Messages in this thread | | | Date | Sat, 12 Apr 2008 12:27:24 -0700 | From | Andrew Morton <> | Subject | Re: [DOC PATCH] semaphore documentation |
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On Sat, 12 Apr 2008 08:12:51 -0600 Matthew Wilcox <matthew@wil.cx> wrote:
> On Fri, Apr 11, 2008 at 10:09:11PM -0700, Andrew Morton wrote: > > On Fri, 11 Apr 2008 13:27:54 -0700 Randy Dunlap <randy.dunlap@oracle.com> wrote: > > > Looks good to me. Thanks. > > > > Yup, most excellent. > > Thanks for the review. > > > btw, down() and friends should have might_sleep() checks in them, shouldn't > > they? They don't seem to be in there, nor in mainline > > lib/semaphore-sleepers.c. Confused. > > Mmm. Ingo gets annoyed when I add additional checks to semaphores -- he > wants them to maintain their current semantics and to get better checking > by migrating more users to mutexes. I've already exposed at least one > problem (in aacraid) by adding the __must_check to down_interruptible(). > > As I wrote in one of the comments, we have places in the kernel which > know that even though they're in a non-sleeping context, there is at > least one more token left in the semaphore. One place this bit me was > in start_kernel(). We disable interrupts and then call lock_kernel() > which calls down(). Since we're in start_kernel(), we know there's > nothing else running and this is perfectly safe. But a might_sleep() > would warn bogusly.
urgh, yes, I'd forgotten about that mess.
I suppose that if might_sleep() checking in down() is useful (and surely it is) we could provide a separate down_im_stupid() (and lock_kernel_im_stupid()) which omits the check, and call that from the problematic sites.
> I'd be open to putting a might_sleep() in __down(). We definitely are > going to sleep at that point, so getting a warning out of it would > be good.
I think it'd be worth playing with some time, but it's off-topic for this current work.
> I thought that schedule() would warn itself in that case, > but I can't see the code that would do that now I check.
schedule() will warn ("scheduling while atomic"), but only if we happened to hit contention.
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