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Subject2.6.0-test4: bug WRT lazy FPU switching and CONFIG_PREEMPT
Hi!

There seems to be a bug with CONFIG_PREEMPT=y and lazy FPU switching. The following small program just unmasks math exceptions
and then continously triggers a divide by zero. When CONFIG_PREEMPT is in effect this eventually leads to other processes
inheriting the math-status of this brain-damaged but legal user level program. Especially, their math-exceptions are unmasked
and so they might catch an unwanted SIGFPE.

######################################### snip ####################################
#include <signal.h>
#include <stdio.h>
#include <fenv.h>
double volatile foobar = 0.0;

static void sigfpe(int sig)
{
static int cnt;
if (++cnt % 100000 == 0) {
fprintf(stderr, ".");
}
}
int main(int argc, char *argv[])
{
signal(SIGFPE, sigfpe);
feenableexcept(FE_ALL_EXCEPT);
foobar = 1.0/foobar;
}
######################################## snap ######################################
The following patch seems to fix the problem. However, I must admit that I didn't fully understand why this happens. The problem
seems to be a preemption occuring between the __save_init_fpu() (i.e. fsave/fxsave) and the stts(). But why should this be a
problem?

The patch touches two files:

include/asm-i386/i387.h:
here save_init_fpu() is defined and this seems to be the location of the problem

arch/i386/kernel/traps.c:
by checking for TS_USEDFPU we spare an additional "device not available" fault (with CONFIG_PREEMPT=y) and thereby a
superflous math_state_restore(). There is not point in restoring the math-state just to save it again (but this didn't cause the
shooting with wild SIGFPEs).

Regards

Claus

######################################### snip ##########################################################################
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
@@ -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"
: "=m" (tsk->thread.i387.fxsave) );
} else {
asm volatile( "fnsave %0 ; fwait"
@@ -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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