From: Roman Penyaev <rpenyaev@suse.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: azat@libevent.org, akpm@linux-foundation.org,
viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring
Date: Fri, 31 May 2019 13:15:21 +0200 [thread overview]
Message-ID: <274e29d102133f3be1f309c66cb0af36@suse.de> (raw)
In-Reply-To: <20190531095607.GC17637@hirez.programming.kicks-ass.net>
On 2019-05-31 11:56, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote:
>> +static inline bool ep_add_event_to_uring(struct epitem *epi, __poll_t
>> pollflags)
>> +{
>> + struct eventpoll *ep = epi->ep;
>> + struct epoll_uitem *uitem;
>> + bool added = false;
>> +
>> + if (WARN_ON(!pollflags))
>> + return false;
>> +
>> + uitem = &ep->user_header->items[epi->bit];
>> + /*
>> + * Can be represented as:
>> + *
>> + * was_ready = uitem->ready_events;
>> + * uitem->ready_events &= ~EPOLLREMOVED;
>> + * uitem->ready_events |= pollflags;
>> + * if (!was_ready) {
>> + * // create index entry
>> + * }
>> + *
>> + * See the big comment inside ep_remove_user_item(), why it is
>> + * important to mask EPOLLREMOVED.
>> + */
>> + if (!atomic_or_with_mask(&uitem->ready_events,
>> + pollflags, EPOLLREMOVED)) {
>> + unsigned int i, *item_idx, index_mask;
>> +
>> + /*
>> + * Item was not ready before, thus we have to insert
>> + * new index to the ring.
>> + */
>> +
>> + index_mask = ep_max_index_nr(ep) - 1;
>> + i = __atomic_fetch_add(&ep->user_header->tail, 1,
>> + __ATOMIC_ACQUIRE);
>
> afaict __atomic_fetch_add() does not exist.
That is gcc extension. I did not find any API just to increment
the variable atomically without using/casting to atomic. What
is a proper way to achieve that?
>
>> + item_idx = &ep->user_index[i & index_mask];
>> +
>> + /* Signal with a bit, which is > 0 */
>> + *item_idx = epi->bit + 1;
>
> Did you just increment the user visible tail pointer before you filled
> the data? That is, can the concurrent userspace observe the increment
> before you put credible data in its place?
No, the "data" is the "ready_events" mask, which was updated before,
using cmpxchg, atomic_or_with_mask() call. All I need is to put an
index of just updated item to the uring.
Userspace, in its turn, gets the index from the ring and then checks
the mask.
>
>> +
>> + /*
>> + * Want index update be flushed from CPU write buffer and
>> + * immediately visible on userspace side to avoid long busy
>> + * loops.
>> + */
>> + smp_wmb();
>
> That's still complete nonsense.
Yes, true. My confusion came from the simple test, where one thread
swaps pointers in a loop, another thread dereferences pointer and
increments a variable:
THR#0
-----------
unsigned vvv1 = 0, vvv2 = 0;
unsigned *ptr;
ptr = &vvv1;
thr_level2 = &vvv2;
while (!stop) {
unsigned *tmp = *thr_level2;
*thr_level2 = ptr;
barrier(); <<<< ????
ptr = tmp;
}
THR#1
-----------
while (!stop) {
ptr = thr_level2;
(*ptr)++;
}
At the end I expect `vvv1` and `vvv2` are approximately equally
incremented. But, without barrier() only one variable is
incremented.
Now I see that barrier() should be defined as a simple compiler
barrier as asm volatile("" ::: "memory"), and there is nothing
related with write buffer as I wrote in the comment.
So indeed garbage and can be removed. Thanks.
--
Roman
next prev parent reply other threads:[~2019-05-31 11:15 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-16 8:57 [PATCH v3 00/13] epoll: support pollable epoll from userspace Roman Penyaev
2019-05-16 8:57 ` [PATCH v3 01/13] epoll: move private helpers from a header to the source Roman Penyaev
2019-05-16 8:57 ` [PATCH v3 02/13] epoll: introduce user structures for polling from userspace Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 03/13] epoll: allocate user header and user events ring " Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 04/13] epoll: some sanity flags checks for epoll syscalls " Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 05/13] epoll: offload polling to a work in case of epfd polled " Roman Penyaev
2019-05-21 7:51 ` Eric Wong
2019-05-22 12:54 ` Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring Roman Penyaev
2019-05-31 9:55 ` Peter Zijlstra
2019-05-31 11:24 ` Roman Penyaev
2019-05-31 13:11 ` Peter Zijlstra
2019-05-31 9:56 ` Peter Zijlstra
2019-05-31 11:15 ` Roman Penyaev [this message]
2019-05-31 12:53 ` Peter Zijlstra
2019-05-31 14:28 ` Roman Penyaev
2019-05-31 16:53 ` Peter Zijlstra
2019-05-31 12:56 ` Peter Zijlstra
2019-05-31 14:21 ` Roman Penyaev
2019-05-31 16:51 ` Peter Zijlstra
2019-05-31 18:58 ` Roman Penyaev
2019-06-03 9:09 ` Peter Zijlstra
2019-06-03 10:02 ` Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 07/13] epoll: call ep_add_event_to_uring() from ep_poll_callback() Roman Penyaev
2019-05-31 9:56 ` Peter Zijlstra
2019-05-31 11:22 ` Roman Penyaev
2019-05-31 13:05 ` Peter Zijlstra
2019-05-31 15:05 ` Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 08/13] epoll: support polling from userspace for ep_insert() Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 09/13] epoll: support polling from userspace for ep_remove() Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 10/13] epoll: support polling from userspace for ep_modify() Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 11/13] epoll: support polling from userspace for ep_poll() Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 12/13] epoll: support mapping for epfd when polled from userspace Roman Penyaev
2019-05-16 8:58 ` [PATCH v3 13/13] epoll: implement epoll_create2() syscall Roman Penyaev
2019-05-16 10:03 ` Arnd Bergmann
2019-05-16 10:20 ` Roman Penyaev
2019-05-16 10:57 ` Arnd Bergmann
2019-05-22 2:33 ` Andrew Morton
2019-05-22 9:11 ` Roman Penyaev
2019-05-22 11:14 ` Arnd Bergmann
2019-05-22 18:36 ` Andrew Morton
2019-05-31 9:55 ` [PATCH v3 00/13] epoll: support pollable epoll from userspace Peter Zijlstra
2019-05-31 14:48 ` Jens Axboe
2019-05-31 16:02 ` Roman Penyaev
2019-05-31 16:54 ` Jens Axboe
2019-05-31 19:45 ` Roman Penyaev
2019-05-31 21:09 ` Jens Axboe
2019-06-05 6:17 ` Roman Penyaev
2019-05-31 16:33 ` Peter Zijlstra
2019-05-31 18:50 ` Roman Penyaev
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