Date: Fri, 8 Mar 2002 09:18:06 -0700 From: Nate Williams <nate@yogotech.com> To: Julian Elischer <julian@elischer.org> Cc: Nate Williams <nate@yogotech.com>, Poul-Henning Kamp <phk@critter.freebsd.dk>, arch@FreeBSD.ORG Subject: Re: Contemplating THIS change to signals. (fwd) Message-ID: <15496.58430.16748.970354@caddis.yogotech.com> In-Reply-To: <Pine.BSF.4.21.0203080017330.46841-100000@InterJet.elischer.org> References: <15496.23508.148366.980354@caddis.yogotech.com> <Pine.BSF.4.21.0203080017330.46841-100000@InterJet.elischer.org>
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> > In effect, the context is actually still active inside the kernel, even > > though the userland process is never given notice. I could see this > > having *really* strange effects for things that do network I/O. > > Often-times I've suspended processes inside of gdb so I could set > > breakpoints knowing full well that *NOTHING* was going to happen, even > > if I was in the middle of a system call. > > You'd be surprised then because once the send() is done, the network IO > will happen independently of the process. I'm more thinking of send. Once the send() system call has queued the data for sending, it's been 'sent' (ie; the stack has it, and will 'DTRT' with it). > this is no different. Except for read() or recvfrom() system calls, and potentially things like 'sendfile()'. Also, write() may behave differently (since write involve disk writing, not network writing). > from the time you do the ^Z to the time the syscall thinks of returning is > how long? If you say 3 seconds then all that is different is that in my > case the data has been taken off the queue but previously it would have > still been on the queue, but since the process is stopped, > who can tell? A lot can happen in 3 seconds. :) > In fact if the data was already present then sleep(0 would > have never been called, so the blocking would (even now) happen > at the user boundary. All I'm doing is making it consitent. Agreed. > > I'm still not getting a warm fuzzy that allowing the userland context to > > complete and then block at the userland boundary is a good idea. I'm > > not saying it's a bad idea, but I'm almost positive there are gremlins > > hiding in the details here. :) > > Userland context? where does userland context come into it? Sorry, I meant 'kernel context' above. My bad. I'll repeat. I'm still not getting a warm fuzzy that allowing the kernel context to complete and then block at the userland boundary is a good idea. I'm not saying it's a bad idea, but I'm almost positive there are gremlins hiding in the details here. :) > > > It waits there for the CONT signal. If the CONT happens before > > > the data arrives or the timeout expires the flag is simply cleared again. > > > > In effect, you're avoiding moving the process from the STOPPED queue and > > back to the SLEEP queue. > > I'm allowing it to STAY in the SLEEEP queue, knowing that when it decides > to continue, it can not get back to userland. because I've blocked that > road with a fallen tree. I know, but prior to this change, it moved to the STOPPED queue, and then back to the SLEEP queue upon receipt of SIGSTOP and SIGCONT signals. > > > It doesn't make such a huge difference now but when there are multiple > > > threads in a process, being able to report to the controlling shell, > > > or the debugger "OK its STOPPED" as soon as I can prove that all > > > threads are in the kernel and not in user space makes a big > > > difference. > > > > Right, but currently, stopping a process/thread stops it *everywhere*, > > including the kernel. We're changing something that has been done this > > way for a very long time, and the POLA may come back and bite us. > > No it doesn't stop it everywhere. if you have not set PCATCH or if you > do not sleep you will proceed forwards oblivious of the pending signal, > until you try return to user land, at which point you will be stopped, > just as I am suggesting, except I am sugesting that we should ALWAYS > stop it there. I'm assuming we're talking about processes with PCATCH set, since that's the behavior that will change. Bu, thanks for clarifying. > > > Also, the bit for "this process is STOPPING" can be set by another > > > thread and all threads will see it. Saving me from having to run > > > around all threads and do all sorts of stuff with each. > > > > Except that there are now locking issues since the 'other thread' is now > > messing around with parts of the thread that may be in use by the thread > > itself. (*ick*) > > No, it is in the proc struct. > All threads sharing the same proc see the same bit. Then we have the potential for race issues, since it could be possible for one thread to get a SIGSTOP, and another to get a SIGCONT very soon after, so we must guarantee that the order these are received is done correctly. Doing this 'safely' almost always involves locking. Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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