PTHREAD_CANCEL_DEFERRED

[Kostik Belousov]

On Thu, Aug 19, 2010 at 06:41:08AM +0800, David Xu wrote:
> Kostik Belousov wrote:
> >On Wed, Aug 18, 2010 at 01:32:39PM +0000, David Xu wrote:
> >  
> >>David Xu wrote:
> >>    
> >>>My idea is to always let close() syscall run, until it will be
> >>>blocked by lower layer, e.g tty or socket layer, as manual of close()
> >>>said it always release the file descriptor despite whether
> >>>it returns EINTR or EWOULDBLOCK, when it is going to sleep,
> >>>a flag will cause it to abort the sleep.
> >>>if the thread does not found the flag at that time,
> >>>a latter signal SIGCANCEL sent by pthread_cancel will unblock it,
> >>>this is how signal works.
> >>>
> >>>      
> >>I have worked out a patch:
> >>http://people.freebsd.org/~davidxu/patch/thread_cancel.patch
> >>
> >>    
> >Ok, the patch is definitely better then my proposal. But it has several
> >details that seems to need correction.
> >
> >First, if TDP_WAKEUP-marked thread receives any non-cancellation signal,
> >then a syscall returns with EINTR. This breaks SA_RESTART.
> >
> >  
> I don't think it breaks SA_RESTART,  there are reasons this is allowed:
I think I need to be more verbose. What I am saying is that if the
posted signal is not SIGCANCEL, then the sleepq_catch_signal() change
does two things wrong, IMO:
1. it will return EINTR while the signal marked as SA_RESTART interrupted
   restartable syscall.
2. TDP_WAKEUP should not be cleared for this case, since we should
   still be prepared for special handling of SIGCANCEL, if it arrives.
In other words, I propose to clear TDP_WAKEUP only when the posted 
signal is SIGCANCEL.

> 1) POSIX explicitly specifies that the EINTR should be returned:
>    
> http://www.opengroup.org/onlinepubs/000095399/functions/xsh_chap02_09.html
> 
>    I copied it here:
>   The side effects of acting upon a cancellation request while 
> suspended during a call
>   of a function are the same as the side effects that may be seen in a 
> single-threaded
>   program when a call to a function is interrupted by a signal and the 
> given function
>   returns [EINTR]. Any such side effects occur before any cancellation 
> cleanup
>   handlers are called.
> 
> 2) Traditional UNIX signal does not delivered in queued order, for example,
>    BSD delivers signal from lowest number to highest, like issignal() does,
>    it only returns lowest number signal which is not masked. a later signal
>    may be delivered first because it has lower number.  so if a lower 
> number
>    signal returns EINTR, but a higher signal returns ERESTART, the 
> final result
>    still is EINTR.
> 
> 3) Some system calls are not restartable, it always return EINTR when 
> interrupted
>    by signal.
> 
> 4) Most of program already prepared for EINTR, it is seldom to find a 
> program
>    does not consider EINTR.
> 
> >Also, I think that a condition to perform cancellation in thr_syscalls.c
> >should be not (ret == -1), but (ret == -1 && errno == EINTR).
> >  
> Yes, you are right, errno may be checked.
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