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>> Paul E. McKenney wrote: >>> On Thu, Aug 21, 2008 at 01:02:48PM +0200, Peter Zijlstra wrote: >>>> On Thu, 2008-08-21 at 12:57 +0200, Stefan Richter wrote: >>>>> Peter Zijlstra wrote: >>>>>> On Wed, 2008-08-20 at 20:50 -0600, jmerkey@wolfmountaingroup.com >>>>>> wrote: >>>>>> >>>>>>> volatiles left in the code due to the previously stated >>>>>>> (and still present) severe breakage of the GNU compiler with SMP >>>>>>> shared data. most of the barrier() functions are just plain >>>>>>> broken >>>>>>> and do not result in proper compiler behavior in this tree. >>>>>> Can you provide explicit detail? >>>>>> >>>>>> By using barrier() the compiler should clobber all its memory and >>>>>> registers therefore forcing a write/reload of the variable. >>>>> I hope Jeff didn't try mere barrier()s only. smp_wmb() and smp_rmb() >>>>> are the more relevant barrier variants for mdb, from what I remember >>>>> when I last looked at it. >>>> Sure, but volatile isn't a replacement for memory barriers. >>> >>> Let's face it, the C standard does not support concurrency, so we are >>> all in a state of sin in any case, forced to rely on combinations of >>> gcc-specific non-standard language extensions and assembly language. >>> >>> Could be worse!!! >> >> Nevertheless, an analysis of which particular parts of code generation >> are insufficient if one particular volatile qualification is removed is >> IMO likely to turn up places in mdb where a clearer or/and more >> efficient implementation is possible. (Based on what I saw a few >> revisions ago; I haven't looked at the current one yet.) >> -- >> Stefan Richter >> -=====-==--- =--- =-=-= >> http://arcgraph.de/sr/ >> > > I used the smp_wmb() functions. I noted a couple of things. a) some of > these macros just emit __asm__ __volatile__ into the code so why not just > say "volatile" to begin with b) smp_wmb() in some cases worked and in > other cases jut optimized away the global reference. c) I can go back and > break the code again by inserting them and building broken assembler d) I > ave been doing hardware and software design since the early 1980;s, I > invented SMP affinity scheduling, and yes, I understand barriers and this > concept of instruction score-boarding and optimization very well -- its > not an excuse for a busted C compiler. > > It did not break all the places in the code, but broke enough for SMP to > lock up and fail, It turned global variables into local variables. If > you want me to reproduce this I can but it will have to wait til this > evening > because I have some product releases to get out the door at Omega 8 today. > > It's simple to reproduce. Take away the volatile declaration for the > rlock_t structure in mdb-ia32.c (rlock_t debug_lock) in all code > references and watch the thing lock up in SMP with multiple processors in > the debugger each stuck with their own local copy of debug_lock. Even if you use the smb_wmb() macros around the debug_lock, the compiler still optimizes the debug_lock into a local variable. After watching the broken behavior, the fact that some of these macro's emit __asm__ __volatile__ anyway, I just left the vars declared volatile. Its a debugger, so its probably ok for the kernel debugger to use some volatile data. Jeff > > The barrier functions do not appear to work in all cases. > > Jeff > > | ||||||||||
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