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Message-ID: <53AD575E.7040902@gmail.com>
Date: Fri, 27 Jun 2014 13:37:02 +0200
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
CC: mtk.manpages@...il.com, lkml <linux-kernel@...r.kernel.org>,
"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
netdev <netdev@...r.kernel.org>,
Ondřej Bílka <neleai@...nam.cz>,
Caitlin Bestler <caitlin.bestler@...il.com>,
Neil Horman <nhorman@...driver.com>,
Elie De Brauwer <eliedebrauwer@...il.com>,
David Miller <davem@...emloft.net>,
Steven Whitehouse <steve@...gwyn.com>,
David Laight <David.Laight@...LAB.COM>,
Paul Moore <paul@...l-moore.com>,
Chris Friesen <chris.friesen@...driver.com>
Subject: Re: [PATCH/RFC] Re: recvmmsg() timeout behavior strangeness [RESEND]
Hi Arnaldo,
On 05/28/2014 02:20 PM, Michael Kerrisk (man-pages) wrote:
> On 05/27/2014 10:30 PM, Arnaldo Carvalho de Melo wrote:
>> Em Tue, May 27, 2014 at 09:28:37PM +0200, Michael Kerrisk (man-pages) escreveu:
>>> On Tue, May 27, 2014 at 9:21 PM, Arnaldo Carvalho de Melo
>>> <acme@...stprotocols.net> wrote:
>>>> Em Tue, May 27, 2014 at 06:35:17PM +0200, Michael Kerrisk (man-pages) escreveu:
>>>>> On 05/26/2014 11:17 PM, Arnaldo Carvalho de Melo wrote:
>>>>>> Can you try the attached patch on top of the first one?
>>>>
>>>>> Patches on patches is a way to make your testers work unnecessarily
>>>>> harder. Also, it means that anyone else who was interested in this
>>>>
>>>> It was meant to highlight the changes with regard to the previous patch,
>>>> i.e. to make things easier for reviewing.
>>>
>>> (I don't think that works...)
>>
>> Lets try both then,
>
> That's better!
>
>> attached goes the updated patch, and this is the
>> diff to the last combined one:
>>
>> diff --git a/net/socket.c b/net/socket.c
>> index 310a50971769..379be43879db 100644
>> --- a/net/socket.c
>> +++ b/net/socket.c
>> @@ -2478,8 +2478,7 @@ SYSCALL_DEFINE5(recvmmsg, int, fd, struct mmsghdr __user *, mmsg,
>>
>> datagrams = __sys_recvmmsg(fd, mmsg, vlen, flags, &timeout_sys);
>>
>> - if (datagrams > 0 &&
>> - copy_to_user(timeout, &timeout_sys, sizeof(timeout_sys)))
>> + if (copy_to_user(timeout, &timeout_sys, sizeof(timeout_sys)))
>> datagrams = -EFAULT;
>>
>> return datagrams;
>>
>> ------------------------------------------
>>
>> This is a quick thing just to show where the problem lies, need to think
>> how to report an -EFAULT at this point properly, i.e. look at
>> __sys_recvmmsg for something related (returning the number of
>> successfully copied datagrams to userspace while storing the error for
>> subsequent reporting):
>>
>> if (err == 0)
>> return datagrams;
>>
>> if (datagrams != 0) {
>> /*
>> * We may return less entries than requested (vlen) if
>> * the
>> * sock is non block and there aren't enough
>> * datagrams...
>> */
>> if (err != -EAGAIN) {
>> /*
>> * ... or if recvmsg returns an error after we
>> * received some datagrams, where we record the
>> * error to return on the next call or if the
>> * app asks about it using getsockopt(SO_ERROR).
>> */
>> sock->sk->sk_err = -err;
>> }
>>
>> return datagrams;
>> }
>>
>> I.e. userspace would have to use getsockopt(SO_ERROR)... need to think
>> more about it, sidetracked now, will be back to this.
>>
>> Anyway, attached goes the current combined patch.
>
> So, I applied against net-next as you suggested offlist.
> Builds and generally tests fine. Some observations:
>
> * In the case that the call is interrupted by a signal handler and no
> datagrams have been received, the call fails with EINTR, as expected.
>
> * The call always updates 'timeout', both in the success case and in the
> EINTR case. (That seems fine.)
So, returning to your recvmmsg-timeout-v3.patch. I think the behavior as
implemented, and described above is okay.
> But, another question...
>
> In the case that the call is interrupted by a signal handler and some
> datagrams have already been received, then the call succeeds, and
> returns the number of datagrams received, and 'timeout' is updated with
> the remaining time. Maybe that's the right behavior, but I just want to
> check. There is at least one other possibility:
>
> * Fetch no datagrams (i.e., the datagrams are left to receive in a
> future call), and the call fails with EINTR, and 'timeout' is updated.
>
> Maybe that possibility is hard to implement (not sure). But my main point
> is to make the current behavior clear, note the alternative, and ask:
> is the current behavior the best choice. (I'm not saying it's not, but I
> do want the choice to be a conscious one.)
So, I think (can't find the mail right now) that you explained elsewhere
that the above would be hard to implement. And in any case, I'm not sure
it's desirable; I only wanted to check that the choice was a deliberate one.
However, there is still a weirdness, which relates to the discussion you
and David Laight had.
Suppose the following scenario.
1. We do a recvmmsg() with 10 second timeout, asking for 5 messages.
2. 3 messages arrive
3. 6 seconds after the call, a signal handler interrupts the call.
4. recvmmsg() returns success, telling us it got 3 messages.
So far, so good. But
5. We make a further recvmmsg() call.
6. That call returns immediately, with an EINTR error.
That really should not be happening. As noted elsewhere in this
thread, EINTR is a property of a specific system call, not of the
thread or the socket. By the time of step 5, the kernel should
already have forgotten about the signal that occurred at step 3.
I don't think I saw any other patch that fixes that behavior.
I recall now that this was why I was waiting for you to follow up
in this thread with a new patch.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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