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SubjectRe: 2.6.0-test4: bug WRT lazy FPU switching and CONFIG_PREEMPT
Claus-Justus Heine wrote:

[snip]
> diff -u --recursive --new-file linux-2.6.0-test4/include/asm-i386/i387.h linux-2.6.0-test4-mine/include/asm-i386/i387.h
[snip]
> --- linux-2.6.0-test4/include/asm-i386/i387.h 2003-07-27 19:06:54.000000000 +0200
> +++ linux-2.6.0-test4-mine/include/asm-i386/i387.h 2003-08-31 15:39:04.000000000 +0200
> @@ -30,7 +30,7 @@
> static inline void __save_init_fpu( struct task_struct *tsk )
> {
> if ( cpu_has_fxsr ) {
> - asm volatile( "fxsave %0 ; fnclex"
> + asm volatile( "fxsave %0 ; fclex"

I'm sorry, changing "fnclex" to "fclex" was a stupid idea, can lead to deadlocks. So that
patch should have looked like this:

#########################################################################################################################
diff -u --recursive --new-file linux-2.6.0-test4/arch/i386/kernel/traps.c linux-2.6.0-test4-mine/arch/i386/kernel/traps.c
--- linux-2.6.0-test4/arch/i386/kernel/traps.c 2003-08-31 15:22:42.000000000 +0200
+++ linux-2.6.0-test4-mine/arch/i386/kernel/traps.c 2003-08-31 15:35:49.000000000 +0200
@@ -605,7 +605,10 @@
* Save the info for the exception handler and clear the error.
*/
task = current;
- save_init_fpu(task);
+ /* don't trigger an unnecessary math_state_restore() */
+ if (task->thread_info->status & TS_USEDFPU) {
+ save_init_fpu(task);
+ }
task->thread.trap_no = 16;
task->thread.error_code = 0;
info.si_signo = SIGFPE;
@@ -667,7 +670,10 @@
* Save the info for the exception handler and clear the error.
*/
task = current;
- save_init_fpu(task);
+ /* don't trigger an unnecessary math_state_restore() */
+ if (task->thread_info->status & TS_USEDFPU) {
+ save_init_fpu(task);
+ }
task->thread.trap_no = 19;
task->thread.error_code = 0;
info.si_signo = SIGFPE;
diff -u --recursive --new-file linux-2.6.0-test4/include/asm-i386/i387.h linux-2.6.0-test4-mine/include/asm-i386/i387.h
--- linux-2.6.0-test4/include/asm-i386/i387.h 2003-07-27 19:06:54.000000000 +0200
+++ linux-2.6.0-test4-mine/include/asm-i386/i387.h 2003-08-31 15:39:04.000000000 +0200
@@ -41,8 +41,10 @@

static inline void save_init_fpu( struct task_struct *tsk )
{
+ preempt_disable();
__save_init_fpu(tsk);
stts();
+ preempt_enable_no_resched();
}


@@ -53,11 +55,13 @@

#define clear_fpu( tsk ) \
do { \
+ preempt_disable(); \
if ((tsk)->thread_info->status & TS_USEDFPU) { \
asm volatile("fwait"); \
(tsk)->thread_info->status &= ~TS_USEDFPU; \
stts(); \
} \
+ preempt_enable_no_resched(); \
} while (0)

/*
##########################################################################################################################################

-
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