Hi, Ruoyao, On Tue, Jan 2, 2024 at 1:23 AM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote: > > There has been a lingering bug in LoongArch Linux systems causing some > GCC tests to intermittently fail (see Closes link). I've made a minimal > reproducer: > > zsh% cat measure.s > .align 4 > .globl _start > _start: > movfcsr2gr $a0, $fcsr0 > bstrpick.w $a0, $a0, 16, 16 > beqz $a0, .ok > break 0 > .ok: > li.w $a7, 93 > syscall 0 > zsh% cc mesaure.s -o measure -nostdlib > zsh% echo $((1.0/3)) > 0.33333333333333331 > zsh% while ./measure; do ; done > > This while loop should not stop as POSIX is clear that execve must set > fenv to the default, where FCSR should be zero. But in fact it will > just stop after running for a while (normally less than 30 seconds). > Note that "$((1.0/3))" is needed to reproduce the issue because it > raises FE_INVALID and makes fcsr0 non-zero. > > The problem is we are relying on SET_PERSONALITY2 to reset > current->thread.fpu.fcsr. But SET_PERSONALITY2 is executed before > start_thread which calls lose_fpu(0). We can see if kernel preempt is > enabled, we may switch to another thread after SET_PERSONALITY2 but > before lose_fpu(0). Then bad thing happens: during the thread switch > the value of the fcsr0 register is stored into current->thread.fpu.fcsr, > making it dirty again. > > The issue can be fixed by setting current->thread.fpu.fcsr after > lose_fpu(0) because lose_fpu clears TIF_USEDFPU, then the thread > switch won't touch current->thread.fpu.fcsr. > > The only other architecture setting FCSR in SET_PERSONALITY2 is MIPS. > They do this for supporting different FP flavors (NaN encodings etc). > which do not exist on LoongArch. I'm not sure how MIPS evades the issue > (or maybe it's just buggy too) as I don't have a running MIPS hardware > now. I think you can use QEMU. :) > > So for LoongArch, just remove the current->thread.fpu.fcsr setting from > SET_PERSONALITY2 and do it in start_thread, after lose_fpu(0). And we > just set it to 0, instead of boot_cpu_data.fpu_csr0 (because we should > provide the userspace a consistent configuration, no matter how hardware > and firmware behave). I still prefer to set fcsr to boot_cpu_data.fpu_csr0, because we will add LoongArch32 later, not sure whether something will change. Huacai > > The while loop failing with the mainline kernel has survived one hour > after this change. > > Closes: https://github.com/loongson-community/discussions/issues/7 > Fixes: 803b0fc5c3f2 ("LoongArch: Add process management") > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx> > --- > arch/loongarch/include/asm/elf.h | 5 ----- > arch/loongarch/kernel/elf.c | 5 ----- > arch/loongarch/kernel/process.c | 1 + > 3 files changed, 1 insertion(+), 10 deletions(-) > > diff --git a/arch/loongarch/include/asm/elf.h b/arch/loongarch/include/asm/elf.h > index 9b16a3b8e706..f16bd42456e4 100644 > --- a/arch/loongarch/include/asm/elf.h > +++ b/arch/loongarch/include/asm/elf.h > @@ -241,8 +241,6 @@ void loongarch_dump_regs64(u64 *uregs, const struct pt_regs *regs); > do { \ > current->thread.vdso = &vdso_info; \ > \ > - loongarch_set_personality_fcsr(state); \ > - \ > if (personality(current->personality) != PER_LINUX) \ > set_personality(PER_LINUX); \ > } while (0) > @@ -259,7 +257,6 @@ do { \ > clear_thread_flag(TIF_32BIT_ADDR); \ > \ > current->thread.vdso = &vdso_info; \ > - loongarch_set_personality_fcsr(state); \ > \ > p = personality(current->personality); \ > if (p != PER_LINUX32 && p != PER_LINUX) \ > @@ -340,6 +337,4 @@ extern int arch_elf_pt_proc(void *ehdr, void *phdr, struct file *elf, > extern int arch_check_elf(void *ehdr, bool has_interpreter, void *interp_ehdr, > struct arch_elf_state *state); > > -extern void loongarch_set_personality_fcsr(struct arch_elf_state *state); > - > #endif /* _ASM_ELF_H */ > diff --git a/arch/loongarch/kernel/elf.c b/arch/loongarch/kernel/elf.c > index 183e94fc9c69..0fa81ced28dc 100644 > --- a/arch/loongarch/kernel/elf.c > +++ b/arch/loongarch/kernel/elf.c > @@ -23,8 +23,3 @@ int arch_check_elf(void *_ehdr, bool has_interpreter, void *_interp_ehdr, > { > return 0; > } > - > -void loongarch_set_personality_fcsr(struct arch_elf_state *state) > -{ > - current->thread.fpu.fcsr = boot_cpu_data.fpu_csr0; > -} > diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c > index 767d94cce0de..caed58770650 100644 > --- a/arch/loongarch/kernel/process.c > +++ b/arch/loongarch/kernel/process.c > @@ -92,6 +92,7 @@ void start_thread(struct pt_regs *regs, unsigned long pc, unsigned long sp) > clear_used_math(); > regs->csr_era = pc; > regs->regs[3] = sp; > + current->thread.fpu.fcsr = 0; > } > > void flush_thread(void) > -- > 2.43.0 > >
