Messages in this thread Patch in this message | | | Date | Sun, 31 Aug 2003 17:42:11 +0200 | From | Claus-Justus Heine <> | Subject | 2.6.0-test4: bug WRT lazy FPU switching and CONFIG_PREEMPT |
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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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