This is the mail archive of the cygwin mailing list for the Cygwin project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

cygserver blocking on semctl(SETVAL) call

[Slightly modified from version previously sent on cygwin-developers, who suggest this is a better forum for discussion]

I've discovered what I believe to be a internal deadlock issue in cygserver.

I have a piece of code:
void SemaphoreManager::setValue(semid_t id, int x) const {
	semun params;
	cout << "SEMCTL..." << flush;
	if(semctl(semid,id,SETVAL,params)<0) {
		perror("ERROR: SemaphoreManager::setValue (semctl)");
	cout << "done" << endl;

This is part of a function which gets called a number of times throughout the life of the program. It works just fine up until one particular call (with x=0) which reliably causes it to block between the two cout's. Not just my program either -- all IPC is blocked at this point. So bringing up new cygwin windows, running 'ipcs', etc., all hang. Once I kill any one process in the group that are using the semaphore, it seems to jump start things a bit and may run a bit more, but usually eventually blocks again until all of my program's processes are killed.

My code runs fine under Linux and Mac OS X, it's only now that we're nearing release that I'm testing under cygwin and finding something has gone wrong in the past 9 months or so -- either something updated on your end, or a change in our code that's now tickling an issue.

The kicker to note here -- is there any reason a *SETVAL* operation could be blocked??? It should either go through or return an error. I'm fairly convinced it's *not* this particular semctl call that's causing the block, it just gets hung up because some *other*, previous, operation has hung cygserver, and it's that operation that's causing the trouble.

One nuisance is that when I run cygserver with -d, it doesn't block in the same place -- something about all that debugging output changes the race conditions. In any case, I've attached the cygserver output leading up to a block, in hopes it means something to you.

Thanks for taking a look -- I'm afraid I'm stumped. (doesn't help gdb only reports '??' for all function calls when I attach to a process, so I can't tell what any of my code is doing. And yes, I do have -g enabled)

Our code can be checked out from CVS, but before running you'll need to increase the semmns and semmsl parameters as described in step 5:

After that's set up:
cvs -d checkout -P Tekkotsu
cd Tekkotsu;
setenv TEKKOTSU_ROOT `pwd` || export TEKKOTSU_ROOT=`pwd`
cd project
make sim
./sim-ERS7 Speed=0

When launched, the simulator forks into four processes, using IPC to communicate between them. 'Speed=0' pauses our simulator so it shouldn't be trying to process anything. When launched, it goes through a series of runlevels CONSTRUCTING, STARTING, RUNNING, STOPPING, DESTRUCTING, DESTRUCTED. Passing InitialRunlevel=X on the command line will stop in a runlevel other than "running", and then you can use the 'runlevel' command within the simulator to advance. It reliably gets into the "starting" runlevel, but something about the "running" runlevel triggers the problem. SemaphoreManager (from the code displayed above) is found in the root IPC directory.
Beware leaked semaphores sets btw, since this problem also causes the signal handler to block when trying to remove the set on being killed, you'll need to kill -9 it, and use 'ipcs' to check for any leftover sets, and then 'ipcrm' them manually between runs. (Actually, I find it easier to just kill/relaunch cygserver itself which releases all of the blocked processes and clears leftover semaphores at the same time)


The following trace corresponds to the 'cygserver -d' activity following entering the 'runlevel' command to move from STARTING to RUNNING, and the block that occurs in that runlevel.

Attachment: cygserverout.txt
Description: Text document

Unsubscribe info:
Problem reports:

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]