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Date: Mon, 26 Oct 2015 09:45:06 +0100
From: Ingo Molnar <mingo@...nel.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...icios.com, josh@...htriplett.org,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
Patrick Marlier <patrick.marlier@...il.com>
Subject: Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use
lockless_dereference()
* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> From: Patrick Marlier <patrick.marlier@...il.com>
>
> The current list_entry_rcu() implementation copies the pointer to a stack
> variable, then invokes rcu_dereference_raw() on it. This results in an
> additional store-load pair. Now, most compilers will emit normal store
> and load instructions, which might seem to be of negligible overhead,
> but this results in a load-hit-store situation that can cause surprisingly
> long pipeline stalls, even on modern microprocessors. The problem is
> that it takes time for the store to get the store buffer updated, which
> can delay the subsequent load, which immediately follows.
>
> This commit therefore switches to the lockless_dereference() primitive,
> which does not expect the __rcu annotations (that are anyway not present
> in the list_head structure) and which, like rcu_dereference_raw(),
> does not check for an enclosing RCU read-side critical section.
> Most importantly, it does not copy the pointer, thus avoiding the
> load-hit-store overhead.
>
> Signed-off-by: Patrick Marlier <patrick.marlier@...il.com>
> [ paulmck: Switched to lockless_dereference() to suppress sparse warnings. ]
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> include/linux/rculist.h | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 17c6b1f84a77..5ed540986019 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -247,10 +247,7 @@ static inline void list_splice_init_rcu(struct list_head *list,
> * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
> */
> #define list_entry_rcu(ptr, type, member) \
> -({ \
> - typeof(*ptr) __rcu *__ptr = (typeof(*ptr) __rcu __force *)ptr; \
> - container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
> -})
> + container_of(lockless_dereference(ptr), type, member)
So this commit:
8db70b132dd5 ("rculist: Make list_entry_rcu() use lockless_dereference()")
when merged with Linus's latest tree, triggers the following build failure on
allyesconfig/allmodconfig x86:
triton:~/tip> make fs/fs-writeback.o
CHK include/config/kernel.release
CHK include/generated/uapi/linux/version.h
CHK include/generated/utsrelease.h
CHK include/generated/bounds.h
CHK include/generated/timeconst.h
CHK include/generated/asm-offsets.h
CALL scripts/checksyscalls.sh
CC fs/fs-writeback.o
In file included from fs/fs-writeback.c:16:0:
fs/fs-writeback.c: In function ‘bdi_split_work_to_wbs’:
include/linux/compiler.h:281:20: error: lvalue required as unary ‘&’ operand
__read_once_size(&(x), __u.__c, sizeof(x)); \
^
include/linux/kernel.h:811:49: note: in definition of macro ‘container_of’
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
^
include/linux/compiler.h:286:22: note: in expansion of macro ‘__READ_ONCE’
#define READ_ONCE(x) __READ_ONCE(x, 1)
^
include/linux/compiler.h:533:26: note: in expansion of macro ‘READ_ONCE’
typeof(p) _________p1 = READ_ONCE(p); \
^
include/linux/rculist.h:250:15: note: in expansion of macro ‘lockless_dereference’
container_of(lockless_dereference(ptr), type, member)
^
fs/fs-writeback.c:782:29: note: in expansion of macro ‘list_entry_rcu’
struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
^
include/linux/compiler.h:283:28: error: lvalue required as unary ‘&’ operand
__read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \
^
include/linux/kernel.h:811:49: note: in definition of macro ‘container_of’
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
^
include/linux/compiler.h:286:22: note: in expansion of macro ‘__READ_ONCE’
#define READ_ONCE(x) __READ_ONCE(x, 1)
^
include/linux/compiler.h:533:26: note: in expansion of macro ‘READ_ONCE’
typeof(p) _________p1 = READ_ONCE(p); \
^
include/linux/rculist.h:250:15: note: in expansion of macro ‘lockless_dereference’
container_of(lockless_dereference(ptr), type, member)
^
fs/fs-writeback.c:782:29: note: in expansion of macro ‘list_entry_rcu’
struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
^
scripts/Makefile.build:258: recipe for target 'fs/fs-writeback.o' failed
make[1]: *** [fs/fs-writeback.o] Error 1
Makefile:1526: recipe for target 'fs/fs-writeback.o' failed
make: *** [fs/fs-writeback.o] Error 2
It's this new usage in fs/fs-writeback.c:
static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
struct wb_writeback_work *base_work,
bool skip_if_busy)
{
struct bdi_writeback *last_wb = NULL;
struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
struct bdi_writeback, bdi_node);
so AFAICS the problem is lockless_dereference() not being able to take this valid
but non-lvalue list head, because READ_ONCE() does not take non-lvalues. All other
existing uses of list_entry_rcu() happen to use lvalues, so this bug didn't get
triggered before.
For now I've reverted this commit locally, to make the kernel build and so, but we
need a cleaner solution I suspect.
Thanks,
Ingo
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