On Thu, 07 Dec 2006 11:03:49 +0000 David Howells <[EMAIL PROTECTED]> wrote:
> David Miller <[EMAIL PROTECTED]> wrote: > > > David, you have to fix the locking scheme used in kernel/workqueue.c, > > you absolutely cannot assume that cmpxchg() is available on all > > platforms. This breaks the build on the platforms that don't > > have such an instruction, and no it cannot emulated. > > Yeah, I've figured that one out. Also, having considered things last night, I > think the use of cmpxchg() is unnecessary. > > I was trying to handle against two possibilities: > > (1) The pending flag may have been unset or may be cleared. However, given > where it's called, the pending flag is _always_ set. I don't think it > can be unset whilst we're in set_wq_data(). > > Once the work is enqueued to be actually run, the only way off the queue > is for it to be actually run. > > If it's a delayed work item, then the bit can't be cleared by the timer > because we haven't started the timer yet. Also, the pending bit can't be > cleared by cancelling the delayed work _until_ the work item has had its > timer started. > > (2) The workqueue pointer might change. This can only happen in two cases: > > (a) The work item has just been queued to actually run, and so we're > protected by the appropriate workqueue spinlock. > > (b) A delayed work item is being queued, and so the timer hasn't been > started yet, and so no one else knows about the work item or can > access it (the pending bit protects us). > > Besides, set_wq_data() _sets_ the workqueue pointer unconditionally, so > it can be assigned instead. > > So, I think replacing the set_wq_data() with a straight assignment would be > okay in most cases. The problem is where we end up tangling with > test_and_set_bit() emulated using spinlocks, and even then it's not a problem > _provided_ test_and_set_bit() doesn't attempt to modify the word if the bit > was set. > I don't see why the 2.6.19 logic needed changing. a) Nobody should be freeing the work_struct itself without running flush_scheduled_work() and b) even if the work_struct _did_ get freed, the callback function won't care, because there's nothing in that work_struct which it's interested in. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
