my patch is not good enough.
Q2:
How about (1)change struct *cdevs to struct **cdevs, and (2)use cdev_alloc()
instead of cdev_init()? :)
Let char_dev core code manage the cdev. when the cdev's ref-count become zero,
cdev will be released by char_dev core code self.
I very appreciate for your comments. and your advices are very important and
helpful for me to solve this issue.
thanks,
xinhui
> thanks,
>
> greg k-h
>
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