Date: Tue, 23 Aug 2011 16:33:35 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: mdf@FreeBSD.org Cc: freebsd-arch@FreeBSD.org Subject: Re: skipping locks, mutex_owned, usb Message-ID: <4E53AC2F.1040006@FreeBSD.org> In-Reply-To: <CAMBSHm-nT5-wTbAFqsJ6ZjCPE9Vfxeva1F3zwWSAK8Ecw=8VsA@mail.gmail.com> References: <4E53986B.5000804@FreeBSD.org> <CAMBSHm-nT5-wTbAFqsJ6ZjCPE9Vfxeva1F3zwWSAK8Ecw=8VsA@mail.gmail.com>
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on 23/08/2011 15:58 mdf@FreeBSD.org said the following: > On Tue, Aug 23, 2011 at 5:09 AM, Andriy Gapon <avg@freebsd.org> wrote: >> III. With my stop_scheduler_on_panic patch ukbd_poll() produces infinite chains >> of 'infinite' recursion because mtx_owned() always returns false. This is because >> I skip all lock/unlock/etc operations in the post-panic context. I think that >> it's a good philosophical question: what operations like mtx_owned(), >> mtx_recursed(), mtx_trylock() 'should' return when we actually act as if no locks >> exist at all? >> >> My first knee-jerk reaction was to change mtx_owned() to return true in this >> "lock-less" context, but _hypothetically_ there could exist some symmetric code >> that does something like: >> func() >> { >> if (mtx_owned(&Giant)) { >> mtx_unlock(&Giant); >> func(); >> mtx_lock(&Giant); >> return; >> } >> >> // etc ... >> >> What do you think about this problem? > > Given that checking for a lock being held is a lot more common than > checking if it's not held (in the context of mtx_assert(9) and > friends), it seems reasonable to report that a lock is held in the > special context of after panic. But, OTOH, there are snippets like this (found with grep, haven't looked at the code): /usr/src/sys/kern/kern_sx.c: while (mtx_owned(&Giant)) { >> That question III actually brings another thought: perhaps the whole of idea of >> skipping locks in a certain context is not a correct direction? >> Perhaps, instead we should identify all the code that could be legitimately >> executed in the after-panic and/or kdb contexts and make that could explicitly >> aware of its execution context. That is, instead of trying to make _lock, >> _unlock, _owned, _trylock, etc do the right thing auto-magically, we should try to >> make the calling code check panicstr and kdb_active and do the right thing on that >> level (which would include not invoking those lock-related operations or other >> inappropriate operations). > > I don't think it's possible to identify all the code, since what runs > after panic isn't tested very much. :-) I don't disagree that one > should think about the issue, but providing reasonable defaults like > skipping locks reduces the burden on the programmer. Yes, I agree with your and John's practical approach to this. Maybe print a warning about each skipped locking operation? But not sure if that would be useful, if there would be no intention of changing the code. -- Andriy Gapon
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