[v8,3/3] printk: fix double printing with earlycon

Message ID 20170320100302.8656-1-aleksey.makarov@linaro.org
State New
Headers show
Series
  • Untitled series #796
Related show

Commit Message

Aleksey Makarov March 20, 2017, 10:03 a.m.
If a console was specified by ACPI SPCR table _and_ command line
parameters like "console=ttyAMA0" _and_ "earlycon" were specified,
then log messages appear twice.

The root cause is that the code traverses the list of specified
consoles (the `console_cmdline` array) and stops at the first match.
But it may happen that the same console is referred by the elements
of this array twice:

	pl011,mmio,0x87e024000000,115200 -- from SPCR
	ttyAMA0 -- from command line

but in this case `preferred_console` points to the second entry and
the flag CON_CONSDEV is not set, so bootconsole is not deregistered.

To fix that, introduce an invariant "The last non-braille console
is always the preferred one" on the entries of the console_cmdline
array.  Then traverse it in reverse order to be sure that if
the console is preferred then it will be the first matching entry.

Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@linaro.org>

---

v7 -> v8:
- add an explanation to the comment how console_cmdline can contain entries
  referring to the same console
- move the body of the function introduced in the previous version to cycle
- don't panic() (Sergey Senozhatsky).  Don't check this condition because
  the loop condition guarantees that it holds.
- use swap() (Sergey Senozhatsky)

v6 -> v7:
- return back to v5 
- leave the check for already registered entries and add a function that
  maintains the invariant.

v5 -> v6:
- drop v5 and continue to work on v4:
- introduce _braille_is_braille_console(). It helps to split original loop
  into three parts: 1) search for braille console, 2) check for
  preferred_console, 3) match other entries so that these three parts do not
  intersect.
- introduce for_each_console_cmdline() macros to traverse console_cmdline
  (Petr Mladek)

 kernel/printk/printk.c | 49 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 42 insertions(+), 7 deletions(-)

-- 
2.12.0

Comments

Petr Mladek March 28, 2017, 12:56 p.m. | #1
On Tue 2017-03-28 11:04:04, Sergey Senozhatsky wrote:
> On (03/27/17 19:28), Aleksey Makarov wrote:

> [..]

> > > > +			/*

> > > > +			 * Maintain an invariant that will help to find if

> > > > +			 * the matching console is preferred, see

> > > > +			 * register_console():

> > > > +			 *

> > > > +			 * The last non-braille console is always

> > > > +			 * the preferred one.

> > > > +			 */

> > > > +			for (last = MAX_CMDLINECONSOLES - 1;

> > > > +			     last >= 0 && !console_cmdline[last].name[0];

> > > > +			     last--)

> > > > +				;

> > > 

> > > This is a rather non-trivial code to find the last element.

> > > I might make sense to count it in a global variable.

> > > Then we might remove the check for console_cmdline[i].name[0]

> > > also in the other for cycles and make them better readable.

> > 

> > Having an additional variable console_cmdline_last pointing to the last element

> > would require maintaining consistency between this variable and

> > contents of console_cmdline.  For the code we have it is not hard, but when code

> > is changed we need to check this.  Also there exists preferred_console that

> > has almost the same meaning but it points not to the last element, but to the

> > last non-braille element.  Also we need to have a special value (-1) for this

> > variable for empty array. So I personally would instead try to rewrite this:

> > 

> > 	for (last = MAX_CMDLINECONSOLES - 1; last >= 0; last--)

> > 		if (console_cmdline[last].name[0])

> > 			break;

> > 

> > Is it better?  If not, I will send a version with console_cmdline_last.

> 

> personally I'm fine with the nested loop. the latest version

> 	"for (last = MAX_CMDLINECONSOLES - 1; last >= 0;..."

> 

> is even easier to read.


The number of elements is bumped on a single location, so there
is not much to synchronize. The old approach was fine because
the for cycles were needed anyway, they started on the 0th element,
and NULL ended arrays are rather common practice.

But we are searching the array from the end now. Also we use the
for cycle just to get the number here. This is not a common
practice and it makes the code more complicated and strange from
my point of view.

If you do not like -1, you could use console_cmdline_cnt and
start with 0. I would actually do so because it is a common
approach and easy to understand.

> 

> so we do not just iterate console_cmdline anymore, but also modify it.

> this, probably, has impact on the following scenario

> 

> CPU0						CPU1

> 

> add_preferred_console()			add_preferred_console()

>   __add_preferred_console()		  __add_preferred_console()

>       swap(i1, last)			    swap(i2, last)

> 

> 	temp1 = i1

> 	i1 = last				temp2 = i2

> 	last = temp1				i2 = last

> 						last = temp2

> 

> so both i1 and i2 will point to 'last' now, IOW, we will have two

> identical entries in console_cmdline, while i1 or i2 will be lost.

> 

> 

> neither add_preferred_console() nor __add_preferred_console() have any

> serialization. and I assume that we can call add_preferred_console()

> concurrently, can't we?


Very good point!

Well, if this race exists, it was racy even before. Of course,
the old race was only when new entry was added. It would
be more visible now because also shuffling would be racy.

OK, most add_preferred_console() calls are in functions
that are defined as console_initcall(). They seem to
be defined only when the respective drivers are built in.
It seems that these initcalls are serialized, see console_init().

add_preferred_console() is used also in uart_add_one_port():

   -> uart_add_one_port()
    -> of_console_check()
     -> add_preferred_console()

But there the calls are synchronized as well via
port_mutex.

Finally, __add_preferred_console() is called also from
console_setup(). It is called via do_early_param()
even before the console_initcall() functions. It is
serialized as well.

If I did not miss anything, it would seem that
__add_preferred_console() are called synchronously
and only during boot by design.

Best Regards,
Petr

Patch hide | download patch | download mbox

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fd752f0c8ef1..462036e7a767 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1909,8 +1909,28 @@  static int __add_preferred_console(char *name, int idx, char *options,
 	     i < MAX_CMDLINECONSOLES && c->name[0];
 	     i++, c++) {
 		if (strcmp(c->name, name) == 0 && c->index == idx) {
-			if (!brl_options)
-				preferred_console = i;
+			int last;
+
+			if (brl_options)
+				return 0;
+
+			/*
+			 * Maintain an invariant that will help to find if
+			 * the matching console is preferred, see
+			 * register_console():
+			 *
+			 * The last non-braille console is always
+			 * the preferred one.
+			 */
+			for (last = MAX_CMDLINECONSOLES - 1;
+			     last >= 0 && !console_cmdline[last].name[0];
+			     last--)
+				;
+
+			if (i != last)
+				swap(console_cmdline[i], console_cmdline[last]);
+
+			preferred_console = last;
 			return 0;
 		}
 	}
@@ -2457,12 +2477,27 @@  void register_console(struct console *newcon)
 	}
 
 	/*
-	 *	See if this console matches one we selected on
-	 *	the command line.
+	 * See if this console matches one we selected on the command line.
+	 *
+	 * There may be several entries in the console_cmdline array matching
+	 * with the same console, one with newcon->match(), another by
+	 * name/index:
+	 *
+	 *	pl011,mmio,0x87e024000000,115200 -- added from SPCR
+	 *	ttyAMA0 -- added from command line
+	 *
+	 * Traverse the console_cmdline array in reverse order to be
+	 * sure that if this console is preferred then it will be the first
+	 * matching entry.  We use the invariant that is maintained in
+	 * __add_preferred_console().
 	 */
-	for (i = 0, c = console_cmdline;
-	     i < MAX_CMDLINECONSOLES && c->name[0];
-	     i++, c++) {
+	for (i = MAX_CMDLINECONSOLES - 1; i >= 0; i--) {
+
+		if (!console_cmdline[i].name[0])
+			continue;
+
+		c = console_cmdline + i;
+
 		if (!newcon->match ||
 		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
 			/* default matching */