Messages in this thread |  | | From | Zong Li <> | Date | Mon, 6 Apr 2020 18:36:42 +0800 | Subject | Re: [PATCH v3 8/9] riscv: introduce interfaces to patch kernel code |
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On Sat, Apr 4, 2020 at 8:12 PM Zong Li <zong.li@sifive.com> wrote: > > On Sat, Apr 4, 2020 at 11:14 AM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > > > Hi Zong, > > > > On Fri, 3 Apr 2020 17:04:51 +0800 > > Zong Li <zong.li@sifive.com> wrote: > > > > > > > > > +{ > > > > > > > + void *waddr = addr; > > > > > > > + bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE; > > > > > > > + unsigned long flags = 0; > > > > > > > + int ret; > > > > > > > + > > > > > > > + raw_spin_lock_irqsave(&patch_lock, flags); > > > > > > > > > > > > This looks a bit odd since stop_machine() is protected by its own mutex, > > > > > > and also the irq is already disabled here. > > > > > > > > > > We need it because we don't always enter the riscv_patch_text_nosync() > > > > > through stop_machine mechanism. If we call the > > > > > riscv_patch_text_nosync() directly, we need a lock to protect the > > > > > page. > > > > > > > > Oh, OK, but it leads another question. Is that safe to patch the > > > > text without sync? Would you use it for UP system? > > > > I think it is better to clarify "in what case user can call _nosync()" > > > > and add a comment on it. > > > > > > The ftrace is one of the cases, as documentation of ftrace said, when > > > dynamic ftrace is initialized, it calls kstop_machine to make the > > > machine act like a uniprocessor so that it can freely modify code > > > without worrying about other processors executing that same code. So > > > the ftrace called the _nosync interface here directly. > > > > Hmm, even though, since it already running under kstop_machine(), no > > other thread will run. > > Could you consider to use text_mutex instead of that? The text_mutex > > is already widely used in x86 and kernel/kprobes.c etc. > > > > (Hmm, it seems except for x86, alternative code don't care about > > racing...) > >
The mutex_lock doesn't seem to work in ftrace context, I think it might be the reason why other architectures didn't use text_mutex in somewhere.
# echo function > current_tracer [ 28.198070] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:281 [ 28.198663] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 11, name: migration/0 [ 28.199491] CPU: 0 PID: 11 Comm: migration/0 Not tainted 5.6.0-00012-gd6f56a7a4be2-dirty #10 [ 28.200330] Call Trace: [ 28.200798] [<ffffffe00060319a>] walk_stackframe+0x0/0xcc [ 28.201395] [<ffffffe000603442>] show_stack+0x3c/0x46 [ 28.200798] [<ffffffe00060319a>] walk_stackframe+0x0/0xcc [ 28.201395] [<ffffffe000603442>] show_stack+0x3c/0x46 [ 28.201898] [<ffffffe000d498b0>] dump_stack+0x76/0x90 [ 28.202329] [<ffffffe00062c3f0>] ___might_sleep+0x100/0x10e [ 28.202720] [<ffffffe00062c448>] __might_sleep+0x4a/0x78 [ 28.203033] [<ffffffe000d61622>] mutex_lock+0x2c/0x54 [ 28.203397] [<ffffffe00060393e>] patch_insn_write+0x32/0xd8 [ 28.203780] [<ffffffe000603a94>] patch_text_nosync+0x10/0x32 [ 28.204139] [<ffffffe0006051b0>] __ftrace_modify_call+0x5c/0x6c [ 28.204497] [<ffffffe0006052c6>] ftrace_update_ftrace_func+0x20/0x4a [ 28.204919] [<ffffffe000697742>] ftrace_modify_all_code+0xa0/0x148 [ 28.205378] [<ffffffe0006977fc>] __ftrace_modify_code+0x12/0x1c [ 28.205793] [<ffffffe0006924b6>] multi_cpu_stop+0xa2/0x158 [ 28.206147] [<ffffffe0006921b0>] cpu_stopper_thread+0xa4/0x13a [ 28.206510] [<ffffffe000629f38>] smpboot_thread_fn+0xf8/0x1da [ 28.206868] [<ffffffe000625f36>] kthread+0xfa/0x12a [ 28.207201] [<ffffffe0006017e2>] ret_from_exception+0x0/0xc
> > Yes, text_mutex seems to be great. I'll change to use text_mutex in > the next version if it works fine after testing. Thanks. > > > Thank you, > > -- > > Masami Hiramatsu <mhiramat@kernel.org>
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