fix for 2.6 scheduler crashes

The patch below fixes the 2.6 scheduler crash. It's another subtle signal problem. This one is caused by siglongjmp restoring the old signal mask before calling longjmp. See the comment for the gory details. I've had the stress test running for more than an hour with this patch. Without it, it would be lucky to go for five minutes.

Jeff

===== arch/um/include/signal_user.h 1.1 vs edited =====

@@ -11,6 +11,8 @@
 extern int change_sig(int signal, int on);
 extern void set_sigstack(void *stack, int size);
 extern void set_handler(int sig, void (*handler)(int), int flags, ...);
+extern int set_signals(int enable);
+extern int get_signals(void);
 
 #endif
 

===== arch/um/kernel/time_kern.c 1.12 vs edited =====

@@ -102,10 +102,12 @@
 
 irqreturn_t um_timer(int irq, void *dev, struct pt_regs *regs)
 {
+       unsigned long flags;
+
        do_timer(regs);
-       write_seqlock_irq(&xtime_lock);
+       write_seqlock_irqsave(&xtime_lock, flags);
        timer();
-       write_sequnlock_irq(&xtime_lock);
+       write_sequnlock_irqrestore(&xtime_lock, flags);
        return(IRQ_HANDLED);
 }
 

===== arch/um/kernel/skas/process.c 1.9 vs edited =====

@@ -188,13 +188,25 @@
 void new_thread(void *stack, void **switch_buf_ptr, void **fork_buf_ptr,
                void (*handler)(int))
 {
+       unsigned long flags;
        jmp_buf switch_buf, fork_buf;
 
        *switch_buf_ptr = &switch_buf;
        *fork_buf_ptr = &fork_buf;
 
+       /* Somewhat subtle - siglongjmp restores the signal mask before doing
+        * the longjmp.  This means that when jumping from one stack to another
+        * when the target stack has interrupts enabled, an interrupt may occur
+        * on the source stack.  This is bad when starting up a process because
+        * it's not supposed to get timer ticks until it has been scheduled.
+        * So, we disable interrupts around the sigsetjmp to ensure that 
+        * they can't happen until we get back here where they are safe.
+        */
+       flags = get_signals();
+       block_signals();
        if(sigsetjmp(fork_buf, 1) == 0)
                new_thread_proc(stack, handler);
+       set_signals(flags);
 
        remove_sigstack();
 }

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Good afternoon, Jeff,

Many thanks for the work!
I've got one vm with 25 copies of "while /bin/true ; do /bin/true
; done &" running and me repeatedly logging in over ssh from the outside world; the load is high, but no crashes. The system even stays responsive.
Only 25 minutes of testing so far, but it looks good. I'd like to
give a tentative thumbs up.
For others hoping to try this patch out, you'll need to use the
"-l" parameter to patch to apply this; the tabs in the actual source were replaced with 8 spaces in Jeff's patch.
If you'd simply like a precompiled binary, the one I'm testing is
at http://www.stearns.org/uml/ ; the config, System.map, and linux all end with norecurs-patch2{,.bz2}.
Cheers,
- Bill

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