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Subject[PATCH] Linux 0.01 disk lockup
Hi.

Linux 0.01 has a bug in disk request sorting - when interrupt happens
while sorting is active, the interrupt routine won't clear do_hd - thus
the disk will stay locked up forever.

Function add_request also lacks memory barriers - the compiler could
reorder writes to variable sorting and writes to request queue - producing
race conditions. Because gcc 1.40 does not have __asm__("":::"memory"), I
had to use dummy function call as a memory barrier.

The following patch fixes both issues.

Mikulas

diff -u -r linux-orig/kernel/hd.c linux/kernel/hd.c
--- linux-orig/kernel/hd.c Tue Sep 17 17:05:21 1991
+++ linux/kernel/hd.c Thu Sep 27 15:14:38 2001
@@ -55,9 +55,9 @@
((s1)->head<(s2)->head || (s1)->head==(s2)->head && \
((s1)->sector<(s2)->sector))))

-static struct hd_request * this_request = NULL;
+struct hd_request * this_request = NULL;

-static int sorting=0;
+int sorting=0;

static void do_request(void);
static void reset_controller(void);
@@ -293,8 +293,10 @@
{
int i,r;

- if (sorting)
+ if (sorting) {
+ do_hd=NULL;
return;
+ }
if (!this_request) {
do_hd=NULL;
return;
@@ -319,6 +321,8 @@
panic("unknown hd-command");
}

+void barrier();
+
/*
* add-request adds a request to the linked list.
* It sets the 'sorting'-variable when doing something
@@ -338,6 +342,7 @@
* disabling interrupts.
*/
sorting=1;
+ barrier();
if (!(tmp=this_request))
this_request=req;
else {
@@ -354,15 +359,19 @@
tmp->next=req;
}
}
+ barrier();
sorting=0;
+ barrier();
/*
* NOTE! As a result of sorting, the interrupts may have died down,
* as they aren't redone due to locking with sorting=1. They might
* also never have started, if this is the first request in the queue,
* so we restart them if necessary.
*/
- if (!do_hd)
+ if (!do_hd) {
+ barrier();
do_request();
+ }
}

void rw_abs_hd(int rw,unsigned int nr,unsigned int sec,unsigned int head,
diff -u -r linux-orig/kernel/system_call.s linux/kernel/system_call.s
--- linux-orig/kernel/system_call.s Tue Sep 17 17:50:52 1991
+++ linux/kernel/system_call.s Thu Sep 27 14:59:37 2001
@@ -47,7 +47,7 @@

nr_system_calls = 67

-.globl _system_call,_sys_fork,_timer_interrupt,_hd_interrupt,_sys_execve
+.globl _system_call,_sys_fork,_timer_interrupt,_hd_interrupt,_sys_execve,_barrier

.align 2
bad_sys_call:
@@ -186,6 +186,9 @@
call _copy_process
addl $20,%esp
1: ret
+
+_barrier:
+ ret

_hd_interrupt:
pushl %eax
-
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