From: Blaisorblade <bla...@ya...> - 2006-10-14 00:14:20
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On Monday 09 October 2006 20:00, Jeff Dike wrote: > On Thu, Oct 05, 2006 at 11:38:52PM +0200, Paolo 'Blaisorblade' Giarrusso wrote: > > From: Paolo 'Blaisorblade' Giarrusso <bla...@ya...> > > > > If enable is moved by GCC in a register its value may not be preserved > > after coming back there with longjmp(). So, mark it as volatile to > > prevent this; this is suggested (it seems) in info gcc, when it talks > > about -Wuninitialized. I re-read this and it seems to say something > > different, but I still believe this may be needed. > > > > Signed-off-by: Paolo 'Blaisorblade' Giarrusso <bla...@ya...> > > --- > > > > arch/um/include/longjmp.h | 3 ++- > > 1 files changed, 2 insertions(+), 1 deletions(-) > > > > diff --git a/arch/um/include/longjmp.h b/arch/um/include/longjmp.h > > index e93c6d3..e860bc5 100644 > > --- a/arch/um/include/longjmp.h > > +++ b/arch/um/include/longjmp.h > > @@ -12,7 +12,8 @@ #define UML_LONGJMP(buf, val) do { \ > > } while(0) > > > > #define UML_SETJMP(buf) ({ \ > > - int n, enable; \ > > + int n; \ > > + volatile int enable; \ > > enable = get_signals(); \ > > n = setjmp(*buf); \ > > if(n != 0) \ > > I agree with this, but not entirely with your reasoning. The > -Wuninitialized documentation just talks about when gcc emits a > warning. > > What we want is a guarantee that enable is not cached in a register, > but is stored in memory. What documentation I can find seems to imply > that is the case ("accesses to volatile objects must have settled > before the next sequence point"). > > However, given the prevailing opinion that essentially all volatile > declarations are hiding bugs, I wouldn't mind a bit of review of this > from someone holding this opinion. I agree with that, so I think Al (whom I cc'ed) will be able to evaluate this. - however I think that the problem with volatile is that it does not have any semantic wrt cache - volatile does not even work with host-to-device operation (the thing it was thought for) if you have reasonable caches - unless the var is in a MMIO region with disabled caches. So relying on volatile for variables shared between threads is not useful. Note that raw_spinlock_t counter is volatile, however - separate caches flushes or atomic operations are needed, but it may avoid the need for a full barrier() operation or a volatile "memory" clobber - GCC knows only the volatile object has changed, while with barrier() it must assume everything must be reloaded. However, at least one barrier operation is needed for a spinlock to prevent GCC from moving code outside of critical sections. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade http://www.user-mode-linux.org/~blaisorblade Chiacchiera con i tuoi amici in tempo reale! http://it.yahoo.com/mail_it/foot/*http://it.messenger.yahoo.com |
From: Jeff D. <jd...@ad...> - 2006-10-10 02:09:37
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On Thu, Oct 05, 2006 at 11:32:12PM +0200, Paolo 'Blaisorblade' Giarrusso wrote: > This is the first set of patches for UML for 2.6.19 I send - I have various > other patches to merge but I'm sending them in separate batches to avoid > flooding. ACK to all of these, except for the first one (moving the inclusion of the arch Makefile), which seems wrong to me. Jeff |