Re: explanation of some pieces of code?
Hi Mike,
Thank you so much for your help!
Regards,
Jack
On Jan 29, 11:56 am, Mike Christie wrote:
> On 01/27/2010 02:41 AM, Jack Z wrote:
>
>
>
> > Hi group,
>
> > I'm looking at the following code and come up with some questions
>
> > In this function,
>
> > static void iscsi_sw_tcp_write_space(struct sock *sk)
> > {
> > struct iscsi_conn *conn = (struct iscsi_conn*)sk->sk_user_data;
> > struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
> > struct iscsi_sw_tcp_conn *tcp_sw_conn = tcp_conn->dd_data;
>
> > tcp_sw_conn->old_write_space(sk);
> > ISCSI_SW_TCP_DBG(conn, "iscsi_write_space\n");
> > iscsi_conn_queue_work(conn);
> > }
>
> > tcp_sw_conn->old_write_space() is called. And in the following
> > function, tcp_sw_conn->old_write_space() seems to be assigned to the
> > original value of "sk->sk_write_space"...
>
> > Could anyone show me which function does "sk->sk_write_space" point to
> > before it is assigned to "iscsi_sw_tcp_write_space;" later in the same
> > function?
>
> I believe it is sk_stream_write_space. For tcp v4 sockets see
> tcp_v4_init_sock.
>
>
>
>
>
> > static void iscsi_sw_tcp_conn_set_callbacks(struct iscsi_conn *conn)
> > {
> > struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
> > struct iscsi_sw_tcp_conn *tcp_sw_conn = tcp_conn->dd_data;
> > struct sock *sk = tcp_sw_conn->sock->sk;
>
> > /* assign new callbacks */
> > ..
> > tcp_sw_conn->old_write_space = sk->sk_write_space;
>
> > // assignment of "sk->sk_write_space" but after its value was assigned
> > to "tcp_sw_conn->old_write_space"...
>
> > sk->sk_write_space = iscsi_sw_tcp_write_space;
> > write_unlock_bh(&sk->sk_callback_lock);
> > }
>
> > Another question is: Now that we have "sk->sk_write_space =
> > iscsi_sw_tcp_write_space" why do we have to call "tcp_sw_conn-
> >> old_write_space(sk);" in "iscsi_sw_tcp_write_space()"? Which function
>
> We override it so the iscsi layer can start up its send code if needed,
> and we call the old tcp function so it can do its processing (possibly
> wake up threads waiting for space to open up).
>
>
>
> > is actually called when we do "tcp_sw_conn->old_write_space(sk);"?
>
> > And also in fuction "static void iscsi_sw_tcp_write_space(struct sock
> > *sk)", the last instruction is "iscsi_conn_queue_work(conn);". When I
> > trace down I found
>
> > inline void iscsi_conn_queue_work(struct iscsi_conn *conn)
> > {
> > struct Scsi_Host *shost = conn->session->host;
> > struct iscsi_host *ihost = shost_priv(shost);
>
> > if (ihost->workq)
> > queue_work(ihost->workq,&conn->xmitwork);
> > }
>
> > But I couldn't find more details about "queue_work(ihost->workq,&conn-
> >> xmitwork);" in the source code (ver 2.0.871). Could anyone maybe
> > point me to the code of "queu_work"?
>
> It is in the kernel source. Get the kernel and look at
> kernel/workqueue.c. It basically wakes up a thread and has it process
> some work. For iscsi it wakes up our xmit thread in case it had slept
> due to there not being any write space.
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Re: explanation of some pieces of code?
On 01/27/2010 02:41 AM, Jack Z wrote:
Hi group,
I'm looking at the following code and come up with some questions
In this function,
static void iscsi_sw_tcp_write_space(struct sock *sk)
{
struct iscsi_conn *conn = (struct iscsi_conn*)sk->sk_user_data;
struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
struct iscsi_sw_tcp_conn *tcp_sw_conn = tcp_conn->dd_data;
tcp_sw_conn->old_write_space(sk);
ISCSI_SW_TCP_DBG(conn, "iscsi_write_space\n");
iscsi_conn_queue_work(conn);
}
tcp_sw_conn->old_write_space() is called. And in the following
function, tcp_sw_conn->old_write_space() seems to be assigned to the
original value of "sk->sk_write_space"...
Could anyone show me which function does "sk->sk_write_space" point to
before it is assigned to "iscsi_sw_tcp_write_space;" later in the same
function?
I believe it is sk_stream_write_space. For tcp v4 sockets see
tcp_v4_init_sock.
static void iscsi_sw_tcp_conn_set_callbacks(struct iscsi_conn *conn)
{
struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
struct iscsi_sw_tcp_conn *tcp_sw_conn = tcp_conn->dd_data;
struct sock *sk = tcp_sw_conn->sock->sk;
/* assign new callbacks */
..
tcp_sw_conn->old_write_space = sk->sk_write_space;
// assignment of "sk->sk_write_space" but after its value was assigned
to "tcp_sw_conn->old_write_space"...
sk->sk_write_space = iscsi_sw_tcp_write_space;
write_unlock_bh(&sk->sk_callback_lock);
}
Another question is: Now that we have "sk->sk_write_space =
iscsi_sw_tcp_write_space" why do we have to call "tcp_sw_conn-
old_write_space(sk);" in "iscsi_sw_tcp_write_space()"? Which function
We override it so the iscsi layer can start up its send code if needed,
and we call the old tcp function so it can do its processing (possibly
wake up threads waiting for space to open up).
is actually called when we do "tcp_sw_conn->old_write_space(sk);"?
And also in fuction "static void iscsi_sw_tcp_write_space(struct sock
*sk)", the last instruction is "iscsi_conn_queue_work(conn);". When I
trace down I found
inline void iscsi_conn_queue_work(struct iscsi_conn *conn)
{
struct Scsi_Host *shost = conn->session->host;
struct iscsi_host *ihost = shost_priv(shost);
if (ihost->workq)
queue_work(ihost->workq,&conn->xmitwork);
}
But I couldn't find more details about "queue_work(ihost->workq,&conn-
xmitwork);" in the source code (ver 2.0.871). Could anyone maybe
point me to the code of "queu_work"?
It is in the kernel source. Get the kernel and look at
kernel/workqueue.c. It basically wakes up a thread and has it process
some work. For iscsi it wakes up our xmit thread in case it had slept
due to there not being any write space.
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RE: explanation of some pieces of code?
How to quit this group? -Original Message- From: open-iscsi@googlegroups.com [mailto:open-is...@googlegroups.com] On Behalf Of Jack Z Sent: 2010年1月28日 15:53 To: open-iscsi Subject: Re: explanation of some pieces of code? Hi Ulrich, Thanks for your reply. That for sure increased my understanding about the "hook"s a lot. I think what I really want to know is the latter one, i.e. " what the routine is supposed to do?". To reorganize the question, what is sk- >sk_write_space usually supposed to do? On Jan 28, 12:16 am, "Ulrich Windl" wrote: > On 27 Jan 2010 at 9:59, Jack Z wrote: > > > Hi Ulrich, > > > Thanks for your comment. > > > So by "implement polymorphism", do you mean sk->sw_write_space is > > platform dependent or it does different jobs when called from > > different functions in the open-iscsi code? > > Hi, > > I mean: If you call a routine indirect through a function pointer, the > reason is that you want to call different routines at runtime. > Otherwise this is unnecessary and shows poor performance. > > So if calling different routines through a pointer, there must exist a > common expectation what the code being called does (i.e. the common > subset of expectations that every possible routine fulfills). > > [If you want to look it up somewhere else, try "dynamic binding", > "polymorphism" or Bertrand Meyers "Object-Oriented software > construction"] > > Therefore you should not ask which specific routine will be called > through the pointer at some moment in time, but what the routine > (whatever it is) is expected to do. > > (I know it's theoretical, but just introducing "hooks" (as function > pointers are frequently called) can be a source of errors unless the > semantics of such a hook are well defined). > > Now: Do you really want to know which specific routine is called, or do > you want to know what the routine is supposed to do? > > [...] > > Regards, > Ulrich -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en. -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en.
Re: explanation of some pieces of code?
Hi Ulrich, Thanks for your reply. That for sure increased my understanding about the "hook"s a lot. I think what I really want to know is the latter one, i.e. " what the routine is supposed to do?". To reorganize the question, what is sk- >sk_write_space usually supposed to do? On Jan 28, 12:16 am, "Ulrich Windl" wrote: > On 27 Jan 2010 at 9:59, Jack Z wrote: > > > Hi Ulrich, > > > Thanks for your comment. > > > So by "implement polymorphism", do you mean sk->sw_write_space is > > platform dependent or it does different jobs when called from > > different functions in the open-iscsi code? > > Hi, > > I mean: If you call a routine indirect through a function pointer, the > reason is that you want to call different routines at runtime. > Otherwise this is unnecessary and shows poor performance. > > So if calling different routines through a pointer, there must exist a > common expectation what the code being called does (i.e. the common > subset of expectations that every possible routine fulfills). > > [If you want to look it up somewhere else, try "dynamic binding", > "polymorphism" or Bertrand Meyers "Object-Oriented software > construction"] > > Therefore you should not ask which specific routine will be called > through the pointer at some moment in time, but what the routine > (whatever it is) is expected to do. > > (I know it's theoretical, but just introducing "hooks" (as function > pointers are frequently called) can be a source of errors unless the > semantics of such a hook are well defined). > > Now: Do you really want to know which specific routine is called, or do > you want to know what the routine is supposed to do? > > [...] > > Regards, > Ulrich -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en.
Re: explanation of some pieces of code?
On 27 Jan 2010 at 9:59, Jack Z wrote: > Hi Ulrich, > > Thanks for your comment. > > So by "implement polymorphism", do you mean sk->sw_write_space is > platform dependent or it does different jobs when called from > different functions in the open-iscsi code? Hi, I mean: If you call a routine indirect through a function pointer, the reason is that you want to call different routines at runtime. Otherwise this is unnecessary and shows poor performance. So if calling different routines through a pointer, there must exist a common expectation what the code being called does (i.e. the common subset of expectations that every possible routine fulfills). [If you want to look it up somewhere else, try "dynamic binding", "polymorphism" or Bertrand Meyers "Object-Oriented software construction"] Therefore you should not ask which specific routine will be called through the pointer at some moment in time, but what the routine (whatever it is) is expected to do. (I know it's theoretical, but just introducing "hooks" (as function pointers are frequently called) can be a source of errors unless the semantics of such a hook are well defined). Now: Do you really want to know which specific routine is called, or do you want to know what the routine is supposed to do? [...] Regards, Ulrich -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en.
Re: explanation of some pieces of code?
Hi Ulrich, Thanks for your comment. So by "implement polymorphism", do you mean sk->sw_write_space is platform dependent or it does different jobs when called from different functions in the open-iscsi code? Thanks a lot! Jack On Jan 27, 2:10 am, "Ulrich Windl" wrote: > On 27 Jan 2010 at 0:41, Jack Z wrote: > > [...] > > > Another question is: Now that we have "sk->sk_write_space = > > iscsi_sw_tcp_write_space" why do we have to call "tcp_sw_conn- > > >old_write_space(sk);" in "iscsi_sw_tcp_write_space()"? Which function > > is actually called when we do "tcp_sw_conn->old_write_space(sk);"? > > [...] > > Hi, > > I cannot answer your question, but I'd like to place a remark on it: > The way you ask is a very low level question (which function is > actually called?). As this mechanism seems to implement polymorphism, > the high-level question probably is: "What is the fuction call supposed > to do (at that point)?" > > Sorry, but I had to say. > > Regards, > Ulrich -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en.
Re: explanation of some pieces of code?
On 27 Jan 2010 at 0:41, Jack Z wrote: [...] > Another question is: Now that we have "sk->sk_write_space = > iscsi_sw_tcp_write_space" why do we have to call "tcp_sw_conn- > >old_write_space(sk);" in "iscsi_sw_tcp_write_space()"? Which function > is actually called when we do "tcp_sw_conn->old_write_space(sk);"? > [...] Hi, I cannot answer your question, but I'd like to place a remark on it: The way you ask is a very low level question (which function is actually called?). As this mechanism seems to implement polymorphism, the high-level question probably is: "What is the fuction call supposed to do (at that point)?" Sorry, but I had to say. Regards, Ulrich -- You received this message because you are subscribed to the Google Groups "open-iscsi" group. To post to this group, send email to open-is...@googlegroups.com. To unsubscribe from this group, send email to open-iscsi+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/open-iscsi?hl=en.
