From: Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>
To: Dmitry Antipov <dmantipov@yandex.ru>
Cc: lttng-dev <lttng-dev@lists.lttng.org>
Subject: Re: [lttng-dev] Userspace RCU data alignment
Date: Fri, 5 Jun 2020 12:09:08 -0400 (EDT) [thread overview]
Message-ID: <168357390.56518.1591373348015.JavaMail.zimbra@efficios.com> (raw)
Message-ID: <20200605160908.SiID8Q859QAiVLCdF_5bEZmgxFvXCBieLwYgO1HmZcQ@z> (raw)
In-Reply-To: <5352eebd-dbb7-70f9-a354-91522170efdf@yandex.ru>
----- On Jun 3, 2020, at 8:36 AM, lttng-dev lttng-dev@lists.lttng.org wrote:
> I've compiled userspace-rcu with:
>
> CC=gcc CFLAGS='-O0 -g3 -fsanitize=undefined' LIBS='-lubsan' ./configure [xxx]
>
> and see a lot of 'misaligned address' runtime errors like:
>
> ./doc/examples/urcu-flavors/bp ==>
>
> ../include/urcu/static/urcu-bp.h:185:6: runtime error: member access within
> misaligned address 0x7fcac376a020 for type 'struct urcu_bp_reader', which
> requires 128 byte alignment
> 0x7fcac376a020: note: pointer points here
> ca 7f 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 00 00 00 00 00 00
> ^
> #0 0x7fcac372a880 in _urcu_bp_read_unlock ../include/urcu/static/urcu-bp.h:185
> #1 0x7fcac372b0d9 in urcu_bp_read_unlock
> /home/antipov/userspace-rcu/src/urcu-bp.c:363
> #2 0x4017d5 in main
> /home/antipov/userspace-rcu/doc/examples/urcu-flavors/bp.c:92
> #3 0x7fcac2bc2041 in __libc_start_main ../csu/libc-start.c:308
> #4 0x40111d in _start
> (/home/antipov/userspace-rcu/doc/examples/urcu-flavors/bp+0x40111d)
>
> ../include/urcu/static/urcu-bp.h:189:2: runtime error: member access within
> misaligned address 0x7fcac376a020 for type 'struct urcu_bp_reader', which
> requires 128 byte alignment
> 0x7fcac376a020: note: pointer points here
> ca 7f 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 00 00 00 00 00 00
> ^
> #0 0x7fcac372a8c1 in _urcu_bp_read_unlock ../include/urcu/static/urcu-bp.h:189
> #1 0x7fcac372b0d9 in urcu_bp_read_unlock
> /home/antipov/userspace-rcu/src/urcu-bp.c:363
> #2 0x4017d5 in main
> /home/antipov/userspace-rcu/doc/examples/urcu-flavors/bp.c:92
> #3 0x7fcac2bc2041 in __libc_start_main ../csu/libc-start.c:308
> #4 0x40111d in _start
> (/home/antipov/userspace-rcu/doc/examples/urcu-flavors/bp+0x40111d)
>
> Is it critical for correctness, or speed, or whatever else?
As I remember this is mainly for speed, so it's not a show-stopper.
> To whom it may be interesting, some of (probably not all) alignment issues may
> be fixed
> with attached patch.
I agree on fixing this, but I recommend a different approach:
Change:
struct registry_chunk {
size_t data_len; /* data length */
size_t used; /* amount of data used */
struct cds_list_head node; /* chunk_list node */
char data[];
};
for:
struct registry_chunk {
size_t nr_readers; /* number of readers */
size_t used; /* amount of data used */
struct cds_list_head node; /* chunk_list node */
struct urcu_bp_reader readers[];
};
And adapt all the readers allocation scheme within urcu-bp to work on
"readers" elements rather than bytes.
By doing so, struct registry_chunk will adapt to have the proper alignment.
Thanks,
Mathieu
>
> Dmitry
>
> _______________________________________________
> lttng-dev mailing list
> lttng-dev@lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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next prev parent reply other threads:[~2020-06-05 16:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-03 12:36 Userspace RCU data alignment Dmitry Antipov via lttng-dev
2020-06-03 12:36 ` [lttng-dev] " Dmitry Antipov via lttng-dev
2020-06-05 16:09 ` Mathieu Desnoyers via lttng-dev [this message]
2020-06-05 16:09 ` Mathieu Desnoyers via lttng-dev
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