From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: libc-alpha@sourceware.org
Subject: Re: [PATCH 3/3] Refactor atfork handlers
Date: Tue, 20 Feb 2018 10:00:19 -0300 [thread overview]
Message-ID: <4aad8145-b06f-4d95-315a-73d5f2253971@linaro.org> (raw)
In-Reply-To: <7b71dd04-afd0-9ff0-79c3-3d47cbd77ee2@redhat.com>
On 20/02/2018 08:29, Florian Weimer wrote:
> On 02/08/2018 01:50 PM, Adhemerval Zanella wrote:
>> +static struct fork_handler *
>> +fork_handler_list_find_if (struct fork_handler_list *fork_handlers,
>> + void *dso_handle)
>
> Should be called _find, not find_if (no callback is involved).
Fixed.
>
>> + struct fork_handler *first = fork_handler_list_find_if (&fork_handlers,
>> + dso_handle);
>> + /* Removing is done by shifting the elements in the way the elements
>> + that are not to be removed appear in the beginning in dynarray.
>> + This avoid the quadradic run-time if a naive strategy to remove and
>> + shift one element at time. */
>> + if (first != NULL)
>> + {
>> + struct fork_handler *result = first;
>
> result should probably be called new_end or something like that.
I changed to new_end.
>
>> + first++;
>> + for (; first != fork_handler_list_end (&fork_handlers); ++first)
>> + {
>> + if (first->dso_handle != dso_handle)
>> + {
>> + memcpy (result, first, sizeof (struct fork_handler));
>
> Wouldn't a simple struct assignment work here?
I think so, I changed it to struct assignment.
>
> I think this patch is a step in the right direction, so it should go in with these changes.
Thanks for the review.
>
> However, I think we should make a few improvements in follow-up fixes:
>
> Reduce RSS usage for the common case that no atfork handlers are ever registered. This will be the case once we remove the bogus __reclaim_stacks function.
>
> Make a temporary copy of the handler array during fork. This has two benefits: We can run the handlers without acquiring the handler lock (to avoid application deadlocks). We also make sure that a handler does not run in a child process which did not run in the parent process. I think the old implementation had both properties.
The temporary copy is problematic because we either need to allocate on the stack using
vla/alloca (current practice and prone of stack overflow) or by malloc (which requires
locking anyway). Also, to temporary copy we will need pretty much the same lock-free
algorithm which adds code complexity.
My understanding is current algorithm tries hard to remove any locking on fork generation
mainly because back then posix_spawn was no specified and suboptimal. Now that we have
a faster way to spawn process in multithread environment I think there is no much gain
in trying to optimizing locking in atfork handlers.
Regarding the handler running in child process the proposed implementation does implement
it.
next prev parent reply other threads:[~2018-02-20 12:58 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-07 13:09 [PATCH 1/3] Refactor Linux ARCH_FORK implementation Adhemerval Zanella
2018-02-07 13:09 ` [PATCH 2/3] dynarray: Implement remove function Adhemerval Zanella
2018-02-07 14:48 ` Alexander Monakov
2018-02-07 16:06 ` Adhemerval Zanella
2018-02-07 13:09 ` [PATCH 3/3] Refactor atfork handlers Adhemerval Zanella
2018-02-07 15:07 ` Florian Weimer
2018-02-07 17:16 ` Adhemerval Zanella
2018-02-08 8:32 ` Florian Weimer
2018-02-08 12:50 ` Adhemerval Zanella
2018-02-20 11:29 ` Florian Weimer
2018-02-20 13:00 ` Adhemerval Zanella [this message]
2018-02-20 13:05 ` Florian Weimer
2018-02-20 13:27 ` Adhemerval Zanella
2018-02-20 13:42 ` Florian Weimer
2018-02-20 13:48 ` Adhemerval Zanella
2018-02-20 13:58 ` Florian Weimer
2018-02-20 14:23 ` Adhemerval Zanella
2018-02-23 10:41 ` Florian Weimer
2018-02-23 12:10 ` Adhemerval Zanella
2018-02-27 8:25 ` Florian Weimer
2018-03-07 16:51 ` [PATCH 1/3] Refactor Linux ARCH_FORK implementation Adhemerval Zanella
2018-03-08 12:05 ` Florian Weimer
2018-03-08 12:58 ` Adhemerval Zanella
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