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Re: [Qemu-devel] [PATCH v2] qemu-coroutine: segfault when restarting co_
From: |
Wen Congyang |
Subject: |
Re: [Qemu-devel] [PATCH v2] qemu-coroutine: segfault when restarting co_queue |
Date: |
Tue, 10 Feb 2015 11:16:22 +0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 |
On 02/09/2015 10:48 PM, Stefan Hajnoczi wrote:
> On Mon, Feb 09, 2015 at 02:50:39PM +0800, Bin Wu wrote:
>> From: Bin Wu <address@hidden>
>>
>> We tested VMs migration with their disk images by drive_mirror. With
>> migration, two VMs copyed large files between each other. During the
>> test, a segfault occured. The stack was as follow:
>>
>> (gdb) bt
>> qemu-coroutine-lock.c:66
>> to=0x7fa5a1798648) at qemu-coroutine.c:97
>> request=0x7fa28c2ffa10, reply=0x7fa28c2ffa30, qiov=0x0, offset=0) at
>> block/nbd-client.c:165
>> sector_num=8552704, nb_sectors=2040, qiov=0x7fa5a1757468, offset=0) at
>> block/nbd-client.c:262
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468) at
>> block/nbd-client.c:296
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block/nbd.c:291
>> req=0x7fa28c2ffbb0, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
>> flags=0) at block.c:3321
>> offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
>> block.c:3447
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown:
>> 0)) at
>> block.c:3471
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block.c:3480
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block/raw_bsd.c:62
>> req=0x7fa28c2ffe30, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
>> flags=0) at block.c:3321
>> offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
>> block.c:3447
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown:
>> 0)) at
>> block.c:3471
>> coroutine-ucontext.c:121
>
> This backtrace is incomplete. Where are the function names? The
> parameter lists appear incomplete too.
>
>> After analyzing the stack and reviewing the code, we find the
>> qemu_co_queue_run_restart should not be put in the coroutine_swap function
>> which
>> can be invoked by qemu_coroutine_enter or qemu_coroutine_yield. Only
>> qemu_coroutine_enter needs to restart the co_queue.
>>
>> The error scenario is as follow: coroutine C1 enters C2, C2 yields
>> back to C1, then C1 ternimates and the related coroutine memory
>> becomes invalid. After a while, the C2 coroutine is entered again.
>> At this point, C1 is used as a parameter passed to
>> qemu_co_queue_run_restart. Therefore, qemu_co_queue_run_restart
>> accesses an invalid memory and a segfault error ocurrs.
>>
>> The qemu_co_queue_run_restart function re-enters coroutines waiting
>> in the co_queue. However, this function should be only used int the
>> qemu_coroutine_enter context. Only in this context, when the current
>> coroutine gets execution control again(after the execution of
>> qemu_coroutine_switch), we can restart the target coutine because the
>> target coutine has yielded back to the current coroutine or it has
>> terminated.
>>
>> First we want to put qemu_co_queue_run_restart in qemu_coroutine_enter,
>> but we find we can not access the target coroutine if it terminates.
>
> This example captures the scenario you describe:
>
> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
> index 525247b..883cbf5 100644
> --- a/qemu-coroutine.c
> +++ b/qemu-coroutine.c
> @@ -103,7 +103,10 @@ static void coroutine_swap(Coroutine *from, Coroutine
> *to)
> {
> CoroutineAction ret;
>
> + fprintf(stderr, "> %s from %p to %p\n", __func__, from, to);
> ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
> + fprintf(stderr, "< %s from %p to %p switch %s\n", __func__, from, to,
> + ret == COROUTINE_YIELD ? "yield" : "terminate");
>
> qemu_co_queue_run_restart(to);
>
> @@ -111,6 +114,7 @@ static void coroutine_swap(Coroutine *from, Coroutine *to)
> case COROUTINE_YIELD:
> return;
> case COROUTINE_TERMINATE:
> + fprintf(stderr, "coroutine_delete %p\n", to);
> trace_qemu_coroutine_terminate(to);
> coroutine_delete(to);
> return;
> diff --git a/tests/test-coroutine.c b/tests/test-coroutine.c
> index 27d1b6f..d44c428 100644
> --- a/tests/test-coroutine.c
> +++ b/tests/test-coroutine.c
> @@ -13,6 +13,7 @@
>
> #include <glib.h>
> #include "block/coroutine.h"
> +#include "block/coroutine_int.h"
>
> /*
> * Check that qemu_in_coroutine() works
> @@ -122,6 +123,35 @@ static void test_yield(void)
> g_assert_cmpint(i, ==, 5); /* coroutine must yield 5 times */
> }
>
> +static void coroutine_fn c2_fn(void *opaque)
> +{
> + fprintf(stderr, "c2 Part 1\n");
> + qemu_coroutine_yield();
> + fprintf(stderr, "c2 Part 2\n");
> +}
> +
> +static void coroutine_fn c1_fn(void *opaque)
> +{
> + Coroutine *c2 = opaque;
> +
> + fprintf(stderr, "c1 Part 1\n");
> + qemu_coroutine_enter(c2, NULL);
> + fprintf(stderr, "c1 Part 2\n");
> +}
> +
> +static void test_co_queue(void)
> +{
> + Coroutine *c1;
> + Coroutine *c2;
> +
> + c1 = qemu_coroutine_create(c1_fn);
> + c2 = qemu_coroutine_create(c2_fn);
> +
> + qemu_coroutine_enter(c1, c2);
> + memset(c1, 0xff, sizeof(Coroutine));
> + qemu_coroutine_enter(c2, NULL);
> +}
> +
> /*
> * Check that creation, enter, and return work
> */
> @@ -343,6 +373,7 @@ static void perf_cost(void)
> int main(int argc, char **argv)
> {
> g_test_init(&argc, &argv, NULL);
> + g_test_add_func("/basic/co_queue", test_co_queue);
> g_test_add_func("/basic/lifecycle", test_lifecycle);
> g_test_add_func("/basic/yield", test_yield);
> g_test_add_func("/basic/nesting", test_nesting);
>
> Here is the output (with printfs in coroutine_swap):
>
> -> coroutine_swap from MAIN to C1
> c1 Part 1
> -> coroutine_swap from C1 to C2
> c2 Part 1
> -> coroutine_swap from C2 to C1
> <- coroutine_swap from C1 to C2 switch yield
> c1 Part 2
> <- coroutine_swap from MAIN to C1 switch terminate
> coroutine_delete C1
> -> coroutine_swap from MAIN to C2
> <- coroutine_swap from C2 to C1 switch yield !!!
> c2 Part 2
> <- coroutine_swap from MAIN to C2 switch terminate
> coroutine_delete C2
>
> I have marked the problematic line with "!!!". The to=C1 variable is
> used after C1 has been deleted.
How to build the test program? make test? I meet the following errors:
make test
make -C tests/tcg test
make[1]: Entering directory '/work/src/qemu/tests/tcg'
CC test_path.o
In file included from /work/src/qemu/include/qemu-common.h:43:0,
from /work/src/qemu/util/cutils.c:24,
from test_path.c:3:
/work/src/qemu/include/glib-compat.h:19:18: fatal error: glib.h: No such file
or directory
#include <glib.h>
^
compilation terminated.
/work/src/qemu/rules.mak:57: recipe for target 'test_path.o' failed
make[1]: *** [test_path.o] Error 1
make[1]: Leaving directory '/work/src/qemu/tests/tcg'
Makefile:428: recipe for target 'test' failed
make: *** [test] Error 2
Do I miss something?
Thanks
Wen Congyang
>
> The test crashes since it writes 0xff to C1 after it has terminated.
>
>> Signed-off-by: Bin Wu <address@hidden>
>> ---
>> qemu-coroutine.c | 16 ++++++++++------
>> 1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
>> index 525247b..cc0bdfa 100644
>> --- a/qemu-coroutine.c
>> +++ b/qemu-coroutine.c
>> @@ -99,29 +99,31 @@ static void coroutine_delete(Coroutine *co)
>> qemu_coroutine_delete(co);
>> }
>>
>> -static void coroutine_swap(Coroutine *from, Coroutine *to)
>> +static CoroutineAction coroutine_swap(Coroutine *from, Coroutine *to)
>> {
>> CoroutineAction ret;
>>
>> ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
>>
>> - qemu_co_queue_run_restart(to);
>> -
>> switch (ret) {
>> case COROUTINE_YIELD:
>> - return;
>> + break;
>> case COROUTINE_TERMINATE:
>> trace_qemu_coroutine_terminate(to);
>> + qemu_co_queue_run_restart(to);
>> coroutine_delete(to);
>> - return;
>> + break;
>> default:
>> abort();
>> }
>> +
>> + return ret;
>> }
>>
>> void qemu_coroutine_enter(Coroutine *co, void *opaque)
>> {
>> Coroutine *self = qemu_coroutine_self();
>> + CoroutineAction ret;
>>
>> trace_qemu_coroutine_enter(self, co, opaque);
>>
>> @@ -132,7 +134,9 @@ void qemu_coroutine_enter(Coroutine *co, void *opaque)
>>
>> co->caller = self;
>> co->entry_arg = opaque;
>> - coroutine_swap(self, co);
>> + ret = coroutine_swap(self, co);
>> + if (ret == COROUTINE_YIELD)
>> + qemu_co_queue_run_restart(co);
>> }
>
> Your fix looks correct although QEMU coding style requires {}.
>
> I tried to think of a simpler solution that keeps a single
> qemu_co_queue_run_restart() call but was unable to find one.
>
> Please send another revision with a test-coroutine.c test case so we can
> protect against regressions.
>
> Thanks,
> Stefan
>