Messages in this thread | | | From | (Eric W. Biederman) | Date | Mon, 07 Dec 2009 18:11:49 -0800 | Subject | Re: [rfc] "fair" rw spinlocks |
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"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> On Mon, Dec 07, 2009 at 03:19:59PM -0800, Eric W. Biederman wrote: >> Andi Kleen <andi@firstfloor.org> writes: >> >> > ebiederm@xmission.com (Eric W. Biederman) writes: >> > >> >> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes: >> >>> >> >>> Is it required that all of the processes see the signal before the >> >>> corresponding interrupt handler returns? (My guess is "no", which >> >>> enables a trick or two, but thought I should ask.) >> >> >> >> Not that I recall. I think it is just an I/O completed signal. >> > >> > Wasn't there the sysrq SAK too? That one definitely would need >> > to be careful about synchronicity. >> >> SAK from sysrq is done through schedule work, I seem to recall the >> locking being impossible otherwise. There is also send_sig_all and a >> few others from sysrq. I expect we could legitimately make them >> schedule_work as well if needed. > > OK, I will chance it... Here is one possible trick: > > o Maintain a list of ongoing group-signal operations, protected > by some suitable lock. These could be in a per-chain-locked > hash table, hashed by the signal target (e.g., pgrp). > > o When a task is created, it scans the above list, committing > suicide (or doing whatever the signal requires) if appropriate. > > o When creating a child task, the parent holds an SRCU across > creation. It acquires SRCU before starting creation, and > releases it when it knows that the child has completed > scanning the above list. > > o The updater does the following: > > o Add its request to the above list. > > o Wait for an SRCU grace period to elapse. > > o Kill off everything currently in the task list, > and then wait for each such task to get to a point > where it can be guaranteed not to spawn additional > tasks. (This might be mediated via a reference > count in the corresponding list element, or by > rescanning the task list, or any of a number of > similar tricks.) > > Of course, if the signal is non-fatal, then it is > necessary only to wait until the child has taken > the signal. > > o If it is possible for a given task's children to > outlive it, despite the fact that the children must > commit suicide upon finding themselves indicated by the > list, wait for another SRCU grace period to elapse. > (This additional SRCU grace period would be required > for a non-fatal pgrp signal, for example.) > > o Remove the element from the list. > > Does this approach make sense, or am I misunderstanding the problem?
I think that is about right. I played with that idea a little bit. I was thinking of simply having new children return -ERESTARTSYS, and retry the fork. I put it down because I decided that seems like a very twisted implementation of a read/write lock.
If we can scale noticeably better a than tasklist_lock it is definitely worth doing. I think it is really easy to tie yourself up in pretzels thinking about this.
An srcu in the pid structure that we hold while signaling tasks. Interesting.
> Either way, one additional question... It seems to me that non-fatal > signals really don't require the above mechanism, because if a task > handles the signal, and then spawns a child, one can argue that the > child came after the signal and should thus be unaffected. Right? > Or more confusion on my part?
SIGSTOP also seems pretty important not to escape. I'm not certain of the others. I think I would get a bit upset if job control signals in the shell stopped working properly. I think asking the question did that app do something wrong with SIGTERM or did the kernel drop it would drive me a bit batty.
It is hard to tell what breaks because most buggy implementations will work correctly most of the time.
Eric
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