Are you sure this hasn't been fix in the latest sources (2.4.16) ?
I have send a patch to Ralf, which I believe solves a similar problem as you describe below.
Ralf have you applied the patch ?
/Carsten
Jun Sun wrote:
> This is an incomplete patch, attempting to fix an old fix FPU context
> corruption problem with signal happens.
>
> The following scenario would cause many problems with current code:
>
> 1. process A runs and uses FPU
> 2. Process A is switched out; Process B runs and uses FPU
> 3. Process A runs again, but have not used FPU this time around
> 4. Process A receives a signal.
> 5. Process A's signal handler runs and uses FPU
> 6. Process B runs again and uses FPU.
>
> At step 4, we have last_task_used_fpu being B, current->used_math being 1.
> The current code would save B's FPU context to A's sc structure, but
> does not mark owned_fp bits.
>
> At step 5, when A's sig handler first time uses FPU, FPU is enabled again.
>
> At step 6, when B tries to use FPU again, a lazy fpu switch happens. The
> current FPU regs are stored into A's thread strucutre, effectively eraseing
> previously stored A's FPU context before the sig handler starts.
>
> In addition, B finds its FPU reg values totally bogus when they are loaded
> from B's thread structure, because the FPU regs were saved there. (See
> step 4).
>
> This patch is meant for concept debugging. But it should be easy
> to complete it:
>
> 1. support the two copy_fpu_context function. (note, need to differentiate
> between CPU_HAS_FPU and !CPU_HAS_FPU cases)
> 2. perhaps change ->sc_ownedfp to ->sc_used_fpu, which is really what
> this flag means now.
>
> Any comments?
>
> Jun
>
> ------------------------------------------------------------------------
>
> This is an incomplete patch, attempting to fix an old fix FPU context
> corruption problem with signal happens.
>
> The following scenario would cause many problems with current code:
>
> 1. process A runs and uses FPU
> 2. Process A is switched out; Process B runs and uses FPU
> 3. Process A runs again, but have not used FPU this time around
> 4. Process A receives a signal.
> 5. Process A's signal handler runs and uses FPU
> 6. Process B runs again and uses FPU.
>
> At step 4, we have last_task_used_fpu being B, current->used_math being 1.
> The current code would save B's FPU context to A's sc structure, but
> does not mark owned_fp bits.
>
> At step 5, when A's sig handler first time uses FPU, FPU is enabled again.
>
> At step 6, when B tries to use FPU again, a lazy fpu switch happens. The
> current FPU regs are stored into A's thread strucutre, effectively eraseing
> previously stored A's FPU context before the sig handler starts.
>
> In addition, B finds its FPU reg values totally bogus when they are loaded
> from B's thread structure, because the FPU regs were saved there. (See
> step 4).
>
> This patch is meant for concept debugging. But it should be easy
> to complete it:
>
> 1. support the two copy_fpu_context function. (note, need to differentiate
> between CPU_HAS_FPU and !CPU_HAS_FPU cases)
> 2. perhaps change ->sc_ownedfp to ->sc_used_fpu, which is really what
> this flag means now.
>
> Any comments?
>
> Jun
>
> diff -Nru linux.link/arch/mips/kernel/signal.c.orig linux.link/arch/mips/kernel/signal.c
> --- linux.link/arch/mips/kernel/signal.c.orig Tue Nov 27 11:03:02 2001
> +++ linux.link/arch/mips/kernel/signal.c Wed Dec 19 16:28:08 2001
> @@ -222,8 +222,20 @@
>
> err |= __get_user(owned_fp, &sc->sc_ownedfp);
> if (owned_fp) {
> - err |= restore_fp_context(sc);
> - last_task_used_math = current;
> + if ((last_task_used_match == NULL) || (last_task_used_math == current)) {
> + /* if nobody owns FPU, or sig handler owns it,
> + * we just restore FPU regs from sc to hardware */
> + err |= restore_fp_context(sc);
> + last_task_used_math = current;
> + enable_fpu();
> + } else {
> + /* we copy FPU values from sc to task structure */
> + err |= copy_fpu_context_from_sigcontext(current, sc);
> + disable_fpu();
> + }
> +
> + /* in either case, we need to set the used flag */
> + current->used_math = 1;
> }
>
> return err;
> @@ -352,13 +364,25 @@
> err |= __put_user(regs->cp0_cause, &sc->sc_cause);
> err |= __put_user(regs->cp0_badvaddr, &sc->sc_badvaddr);
>
> - owned_fp = (current == last_task_used_math);
> - err |= __put_user(owned_fp, &sc->sc_ownedfp);
> + /* if we ever used FPU, we need to get FPU regs to sc */
> + if (current->used_math) {
> +
> + if (current == last_task_used_math) {
> + /* if we own FPU now, just save fpu to sc directly */
> + enable_fpu();
> + err |= save_fp_context(sc);
> + } else {
> + /* otherwise, we need copy fpu regs from current task
> + * structure to sc structre.
> + */
> + err |= copy_fpu_context_to_sigcontext(current, sc);
> + }
> +
> + /* put remember that we have valid FPU regs in sc */
> + err |= __put_user(current->used_math, &sc->sc_ownedfp);
>
> - if (current->used_math) { /* fp is active. */
> - enable_fpu();
> - err |= save_fp_context(sc);
> - last_task_used_math = NULL;
> + /* we now pretend we have never used FPU before we start sig
> + * handler */
> regs->cp0_status &= ~ST0_CU1;
> current->used_math = 0;
> }
--
_ _ ____ ___ Carsten Langgaard Mailto:carstenl@mips.com
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