Linux-kernel mailing list archive 1999-23,: reentrancy of console drivers - source of problems found (?)

# reentrancy of console drivers - source of problems found (?)

Petr Vandrovec (vandrove@vc.cvut.cz)
Fri, 11 Jun 1999 12:52:30 +0200

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--X1bOJ3K7DJ5YkBrT
Content-Type: text/plain; charset=us-ascii
Content-Description: console.c problems

Hi,
on Monday I announced that there is reentrancy bug in console subsystem -
- putc procedure gets reentered. After some investigation I found that this
comes from problem that '*_cursor' can now cause screen redraw after JJ
scrollback patches. So I applied this fix:

diff -urN linux/drivers/char/console.c linux/drivers/char/console.c
--- linux/drivers/char/console.c Tue May 11 23:37:40 1999
+++ linux/drivers/char/console.c Thu Jun 10 22:45:32 1999
@@ -503,6 +503,9 @@
{
if (!IS_FG || console_blanked || vcmode == KD_GRAPHICS)
return;
+
+ disable_bh(CONSOLE_BH);
+
if (deccm) {
if (currcons == sel_cons)
clear_selection();
@@ -511,6 +514,8 @@
sw->con_cursor(vc_cons[currcons].d,CM_DRAW);
} else
hide_cursor(currcons);
+
+ enable_bh(CONSOLE_BH);
}
static void set_origin(int currcons)
@@ -1809,11 +1814,12 @@
himask = hi_font_mask;
charmask = himask ? 0x1ff : 0xff;
+ disable_bh(CONSOLE_BH);
+
/* undraw cursor first */
if (IS_FG)
hide_cursor(currcons);
- disable_bh(CONSOLE_BH);
while (!tty->stopped && count) {
enable_bh(CONSOLE_BH);
if (from_user)
\ End

over my 2.3.5 kernel... After that I was not able to cause reentrancy of
matroxfb - but I cannot say that drivers/char/console.c is now safe - there
is couple of calls to *_cursor, some of them are guarded by different console
locks and by disable_bh, some of them maybe are not - Martin, could you verify
that all calls to hide_cursor() are guarded by something on upper level?

When I was debugging it, I got one strange reentrancy:

CPU: 0
EIP: 0010:[printstate+9/48]
EFLAGS: 00000202
eax: 00000001 ebx: 00000001 ecx: 00000008 edx: 0000007f
esi: 00000000 edi: 00000010 ebp: 00000010 esp: c4ed5c3c
ds: 0018 es: 0018 ss: 0018
Process x (pid: 259, process nr: 27, stackpage=c4ed5000)
Stack: c01a6de1 c01a8049 c0231d00 00aaaaaa c7fe0000 c0008000 00000010 00000050
00000008 c0008000 00000001 00160000 0000007f 00000010 00500010 00000001
c01a83d3 00aaaaaa 00000000 c0231d00 c7fe0000 0000000a 00000005 00000000
Call Trace: [guard_start+33/48] [matrox_cfbX_putcs+201/720]
[matrox_cfb32_putcs+99/112] [fbcon_redraw_softback+503/720]
[fbcon_scrolldelta+355/704] [do_8259A_IRQ+141/208]
[console_bh+246/272] [do_bottom_half+153/192] [do_IRQ+82/96]
[ret_from_intr+0/32] [vfat:vfat_lookup+-8754800/288]
[vfat:vfat_lookup+-8754800/288] [vfat:vfat_lookup+-8754776/288]
[matrox_cfbX_putc+272/880] [matrox_cfb32_putc+90/112]
[fbcon_putc+192/224] [add_softcursor+198/208] [set_cursor+79/128]
[con_flush_chars+18/32] [con_write+35/48] [opost_block+395/416]
[console_bh+186/272] [do_bottom_half+153/192]
[do_IRQ+82/96] [set_cursor+79/128] [write_chan+604/880]
[tty_write+550/752] [write_chan+0/880] [sys_write+315/432]
[tty_write+0/752] [system_call+52/64]
Code: 50 1e 06 50 55 57 56 52 51 53 89 e0 50 e8 65 fd ff ff 83 c4

I see console_bh TWICE in this list - but it was UNIPROCESSOR machine
(with SMP kernel), so I cannot imagine, what could cause that. After
I added disable_bh() to set_cursor, I cannot repeat it. I added couple
of checks (cpu_bh_endlock test whether local_cpu_count == 1, check whether
smp_processor_id() returns always 0 on my uni box...) but they were not
trigerred, so I do not know, what happened.

I'm sure that patch is correct, but maybe that it is not complete. It should
be applied to both 2.3.6 and 2.2.10, I think.
Best regards,
Petr Vandrovec
vandrove@vc.cvut.cz

--X1bOJ3K7DJ5YkBrT
Content-Type: text/plain; charset=us-ascii
Content-Description: drivers/char/console.c reentrancy patch
Content-Disposition: attachment; filename="2.3.5fix"

diff -urN linux/drivers/char/console.c linux/drivers/char/console.c
--- linux/drivers/char/console.c Tue May 11 23:37:40 1999
+++ linux/drivers/char/console.c Thu Jun 10 22:45:32 1999
@@ -503,6 +503,9 @@
{
if (!IS_FG || console_blanked || vcmode == KD_GRAPHICS)
return;
+
+ disable_bh(CONSOLE_BH);
+
if (deccm) {
if (currcons == sel_cons)
clear_selection();
@@ -511,6 +514,8 @@
sw->con_cursor(vc_cons[currcons].d,CM_DRAW);
} else
hide_cursor(currcons);
+
+ enable_bh(CONSOLE_BH);
}
static void set_origin(int currcons)
@@ -1809,11 +1814,12 @@
himask = hi_font_mask;
charmask = himask ? 0x1ff : 0xff;
+ disable_bh(CONSOLE_BH);
+
/* undraw cursor first */
if (IS_FG)
hide_cursor(currcons);
- disable_bh(CONSOLE_BH);
while (!tty->stopped && count) {
enable_bh(CONSOLE_BH);
if (from_user)

--X1bOJ3K7DJ5YkBrT--

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