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    Subject[patch] disable_bh/enable_bh race fix [Re: Program to freeze keyboard in 2.1.131]
    On Sat, 19 Dec 1998, George wrote:

    >#include <stdio.h>
    >#include <pthread.h>
    >#define NUM_THREADS 60
    >pthread_t teds[NUM_THREADS];
    >void *thread_run(void *data)
    > int x;
    > for (;;)
    > printf("%d:%d ", pthread_self(), ++x);
    >int main(void)
    > int i;
    > for (i = 0; i < NUM_THREADS; i++)
    > pthread_create(&teds[i], NULL, thread_run, NULL);
    > for (;;)
    > pause();
    > return 0;
    >Compiled with egcs 1.1.1 and 'gcc -O2 -o spam spam.c -lpthread'
    >Now, just pick your favorite virtual terminal and run this program. It'll
    >spit a bunch of numbers on the screen and shoot your load average up. Now
    >just hold down ALT+D or CTRL+C, or any other key combination that will
    >generate a lot of interrupts really fast. Watch the screen while holding
    >the keys down for a few minutes or so, 10 should be enough in most cases to
    >trigger the race.


    >Tested with 2.1.131-ac11 SMP and 'init=/bin/sh' with a 'vmstat 1 >
    >/dev/tty2' running in the background to give me something to attempt to
    >switch to. Tyan Tomcat IV, dual Pentium 233 MMX, RAM and hard drive
    >shouldn't matter in this test. Don't know if UP does it, but SMP does it
    >enough to irritate me if I run a program that causes a lot of output.
    >Previous kernels have done the same thing. AT keyboard and nothing
    >otherwise PS/2 attached motherboard.

    This evening I tried to reproduce and worked ;) and it' s also very easier
    to lockup the console in 2.1.132-3 and previous:

    main() { for (;;) printf("ciao\n"); }

    Running this proggy and pressing a character continously will cause the
    console to lockup.

    I' ll explain why:

    Both the console_bh (from the keyboard irq handler to print the
    pressed char) and the process (from sys_write() to print ciao),
    was running at the same time do_con_write() and so they was
    executing at the same time disable_bh(). Since bh_mask_count was
    not accessed in atomic way it was racing and at some time the
    CONSOLE_BH was disabled all the time because bh_mask_count[] was 1
    instead of 0 while nobody was writting to the console.

    As proof of my thought I added a enable_bh() in a SYS-RQ (sysrq
    still work even if CONSOLE_BH is disabled because they are handled
    by the raw keyboard-irq while the console is handled by console_bh
    that runs after the keyboard irq has full the queue with the
    incoming char) and I was able to restore perfectly from the
    console lockup using the SYS-RQ.

    The race was really very subtle...

    This my patch I developed in the last hours (ugh even if it' s very late
    seems to work, strange ;)) fixes perfectly the console race here.

    Patch against 2.1.132-3.

    Please try the patch and feedback ;)).

    Linus I don't know a lot about Copyright and laws, and I don' t know if
    it' s right to add it where I placed it and/or for the changes I done. I
    added it just to be sure that if somebody will want to change Linux
    licence, he will have also to ask to me so I' ll reply NO ;)

    The ugly thing of the patch is that all archs will have to be updated by
    DaveM and other friends...

    Index: linux/include/asm-i386/softirq.h
    diff -u linux/include/asm-i386/softirq.h: linux/include/asm-i386/softirq.h:
    --- linux/include/asm-i386/softirq.h: Fri Nov 20 00:01:22 1998
    +++ linux/include/asm-i386/softirq.h Tue Dec 22 01:34:47 1998
    @@ -12,7 +12,7 @@
    extern inline void init_bh(int nr, void (*routine)(void))
    bh_base[nr] = routine;
    - bh_mask_count[nr] = 0;
    + atomic_set(&bh_mask_count[nr], 0);
    bh_mask |= 1 << nr;

    @@ -97,13 +97,13 @@
    extern inline void disable_bh(int nr)
    bh_mask &= ~(1 << nr);
    - bh_mask_count[nr]++;
    + atomic_inc(&bh_mask_count[nr]);

    extern inline void enable_bh(int nr)
    - if (!--bh_mask_count[nr])
    + if (atomic_dec_and_test(&bh_mask_count[nr]))
    bh_mask |= 1 << nr;

    Index: linux/include/linux/interrupt.h
    diff -u linux/include/linux/interrupt.h: linux/include/linux/interrupt.h:
    --- linux/include/linux/interrupt.h: Thu Dec 17 16:38:10 1998
    +++ linux/include/linux/interrupt.h Tue Dec 22 01:34:48 1998
    @@ -17,7 +17,7 @@

    extern volatile unsigned char bh_running;

    -extern int bh_mask_count[32];
    +extern atomic_t bh_mask_count[32];
    extern unsigned long bh_active;
    extern unsigned long bh_mask;
    extern void (*bh_base[32])(void);
    Index: linux/kernel/softirq.c
    diff -u linux/kernel/softirq.c: linux/kernel/softirq.c:
    --- linux/kernel/softirq.c: Fri Nov 27 11:19:09 1998
    +++ linux/kernel/softirq.c Tue Dec 22 01:53:26 1998
    @@ -6,6 +6,9 @@
    * do_bottom_half() runs at normal kernel priority: all interrupts
    * enabled. do_bottom_half() is atomic with respect to itself: a
    * bottom_half handler need not be re-entrant.
    + *
    + * Fixed a disable_bh()/enable_bh() race (was causing a console lockup)
    + * due bh_mask_count not atomic handling. Copyright (C) 1998 Andrea Arcangeli

    #include <linux/mm.h>
    @@ -17,7 +20,7 @@

    /* intr_count died a painless death... -DaveM */

    -int bh_mask_count[32];
    +atomic_t bh_mask_count[32];
    unsigned long bh_active = 0;
    unsigned long bh_mask = 0;
    void (*bh_base[32])(void);

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