Re: current_thread_info() not respecting program order with gcc 4.8.x

From: Luis Lozano
Date: Fri Nov 22 2013 - 03:33:19 EST

On Fri, Nov 22, 2013 at 12:18 AM, Luis Lozano <llozano@xxxxxxxxxxxx> wrote:
> On Thu, Nov 21, 2013 at 7:38 PM, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>
>> ----- Original Message -----
>> > From: "Luis Lozano" <llozano@xxxxxxxxxx>
>> > To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
>> > Cc: "Jakub Jelinek" <jakub@xxxxxxxxxx>, "Alexander Holler" <holler@xxxxxxxxxxxxx>, "Linus Torvalds"
>> > <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Richard Henderson" <rth@xxxxxxxxxxx>, "Linux Kernel Mailing List"
>> > <linux-kernel@xxxxxxxxxxxxxxx>, "Will Deacon" <will.deacon@xxxxxxx>, "Catalin Marinas" <catalin.marinas@xxxxxxx>,
>> > "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, lttng-dev@xxxxxxxxxxxxxxx, "Nathan Lynch" <Nathan_Lynch@xxxxxxxxxx>, "Paul
>> > E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, "Andrew Morton" <akpm@xxxxxxxxxxxxxxxxxxxx>, "Bhaskar Janakiraman"
>> > <bjanakiraman@xxxxxxxxxxxx>, "Han Shen" <shenhan@xxxxxxxxxxxx>
>> > Sent: Thursday, November 21, 2013 9:36:27 PM
>> > Subject: Re: current_thread_info() not respecting program order with gcc 4.8.x
>> >
>> > Hi Mathieu,
>> >
>> > Yes the problem we were seeing with GCC bug 58854 is that an interrupt
>> > handler was corrupting the stack of a routine which had an invalid
>> > value of SP (not really on "top" of the stack) for part of the
>> > routine.
>> > But in the assembly code you sent, I don't see where sp is being
>> > modified... or where the access to "below" sp is happening.
>>
>> The following instruction
>>
>> f1c: e50b1048 str r1, [fp, #-72] ; 0xffffffb8
>>
>> appears in the assembly generated by gcc 4.8.2, but not in the one
>> generated by 4.7.3,
>
>
>
> It is there in 4.7.3 just a few lines difference.
> I don't see any meaningful difference between the 2 assembly snippets...
> Just the instructions ordered a little different?
>

unless there is a dependency between the following 2 instructions?
- f1c: e50b1048 str r1, [fp, #-72] ; 0xffffffb8
...
f20: e5936030 ldr r6, [r3, #48] ; 0x30

and moving the str across the ldr makes a difference...


> Luis
>
>>
>> which makes me wonder if it's good or not. With
>> slightly different build output from the diff (probably a different config
>> from Nathan), the first function instructions look like:
>>
>> 00000efc <lttng_event_reserve>:
>> efc: e1a0c00d mov ip, sp
>> f00: e92ddff0 push {r4, r5, r6, r7, r8, r9, sl, fp, ip, lr, pc}
>> f04: e24cb004 sub fp, ip, #4
>> f08: e24dd03c sub sp, sp, #60 ; 0x3c
>> f0c: e52de004 push {lr} ; (str lr, [sp, #-4]!)
>> f10: ebfffffe bl 0 <__gnu_mcount_nc>
>> f14: e5903000 ldr r3, [r0]
>> f18: e1a04000 mov r4, r0
>> f1c: e1a0000d mov r0, sp
>> f20: e5936030 ldr r6, [r3, #48] ; 0x30
>> f24: e3c03d7f bic r3, r0, #8128 ; 0x1fc0
>> f28: e3c3303f bic r3, r3, #63 ; 0x3f
>> f2c: e50b104c str r1, [fp, #-76] ; 0xffffffb4
>> f30: e5932004 ldr r2, [r3, #4]
>> f34: e2822001 add r2, r2, #1
>> f38: e5832004 str r2, [r3, #4]
>> f3c: ebfffffe bl 0 <debug_smp_processor_id>
>> f40: e59f2e44 ldr r2, [pc, #3652] ; 1d8c <lttng_event_reserve+0xe90>
>> f44: e59f3e44 ldr r3, [pc, #3652] ; 1d90 <lttng_event_reserve+0xe94>
>> f48: e7921100 ldr r1, [r2, r0, lsl #2]
>> f4c: e1a05000 mov r5, r0
>> f50: e7932001 ldr r2, [r3, r1]
>>
>> My rusty ARM assembler knowledge analyzes this like:
>>
>> ip = sp (ip being the scratch register)
>> push 11 registers * 4 bytes = 44 bytes onto the stack
>> fp = ip - 4
>> sp = sp - 60 (sp = ip - 104)
>> push {lr} bl
>> 0 <__gnu_mcount_nc> (dynamically patched to a pop {lr})
>> [...]
>> store r1 into mem location fp - 76
>>
>> So yes, it does look like the -76 from fp is within the stack.
>>
>>
>> > In GCC bug 58854 it was pretty clear that the restore of the sp
>> > register in the epilogue is moved to somewhere close to the prologue
>> > of the routine.
>> > Are we missing some diffs from the assembly comparison?
>>
>> My next step is to setup my own 4.8.2 ARM cross-compiler. I've tried this morning,
>> did not manage to get one working even after following 2 howtos, trying with Debian
>> packages, etc. Nathan told me this diff was the full comparison between the two
>> functions, but you'll understand that at this point I want to reproduce
>> everything myself, because this is a _weird_ issue.
>>
>> But after a long day of debugging, it's time for some sleep. I will get back to this
>> tomorrow,
>>
>> Thanks,
>>
>> Mathieu
>>
>> >
>> > Thanks
>> >
>> > Luis
>> >
>> >
>> >
>> >
>> > On Thu, Nov 21, 2013 at 5:57 PM, Mathieu Desnoyers
>> > <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> > > ----- Original Message -----
>> > >> From: "Jakub Jelinek" <jakub@xxxxxxxxxx>
>> > >> To: "Luis Lozano" <llozano@xxxxxxxxxx>
>> > >> Cc: "Alexander Holler" <holler@xxxxxxxxxxxxx>, "Linus Torvalds"
>> > >> <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Mathieu Desnoyers"
>> > >> <mathieu.desnoyers@xxxxxxxxxxxx>, "Richard Henderson" <rth@xxxxxxxxxxx>,
>> > >> "Linux Kernel Mailing List"
>> > >> <linux-kernel@xxxxxxxxxxxxxxx>, "Will Deacon" <will.deacon@xxxxxxx>,
>> > >> "Catalin Marinas" <catalin.marinas@xxxxxxx>,
>> > >> "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, lttng-dev@xxxxxxxxxxxxxxx,
>> > >> "Nathan Lynch" <Nathan_Lynch@xxxxxxxxxx>, "Paul
>> > >> E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, "Andrew Morton"
>> > >> <akpm@xxxxxxxxxxxxxxxxxxxx>, "Bhaskar Janakiraman"
>> > >> <bjanakiraman@xxxxxxxxxxxx>, "Han Shen" <shenhan@xxxxxxxxxxxx>
>> > >> Sent: Thursday, November 21, 2013 7:39:04 PM
>> > >> Subject: Re: current_thread_info() not respecting program order with gcc
>> > >> 4.8.x
>> > >>
>> > >> On Thu, Nov 21, 2013 at 03:45:35PM -0800, Luis Lozano wrote:
>> > >> > I think we need a reproducer. Without this we may all be going on the
>> > >> > wrong path. This whole conversation started on an *assumption* that
>> > >> > some accesses were being reordered.
>> > >> >
>> > >> > evidence of the reorder or reproducer please?
>> > >>
>> > >> Yeah, if a compiler bug is suspected, can anybody please open
>> > >> a bugreport in http://gcc.gnu.org/bugzilla/ with the preprocessed source,
>> > >> compiler version, flags and how it was configured and some hint in which
>> > >> function to look for what exactly? We don't necessarily need a runtime
>> > >> small reproducer, but if it can be shown in the assembly what has been
>> > >> reordered and why you think it shouldn't, with the above mentioned input
>> > >> that ought to be sufficient. Thanks.
>> > >
>> > > OK OK, let me reply on list first so I can share the result of a full day
>> > > of bug hunting. We're not there yet, but many options have been eliminated.
>> > >
>> > > The issue shows up in stress test, when tracing with lttng-modules 2.4-rc1,
>> > > on ARM. It's been reproduced with a Linux kernel 3.12 so far, with
>> > > lttng-modules
>> > > compiled against that kernel.
>> > >
>> > > First, I asked Nathan to compile his kernel with gcc 4.7, and lttng-modules
>> > > with gcc 4.8.x (and vice-versa). The problem only appears when
>> > > lttng-modules
>> > > are compiled with gcc 4.8.x. The compiler version used to compile the rest
>> > > of the kernel does not matter.
>> > >
>> > > Then I looked at gcc 4.8 changelog for ARM, new feature:
>> > > -fno-sched-pressure
>> > > (sched pressure is there by default). Nathan tried compiling lttng-modules
>> > > with
>> > > -fno-sched-pressure. The problem still reproduces.
>> > >
>> > > Knowing that adding barrier() outside of preempt_disable()/enable() was
>> > > fixing the issue, we tried identifying which code location was responsible
>> > > for working around the issue. Skipping a long investigation, here is the
>> > > executive summary:
>> > >
>> > > http://git.lttng.org/?p=lttng-modules.git;a=blob;f=lttng-ring-buffer-client.h;h=50c47b3bf49f6c2dd24e250cf1a9b97808cd8e27;hb=HEAD
>> > >
>> > > Has the following function. We identified that adding a barrier() as shown
>> > > below
>> > > works around the issue:
>> > >
>> > > static
>> > > int lttng_event_reserve(struct lib_ring_buffer_ctx *ctx,
>> > > uint32_t event_id)
>> > > {
>> > > struct lttng_channel *lttng_chan = channel_get_private(ctx->chan);
>> > > int ret, cpu;
>> > >
>> > > cpu = lib_ring_buffer_get_cpu(&client_config);
>> > > if (cpu < 0)
>> > > return -EPERM;
>> > > ctx->cpu = cpu;
>> > >
>> > > switch (lttng_chan->header_type) {
>> > > case 1: /* compact */
>> > > if (event_id > 30)
>> > > ctx->rflags |= LTTNG_RFLAG_EXTENDED;
>> > > break;
>> > > case 2: /* large */
>> > > if (event_id > 65534)
>> > > ctx->rflags |= LTTNG_RFLAG_EXTENDED;
>> > > break;
>> > > default:
>> > > WARN_ON_ONCE(1);
>> > > }
>> > >
>> > > ret = lib_ring_buffer_reserve(&client_config, ctx);
>> > > if (ret)
>> > > goto put;
>> > > lttng_write_event_header(&client_config, ctx, event_id);
>> > > return 0;
>> > > put:
>> > > lib_ring_buffer_put_cpu(&client_config);
>> > > ---------------> barrier() added here;
>> > > <----------------------------
>> > > return ret;
>> > > }
>> > >
>> > > Where barrier() is the usual asm volatile with "memory" clobber, nothing
>> > > else.
>> > >
>> > > Nathan gave me the binary diff for the assembly generated for this function
>> > > without
>> > > the barrier and with the barrier:
>> > >
>> > > --- /tmp/lttng_event_reserve-4.8.2.dump 2013-11-21 11:14:14.536495079 -0600
>> > > +++ /tmp/lttng_event_reserve-with-barrier-4.8.2.dump 2013-11-21
>> > > 14:12:52.997355907 -0600
>> > > @@ -7,11 +7,11 @@
>> > > f10: ebfffffe bl 0 <__gnu_mcount_nc>
>> > > f14: e5903000 ldr r3, [r0]
>> > > f18: e1a04000 mov r4, r0
>> > > - f1c: e50b1048 str r1, [fp, #-72] ; 0xffffffb8
>> > > + f1c: e1a0000d mov r0, sp
>> > > f20: e5936030 ldr r6, [r3, #48] ; 0x30
>> > > - f24: e1a0000d mov r0, sp
>> > > - f28: e3c03d7f bic r3, r0, #8128 ; 0x1fc0
>> > > - f2c: e3c3303f bic r3, r3, #63 ; 0x3f
>> > > + f24: e3c03d7f bic r3, r0, #8128 ; 0x1fc0
>> > > + f28: e3c3303f bic r3, r3, #63 ; 0x3f
>> > > + f2c: e50b1048 str r1, [fp, #-72] ; 0xffffffb8
>> > > f30: e5932004 ldr r2, [r3, #4]
>> > > f34: e2822001 add r2, r2, #1
>> > > f38: e5832004 str r2, [r3, #4]
>> > > @@ -53,7 +53,7 @@
>> > > fc8: e3c3303f bic r3, r3, #63 ; 0x3f
>> > > fcc: e5933000 ldr r3, [r3]
>> > > fd0: e3130002 tst r3, #2
>> > > - fd4: 0a000000 beq fdc <lttng_event_reserve+0xe0>
>> > > + fd4: 0a0002be beq 1ad4 <lttng_event_reserve+0xbd8>
>> > > fd8: ebfffffe bl 0 <preempt_schedule>
>> > > fdc: ea0002bc b 1ad4 <lttng_event_reserve+0xbd8>
>> > > fe0: e3500000 cmp r0, #0
>> > >
>> > > We tried disabling the ftrace function tracing to get mcount out of the
>> > > way,
>> > > and the problem still reproduces.
>> > >
>> > > I'm stopping here in terms of details about the disassembly, because I
>> > > need to double check with Nathan that I get the right disassembly for the
>> > > right
>> > > cases. I also terribly need to setup a 4.8.2 ARM cross-compiler on my
>> > > machine.
>> > >
>> > > I'm attaching Nathan's ARM configuration.
>> > >
>> > > It does look behave a bit like this bug pointed out by Luis:
>> > >
>> > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58854
>> > >
>> > > AFAIU (please correct me if I am wrong), ARM's interrupt handler run
>> > > on top of the thread stack (?). If it's the case, then anything stored
>> > > on the stack below "sp" could be overwritten by an interrupt handler.
>> > > This would fit well the reproduction scenario for this bug: Nathan runs
>> > > LTTng tracing of kmem_cache_free tracepoint with hackbench running.
>> > > A race between a short window of stack use below sp and interrupt handlers
>> > > would trigger with this kind of stress-test.
>> > >
>> > > Thoughts ?
>> > >
>> > > Thanks,
>> > >
>> > > Mathieu
>> > >
>> > >
>> > > --
>> > > Mathieu Desnoyers
>> > > EfficiOS Inc.
>> > > http://www.efficios.com
>> >
>> >
>> >
>> > --
>> >
>> > Luis A. Lozano | Software Engineer | llozano@xxxxxxxxxx | +1 (408)431-5164
>> >
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> http://www.efficios.com



--

Luis A. Lozano | Software Engineer | llozano@xxxxxxxxxx | +1 (408)431-5164
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