Hi, Paul, You means my patch (http://patchwork.linux-mips.org/patch/7297/) is the correct way? Another question: Your patch (http://patchwork.linux-mips.org/patch/7307/) remove preempt_disable()/preempt_enable() in init_fpu(). It will cause problems if there is another function call init_fpu() because it is previously preempt-safe. Maybe introduce a new function (e.g. __init_fpu()) is a better way? Huacai On Sat, Jul 12, 2014 at 7:28 AM, Chen Jie <chenj@xxxxxxxxxx> wrote: > 2014-07-11 23:56 GMT+08:00 Paul Burton <paul.burton@xxxxxxxxxx>: >> On Fri, Jul 11, 2014 at 11:14:13AM +0800, chenj wrote: >>> do_ade may be invoked with preempt enabled. do_cpu will be invoked with >>> preempt enabled. When it's preempted(in do_ade/do_cpu), TIF_USEDFPU will be >>> cleared, when it returns to do_ade/do_cpu, the fpu is actually disabled. >>> >>> e.g. >>> In do_ade() >>> emulate_load_store_insn(): >>> BUG_ON(!is_fpu_owner()); <-- This assertion may be breaked. >>> >>> In do_cpu() >>> enable_restore_fp_context(): >>> was_fpu_owner = is_fpu_owner(); >> >> Preemption should indeed be disabled around the assignment & use of the >> was_fpu_owner variable, but note that you can only hit the problem if >> using MSA. One of the MSA fixes I just submitted also fixes this along >> with another instance of the problem: >> >> http://patchwork.linux-mips.org/patch/7307/ >> >> I prefer my patch to this since it disables preemption for less time, >> in addition to fixing the !used_math() case. >> >> In emulate_load_store_insn I believe the correct fix is simply to remove >> that BUG_ON. The code is about to give up FPU ownership anyway, so it's >> not like there is any requirement being violated if it was already lost. > Yes, you're right. > > """ /* arch/mips/kernel/unaligned.c */ > lose_fpu(1); /* Save FPU state for the emulator. */ > res = fpu_emulator_cop1Handler(regs, ¤t->thread.fpu, 1, &fault_addr); > own_fpu(1); /* Restore FPU state. */ > """ > > Going deep into the code, I find lost_fpu(1) will save fpu context if > owns fpu (otherwise, if preempted, the fpu context will be saved in > process switch), then fpu_emulator_cop1Handler manipulates the saved > fpu context, own_fpu(1) restores it. > > So, remove "BUG_ON(!is_fpu_owner())" is OK. >
