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* DevNotes: Add entry cgf-000010.

Browse Source 
* select.cc (set_handle_or_return_if_not_open): Remove unneeded final backslash
from definition.
(cygwin_select): Reorganize to incorporate outer retry loop.  Move remaining
time recalculation here for retry case.  Use select_stuff::wait_states for loop
control.
(select_stuff::cleanup): Avoid unneeded initialization.
(select_stuff::wait): Modify definition to return select_stuff::wait_states.
Eliminate is_cancelable.  Don't element 1 of an array if it is a cancel handle.
Remove loop.  Rely on being called from enclosing loop in cygwin_select.
Remove time recalculation when restarting.  Try harder to always return from
the bottom.
* select.h (select_stuff::wait_state): New enum.
(select_stuff::wait): Modify declaration to return select_stuff::wait_states.
This commit is contained in:
Christopher Faylor 2012-06-03 02:59:20 +00:00
parent faab45455a
commit 45b61a88be
5 changed files with 219 additions and 152 deletions
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18
winsup/cygwin/ChangeLog
View File

@ -1,3 +1,21 @@
2012-06-02 Christopher Faylor <me.cygwin2012@cgf.cx>
* DevNotes: Add entry cgf-000010.
* select.cc (set_handle_or_return_if_not_open): Remove unneeded final
backslash from definition.
(cygwin_select): Reorganize to incorporate outer retry loop. Move
remaining time recalculation here for retry case. Use
select_stuff::wait_states for loop control.
(select_stuff::cleanup): Avoid unneeded initialization.
(select_stuff::wait): Modify definition to return
select_stuff::wait_states. Eliminate is_cancelable. Don't element 1
of an array if it is a cancel handle. Remove loop. Rely on being
called from enclosing loop in cygwin_select. Remove time recalculation
when restarting. Try harder to always return from the bottom.
* select.h (select_stuff::wait_state): New enum.
(select_stuff::wait): Modify declaration to return
select_stuff::wait_states.
2012-06-02 Christopher Faylor <me.cygwin2012@cgf.cx>
* exceptions.cc (setup_handler): Make debugging output a little more

42
winsup/cygwin/DevNotes
View File

@ -1,3 +1,45 @@
2012-06-02 cgf-000010
<1.7.16>
- Fix emacs problem which exposed an issue with Cygwin's select() function.
If a signal arrives while select is blocking and the program longjmps
out of the signal handler then threads and memory may be left hanging.
Fixes: http://cygwin.com/ml/cygwin/2012-05/threads.html#00275
</1.7.16>
This was try #4 or #5 to get select() signal handling working right.
It's still not there but it should now at least not leak memory or
threads.
I mucked with the interface between cygwin_select and select_stuff::wait
so that the "new" loop in select_stuff::wait() was essentially moved
into the caller. cygwin_select now uses various enum states to decide
what to do. It builds the select linked list at the beginning of the
loop, allowing wait() to tear everything down and restart. This is
necessary before calling a signal handler because the signal handler may
longjmp away.
I initially had this all coded up to use a special signal_cleanup
callback which could be called when a longjmp is called in a signal
handler. And cygwin_select() set up and tore down this callback. Once
I got everything compiling it, of course, dawned on me that just because
you call a longjmp in a signal handler it doesn't mean that you are
jumping *out* of the signal handler. So, if the signal handler invokes
the callback and returns it will be very bad for select(). Hence, this
slower, but hopefully more correct implementation.
(I still wonder if some sort of signal cleanup callback might still
be useful in the future)
TODO: I need to do an audit of other places where this problem could be
occurring.
As alluded to above, select's signal handling is still not right. It
still acts as if it could call a signal handler from something other
than the main thread but, AFAICT, from my STC, this doesn't seem to be
the case. It might be worthwhile to extend cygwait to just magically
figure this out and not even bother using w4[0] for scenarios like this.
2012-05-16 cgf-000009
<1.7.16>

5
winsup/cygwin/release/1.7.16
View File

@ -24,3 +24,8 @@ Bug fixes:
- Handle inode numbers returned by Samba >= 3.5.4.
Fixes: http://cygwin.com/ml/cygwin/2012-05/msg00236.html
- Fix emacs problem which exposed an issue with Cygwin's select() function.
If a signal arrives while select is blocking and the program longjmps
out of the signal handler then threads and memory may be left hanging.
Fixes: http://cygwin.com/ml/cygwin/2012-05/threads.html#00275

293
winsup/cygwin/select.cc
View File

@ -81,7 +81,7 @@ typedef long fd_mask;
{ \
(s)->thread_errno = EBADF; \
return -1; \
} \
}
/* The main select code.
*/
@ -89,14 +89,17 @@ extern "C" int
cygwin_select (int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *to)
{
select_printf ("select(%d, %p, %p, %p, %p)", maxfds, readfds, writefds, exceptfds, to);
select_stuff sel;
fd_set *dummy_readfds = allocfd_set (maxfds);
fd_set *dummy_writefds = allocfd_set (maxfds);
fd_set *dummy_exceptfds = allocfd_set (maxfds);
select_printf ("select(%d, %p, %p, %p, %p)", maxfds, readfds, writefds, exceptfds, to);
pthread_testcancel ();
/* Allocate some fd_set structures using the number of fds as a guide. */
fd_set *r = allocfd_set (maxfds);
fd_set *w = allocfd_set (maxfds);
fd_set *e = allocfd_set (maxfds);
if (!readfds)
readfds = dummy_readfds;
@ -105,12 +108,7 @@ cygwin_select (int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
if (!exceptfds)
exceptfds = dummy_exceptfds;
for (int i = 0; i < maxfds; i++)
if (!sel.test_and_set (i, readfds, writefds, exceptfds))
{
select_printf ("aborting due to test_and_set error");
return -1; /* Invalid fd, maybe? */
}
pthread_testcancel ();
/* Convert to milliseconds or INFINITE if to == NULL */
DWORD ms = to ? (to->tv_sec * 1000) + (to->tv_usec / 1000) : INFINITE;
@ -122,46 +120,74 @@ cygwin_select (int maxfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
else
select_printf ("to NULL, ms %x", ms);
select_printf ("sel.always_ready %d", sel.always_ready);
/* Allocate some fd_set structures using the number of fds as a guide. */
fd_set *r = allocfd_set (maxfds);
fd_set *w = allocfd_set (maxfds);
fd_set *e = allocfd_set (maxfds);
int res = 0;
sel.return_on_signal = &_my_tls == _main_tls;
/* Degenerate case. No fds to wait for. Just wait. */
if (sel.start.next == NULL)
while (!res)
switch (cygwait (ms))
{
case WAIT_OBJECT_0:
select_printf ("signal received");
_my_tls.call_signal_handler ();
if (!sel.return_on_signal)
continue; /* Emulate linux behavior */
set_sig_errno (EINTR);
res = -1;
break;
case WAIT_OBJECT_0 + 1:
sel.destroy ();
pthread::static_cancel_self ();
/*NOTREACHED*/
default:
res = 1; /* temporary flag. Will be set to zero below. */
break;
}
else if (sel.always_ready || ms == 0)
res = 0;
else
res = sel.wait (r, w, e, ms);
if (res >= 0)
int res = select_stuff::select_loop;
LONGLONG start_time = gtod.msecs (); /* Record the current time for later use. */
while (res == select_stuff::select_loop)
{
copyfd_set (readfds, r, maxfds);
copyfd_set (writefds, w, maxfds);
copyfd_set (exceptfds, e, maxfds);
res = (res > 0) ? 0 : sel.poll (readfds, writefds, exceptfds);
for (int i = 0; i < maxfds; i++)
if (!sel.test_and_set (i, readfds, writefds, exceptfds))
{
select_printf ("aborting due to test_and_set error");
return -1; /* Invalid fd, maybe? */
}
select_printf ("sel.always_ready %d", sel.always_ready);
/* Degenerate case. No fds to wait for. Just wait for time to run out
or signal to arrive. */
if (sel.start.next == NULL)
switch (cygwait (ms))
{
case WAIT_OBJECT_0:
select_printf ("signal received");
_my_tls.call_signal_handler ();
if (!sel.return_on_signal)
res = select_stuff::select_loop; /* Emulate linux behavior */
else
{
set_sig_errno (EINTR);
res = select_stuff::select_error;
}
break;
case WAIT_OBJECT_0 + 1:
sel.destroy ();
pthread::static_cancel_self ();
/*NOTREACHED*/
default:
res = select_stuff::select_set_zero; /* Set res to zero below. */
break;
}
else if (sel.always_ready || ms == 0)
res = 0;
else
res = sel.wait (r, w, e, ms);
if (res == select_stuff::select_timeout)
res = 0;
else if (res >= 0)
{
copyfd_set (readfds, r, maxfds);
copyfd_set (writefds, w, maxfds);
copyfd_set (exceptfds, e, maxfds);
res = (res == select_stuff::select_set_zero) ? 0 : sel.poll (readfds, writefds, exceptfds);
}
sel.cleanup ();
sel.destroy ();
if (res == select_stuff::select_loop && ms != INFINITE)
{
select_printf ("recalculating ms");
LONGLONG now = gtod.msecs ();
if (now > (start_time + ms))
select_printf ("timed out after verification");
else
{
ms -= (now - start_time);
start_time = now;
select_printf ("ms now %u", ms);
}
}
}
syscall_printf ("%R = select(%d, %p, %p, %p, %p)", res, maxfds, readfds,
@ -213,7 +239,7 @@ select_stuff::cleanup ()
inline void
select_stuff::destroy ()
{
select_record *s = &start;
select_record *s;
select_record *snext = start.next;
select_printf ("deleting select records");
@ -222,6 +248,7 @@ select_stuff::destroy ()
snext = s->next;
delete s;
}
start.next = NULL;
}
select_stuff::~select_stuff ()
@ -268,24 +295,19 @@ err:
}
/* The heart of select. Waits for an fd to do something interesting. */
int
select_stuff::wait_states
select_stuff::wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
DWORD ms)
{
int wait_ret;
HANDLE w4[MAXIMUM_WAIT_OBJECTS];
select_record *s = &start;
int m = 0;
int res = 0;
bool is_cancelable = false;
DWORD m = 0;
w4[m++] = signal_arrived; /* Always wait for the arrival of a signal. */
if ((w4[m] = pthread::get_cancel_event ()) != NULL)
{
++m;
is_cancelable = true;
}
m++;
int startfds = m;
/* Loop through the select chain, starting up anything appropriate and
counting the number of active fds. */
while ((s = s->next))
@ -293,85 +315,75 @@ select_stuff::wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
if (m >= MAXIMUM_WAIT_OBJECTS)
{
set_sig_errno (EINVAL);
return -1;
return select_error;
}
if (!s->startup (s, this))
{
s->set_select_errno ();
return -1;
return select_error;
}
if (s->h == NULL)
continue;
for (int i = 1; i < m; i++)
if (w4[i] == s->h)
goto next_while;
w4[m++] = s->h;
next_while:
continue;
if (s->h != NULL)
for (DWORD i = startfds; i <= m; i++)
if (i == m)
w4[m = i] = s->h;
else if (w4[i] == s->h)
break;
}
bool gotone;
LONGLONG start_time = gtod.msecs (); /* Record the current time for later use. */
debug_printf ("m %d, ms %u", m, ms);
for (;;)
DWORD wait_ret;
if (!windows_used)
wait_ret = WaitForMultipleObjects (m, w4, FALSE, ms);
else
/* Using MWMO_INPUTAVAILABLE is the officially supported solution for
the problem that the call to PeekMessage disarms the queue state
so that a subsequent MWFMO hangs, even if there are still messages
in the queue. */
wait_ret = MsgWaitForMultipleObjectsEx (m, w4, ms,
QS_ALLINPUT | QS_ALLPOSTMESSAGE,
MWMO_INPUTAVAILABLE);
select_printf ("wait_ret %d. verifying", wait_ret);
wait_states res;
switch (wait_ret)
{
if (!windows_used)
wait_ret = WaitForMultipleObjects (m, w4, FALSE, ms);
case WAIT_OBJECT_0:
select_printf ("signal received");
cleanup ();
destroy ();
_my_tls.call_signal_handler ();
if (!return_on_signal)
res = select_loop;
else
/* Using MWMO_INPUTAVAILABLE is the officially supported solution for
the problem that the call to PeekMessage disarms the queue state
so that a subsequent MWFMO hangs, even if there are still messages
in the queue. */
wait_ret = MsgWaitForMultipleObjectsEx (m, w4, ms,
QS_ALLINPUT | QS_ALLPOSTMESSAGE,
MWMO_INPUTAVAILABLE);
switch (wait_ret)
{
case WAIT_OBJECT_0:
select_printf ("signal received");
#if 0
/* FIXME? Partial revert of change from 2012-01-22. If the signal
handler is called before the threads are stopped via cleanup,
emacs 24.x crashes in thread_pipe. Just restarting without
calling the signal handler makes select entirely uninterruptible
when called from a thread not the main thread, see
http://cygwin.com/ml/cygwin/2012-05/msg00580.html
So, for now, just disable restarting entirely. */
if (!return_on_signal)
goto looping; /* Emulate linux behavior */
#endif
cleanup ();
_my_tls.call_signal_handler ();
set_sig_errno (EINTR);
return -1;
case WAIT_OBJECT_0 + 1:
if (is_cancelable)
{
cleanup ();
destroy ();
pthread::static_cancel_self ();
}
/* This wasn't a cancel event. It was just a normal object to wait
for. */
break;
case WAIT_FAILED:
cleanup ();
system_printf ("WaitForMultipleObjects failed");
s = &start;
s->set_select_errno ();
return -1;
case WAIT_TIMEOUT:
cleanup ();
select_printf ("timed out");
res = 1;
goto out;
res = select_signalled;
}
select_printf ("woke up. wait_ret %d. verifying", wait_ret);
break;
case WAIT_FAILED:
system_printf ("WaitForMultipleObjects failed");
s = &start;
gotone = false;
s->set_select_errno ();
res = select_error;
break;
case WAIT_TIMEOUT:
select_printf ("timed out");
res = select_timeout;
break;
case WAIT_OBJECT_0 + 1:
if (startfds > 1)
{
cleanup ();
destroy ();
pthread::static_cancel_self ();
/*NOTREACHED*/
}
/* Fall through. This wasn't a cancel event. It was just a normal object
to wait for. */
default:
s = &start;
bool gotone = false;
/* Some types of objects (e.g., consoles) wake up on "inappropriate" events
like mouse movements. The verify function will detect these situations.
If it returns false, then this wakeup was a false alarm and we should go
@ -379,42 +391,21 @@ select_stuff::wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
while ((s = s->next))
if (s->saw_error ())
{
cleanup ();
set_errno (s->saw_error ());
return -1; /* Somebody detected an error */
res = select_error; /* Somebody detected an error */
goto out;
}
else if ((((wait_ret >= m && s->windows_handle) || s->h == w4[wait_ret]))
&& s->verify (s, readfds, writefds, exceptfds))
gotone = true;
if (!gotone)
res = select_loop;
else
res = select_ok;
select_printf ("gotone %d", gotone);
if (gotone)
{
cleanup ();
goto out;
}
#if 0
looping:
#endif
if (ms == INFINITE)
{
select_printf ("looping");
continue;
}
select_printf ("recalculating ms");
LONGLONG now = gtod.msecs ();
if (now > (start_time + ms))
{
cleanup ();
select_printf ("timed out after verification");
goto out;
}
ms -= (now - start_time);
start_time = now;
select_printf ("ms now %u", ms);
break;
}
out:
select_printf ("returning %d", res);
return res;

13
winsup/cygwin/select.h
View File

@ -69,6 +69,16 @@ struct select_mailslot_info: public select_info
class select_stuff
{
public:
enum wait_states
{
select_timeout = -4,
select_signalled = -3,
select_loop = -2,
select_error = -1,
select_ok = 0,
select_set_zero = 1
};
~select_stuff ();
bool return_on_signal;
bool always_ready, windows_used;
@ -82,9 +92,10 @@ public:
bool test_and_set (int i, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds);
int poll (fd_set *readfds, fd_set *writefds, fd_set *exceptfds);
int wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds, DWORD ms);
wait_states wait (fd_set *readfds, fd_set *writefds, fd_set *exceptfds, DWORD ms);
void cleanup ();
void destroy ();
select_stuff (): return_on_signal (false), always_ready (false),
windows_used (false), start (0),
device_specific_pipe (0),