You should *not* clear USEDFPU in copy_thread(). Imagine you assign a floating point variable f=0.3, then do fork() and then in the child process, alas, f is undefined (zero most likely). If you really want to do it, it should be in start_thread(). Even if you don't have it cleared in start_thread(), things should be generally OK. You will have some dirty FPU content instead of a all-zero one when you start a new program. But then since all sane program should assign register values before they first time use them, so this bug should be well hidden. I am still curious whether this is a bug in i386 as well or they do clear the flag in some non-obvious way. Note that the unlazy_fpu() in copy_thread won't do it. It only clears the current process's USEDFPU flag, while the child process's flag is set way back in copy_flags() calls inside do_fork(). Jun On Tue, Feb 04, 2003 at 11:54:38PM +0100, Vivien Chappelier wrote: > Hi, > > When copying a thread, access to the FPU is disabled by clearing > the ST0_CU1 bit in the new thread saved CP0_STATUS register. However, the > corresponding TIF_USEDFPU flag is not cleared at it should to indicate the > FPU has not yet been used by the new process. > This patch also clears user_tid in mips64 code, and moves it away > from the FPU comment in the mips code. > > Vivien. > > Index: arch/mips64/kernel/process.c > =================================================================== > RCS file: /home/cvs/linux/arch/mips64/kernel/process.c,v > retrieving revision 1.36 > diff -u -r1.36 process.c > --- arch/mips64/kernel/process.c 9 Jan 2003 19:16:50 -0000 1.36 > +++ arch/mips64/kernel/process.c 4 Feb 2003 22:47:00 -0000 > @@ -114,6 +114,8 @@ > p->thread.reg29 = (unsigned long) childregs; > p->thread.reg31 = (unsigned long) ret_from_fork; > > + p->user_tid = NULL; > + > /* > * New tasks loose permission to use the fpu. This accelerates context > * switching for most programs since they don't use the fpu. > @@ -121,6 +123,7 @@ > p->thread.cp0_status = read_c0_status() & > ~(ST0_CU3|ST0_CU2|ST0_CU1|ST0_KSU); > childregs->cp0_status &= ~(ST0_CU3|ST0_CU2|ST0_CU1); > + clear_ti_thread_flag(ti, TIF_USEDFPU); > > return 0; > } > Index: arch/mips/kernel/process.c > =================================================================== > RCS file: /home/cvs/linux/arch/mips/kernel/process.c,v > retrieving revision 1.50 > diff -u -r1.50 process.c > --- arch/mips/kernel/process.c 9 Jan 2003 19:16:50 -0000 1.50 > +++ arch/mips/kernel/process.c 4 Feb 2003 22:47:04 -0000 > @@ -117,6 +117,8 @@ > p->thread.reg29 = (unsigned long) childregs; > p->thread.reg31 = (unsigned long) ret_from_fork; > > + p->user_tid = NULL; > + > /* > * New tasks loose permission to use the fpu. This accelerates context > * switching for most programs since they don't use the fpu. > @@ -124,7 +126,7 @@ > p->thread.cp0_status = read_c0_status() & > ~(ST0_CU2|ST0_CU1|KU_MASK); > childregs->cp0_status &= ~(ST0_CU2|ST0_CU1); > - p->user_tid = NULL; > + clear_ti_thread_flag(ti, TIF_USEDFPU); > > return 0; > } >
