Messages in this thread | | | Subject | Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07 | From | Steven Rostedt <> | Date | Wed, 30 Mar 2005 11:46:34 -0500 |
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On Wed, 2005-03-30 at 08:50 +0200, Ingo Molnar wrote: > * Steven Rostedt <rostedt@goodmis.org> wrote: > > > OK, I'm declaring defeat here. I've been fighting race conditions all > > day, and it's now 1 in the morning where I live. It looks like this > > implementation has no other choice but to have the waking up "pending > > owner" take the wait_list lock once again. How heavy of a overhead is > > that really? > > as i mentioned it before, taking a lock is not a big issue at all. Since > you have to touch the lock data structure anyway (and all of it fits > into a single cacheline), it doesnt really matter whether it's atomic > flag setting/clearing, or raw spinlock based.c0267ad4 >
OK, I've implemented adding the write_lock, but I still have a nasty race condition, and I finally figured out why!
The damn BKL! Here's the situation I'm having.
Process A grabs BKL then tries to grab another semaphore (say sem X) But process B has sem X so process A sleeps. Since semaphores don't have the restriction of saving the BKL, the BKL gets released from process A. Process C comes along and grabs the BKL. Finally B releases sem X and process A becomes the new "pending owner" and wakes up. When A schedules in it tries to grab the BKL but blocks. Now it is at the point where A doesn't actually own either the BKL or sem X. When C releases the BKL and gives it to A, A is now the pending owner of both the BKL and sem X.
When this occurs, all hell breaks loose.
I believe I can solve this by making the BKL a special case and not implement the pending owner at all for it.
> later on, once things are stable and well-understood, we can still > attempt to micro-optimize it. >
Heck, I'll make it bloat city till I get it working, and then tone it down a little :-) And maybe later we can have a better solution for the BKL.
-- Steve
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