Question on continuous EIOs in CIFS protocol

[Richard Sharpe]

On Thu, Sep 18, 2014 at 11:49 AM, Volker Lendecke
<Volker.Lendecke at sernet.de> wrote:
> Hi!
>
> From this snipped I can't really tell what your big picture
> looks like. Can you please post the whole thing somewhere?
> I'd be happy with your patchset, as long as I can cleanly
> apply this to 3.5.15.

>From what I recall this is deep in their VFS module ...

In any event, why is read getting EINTR so many times in a row?

Is it because of the SIGNALs they are using for write completions?

> Thanks,
>
> Volker
>
> On Thu, Sep 18, 2014 at 11:28:34AM -0700, sandeep nag wrote:
>> We are using 3.5.15 + Async IO + few patches.
>>
>> Here is the code:
>> In the write() call path we call ' cifs_wd_to_buf()'
>>
>> static cifs_error_t
>> cifs_wd_to_buf(
>>     wdata_t         *wd,
>>     char            *buf,
>>     size_t          n)
>> {
>>     size_t          bytes_read = 0;
>>     size_t          num_continuous_interupts = 0;
>>     ssize_t         read_ret = -1;
>>     cifs_error_t    err = CIFS_OK;
>>
>>     if (wd->recv_data == false)
>>     {
>>         if (buf != wd->u.data_buf)
>>         {
>>             memcpy(buf, wd->u.data_buf, n);
>>         }
>>         CIFS_DONE();
>>     }
>>
>>     bytes_read = 0;
>>     while(bytes_read != n)
>>     {
>>         read_ret = read(wd->u.rfd, buf + bytes_read, n - bytes_read);
>>         if (read_ret == 0)
>>         {
>>             read_ret = -1;
>>             CIFS_NOTICE("%s: Short read from socket, n %ld, bytes_read %ld,"
>>                         " errno %d", __FUNCTION__, n, bytes_read, errno);
>>             if (errno == 0)
>>             {
>>                 errno = EIO;
>>             }
>>         }
>>
>>         if (read_ret == -1)
>>         {
>>              if (errno == EINTR)
>>             {
>>                 num_continuous_interupts++;
>>                 if(num_continuous_interupts > 100)
>>                 {
>>                     errno =* EIO;*
>>                    *<====  This is the EIO error I was speaking about*
>>                 }
>>                 else
>>                     continue;
>>             }
>>
>>             assert(errno);
>>             err = ERRNO_TO_CIFS_ERROR(errno);
>>             CIFS_ERROR_DONE(err, "%s: Error reading file data from socket",
>>                                 __FUNCTION__);
>>         }
>>
>>         bytes_read += read_ret;
>>     }
>>
>>     assert(bytes_read == n);
>>
>> done:
>>     return err;
>> }
>>
>>
>> On Thu, Sep 18, 2014 at 10:17 AM, Richard Sharpe <
>> realrichardsharpe at gmail.com> wrote:
>>
>> > On Thu, Sep 18, 2014 at 10:08 AM, Volker Lendecke
>> > <Volker.Lendecke at sernet.de> wrote:
>> > > Hi!
>> > >
>> > > Windows servers expect writes to succeed unless there's a
>> > > real error like a full disk or permission denied or so.
>> > > There won't be a retry. The retry logic needs to live within
>> > > smbd completely.
>> > >
>> > > For a VFS that goes out to a socket and not to a local
>> > > syscall, I would recommend that you look at our async I/O.
>> > > With Samba 4.1 we have implemented very flexible VFS
>> > > operations (pread_send/recv and pwrite_send/recv) that are
>> > > able to handle this nicely. In case you're sitting on an
>> > > earlier version, you need to take a look at
>> > > aio_read/aio_write/aio_return. Not as flexibile, but you can
>> > > also implement async retry logic there.
>> > >
>> > > It would be best if you would post your VFS module source
>> > > somewhere (it's GPL in the end :-)) so that we can make
>> > > recommendations.
>> >
>> > What he didn't tell you is that it is Samba 3.5.15 or so and they are
>> > using the Async IO stuff in that version.
>> >
>> > --
>> > Regards,
>> > Richard Sharpe
>> > (何以解憂？唯有杜康。--曹操)
>> >
>
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-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)