Messages in this thread | | | Date | Thu, 28 May 2020 16:57:22 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH v3 4/4] kdb: Switch kdb_msg_write() to use safer polling I/O |
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On Thu 2020-05-28 12:26:20, Daniel Thompson wrote: > On Thu, May 28, 2020 at 11:48:48AM +0530, Sumit Garg wrote: > > On Wed, 27 May 2020 at 19:01, Daniel Thompson > > <daniel.thompson@linaro.org> wrote: > > > > > > On Wed, May 27, 2020 at 11:55:59AM +0530, Sumit Garg wrote: > > > > In kgdb NMI context, calling console handlers isn't safe due to locks > > > > used in those handlers which could lead to a deadlock. Although, using > > > > oops_in_progress increases the chance to bypass locks in most console > > > > handlers but it might not be sufficient enough in case a console uses > > > > more locks (VT/TTY is good example). > > > > > > > > Currently when a driver provides both polling I/O and a console then kdb > > > > will output using the console. We can increase robustness by using the > > > > currently active polling I/O driver (which should be lockless) instead > > > > of the corresponding console. For several common cases (e.g. an > > > > embedded system with a single serial port that is used both for console > > > > output and debugger I/O) this will result in no console handler being > > > > used. > > > > > > > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h > > > > index b072aeb..05d165d 100644 > > > > --- a/include/linux/kgdb.h > > > > +++ b/include/linux/kgdb.h > > > > @@ -275,6 +275,7 @@ struct kgdb_arch { > > > > * for the I/O driver. > > > > * @is_console: 1 if the end device is a console 0 if the I/O device is > > > > * not a console > > > > + * @tty_drv: Pointer to polling tty driver. > > > > */ > > > > struct kgdb_io { > > > > const char *name; > > > > @@ -285,6 +286,7 @@ struct kgdb_io { > > > > void (*pre_exception) (void); > > > > void (*post_exception) (void); > > > > int is_console; > > > > + struct tty_driver *tty_drv; > > > > > > Should this be a struct tty_driver or a struct console? > > > > > > In other words if the lifetime the console structure is the same as the > > > tty_driver then isn't it better to capture the console instead > > > (easier to compare and works with non-tty devices such as the > > > USB debug mode). > > > > > > > IIUC, you mean to say we can easily replace "is_console" with "struct > > console". This sounds feasible and should be a straightforward > > comparison in order to prefer "dbg_io_ops" over console handlers. So I > > will switch to use "struct console" instead. > > My comment contains an if ("if the lifetime of the console structure is > the same") so you need to check that it is true before sharing a patch to > make the change.
Honestly, I am not completely familiar with the console an tty drivers code.
Anyway, struct console is typically statically defined by the console driver code. It is not must to have but I am not aware of any driver where it would be dynamically defined.
On the other hand, struct tty_driver is dynamically allocated when the driver gets initialized.
So I would say that it is pretty safe to store struct console. Well, you need to call con->device() to see if the tty_driver is actually initialized.
Best Regards, Petr
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