Messages in this thread | | | From | Keith Owens <> | Subject | Re: [PATCH][2.6] Allow x86_64 to reenable interrupts on contention | Date | Wed, 28 Jul 2004 11:48:05 +1000 |
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On Tue, 27 Jul 2004 17:40:30 -0700, William Lee Irwin III <wli@holomorphy.com> wrote: >It's good to know there is a precedent and that the backtrace issue has >been looked at on other architectures. > >On Wed, Jul 28, 2004 at 10:35:08AM +1000, Keith Owens wrote: >> Are you are planning to consolidate the out of line code for i386? Is >> there a patch (even work in progress) so I can start thinking about >> doing reliable backtraces? > >The experiments were carried out using the standard calling convention. >We may investigate less standard calling conventions, but they should >actually already be there given __write_lock_failed/__read_lock_failed. >i.e. if reliable backtraces are going to be an issue they should >already be an issue for rwlocks.
rwlocks are already a issue on i386, but not a big one. The return address from __write_lock_failed/__read_lock_failed is pointing into the out of line code, not the code that really took the rwlock. The nearest label is LOCK_SECTION_NAME, i.e. .text.lock.KBUILD_BASENAME. A backtrace through a contended rwlock on i386 looks like this
interrupt __write_lock_failed/__read_lock_failed .text.lock.KBUILD_BASENAME caller of routine that took the lock ...
when it should really be
interrupt __write_lock_failed/__read_lock_failed .text.lock.KBUILD_BASENAME routine that took the look <=== missing information caller of routine that took the lock ...
IOW we only get the name of the object, not the function within the object that took the lock. i386 gets away with this because .text.lock.KBUILD_BASENAME is usually enough information to determine which lock is the problem, and the i386 backtrace algorithm has enough redundancy to get back in sync for the rest of the trace, even with the missing function entry.
OTOH, ia64 unwind is incredibly sensitive to the exact instruction pointer and there is zero redundancy in the unwind data. If the return ip is not known to the unwind code, then the ia64 unwinder cannot backtrace correctly. Which meant that I had to get the ia64 out of line code exactly right, close enough was not good enough.
With Zwane's patch, any contended spinlock on i386 will look like rwsem, it will be missing the routine that took the look. Good enough for most cases.
kdb does unwind through out of line spinlock code exactly right, simply because I added extra heuristics to kdb to cope with this special case. When people complain that the kdb i386 backtrace code is too messy, they are really saying that they do not care about getting exact data for all the hand crafted assembler in the kernel.
BTW, if anybody is planning to switch to dwarf for debugging the i386 kernel then you _must_ have valid dwarf data for the out of line code.
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