Messages in this thread |  | | From | Mike Frysinger <> | Date | Wed, 13 Apr 2011 17:11:10 -0400 | Subject | Re: [uclinux-dist-devel] freezer: should barriers be smp ? |
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On Wed, Apr 13, 2011 at 17:05, Pavel Machek wrote: > On Wed 2011-04-13 17:02:45, Mike Frysinger wrote: >> On Wed, Apr 13, 2011 at 16:58, Rafael J. Wysocki wrote: >> > On Wednesday, April 13, 2011, Mike Frysinger wrote: >> >> when we suspend/resume Blackfin SMP systems, we notice that the >> >> freezer code runs on multiple cores. this is of course what you want >> >> -- freeze processes in parallel. however, the code only uses non-smp >> >> based barriers which causes us problems ... our cores need software >> >> support to keep caches in sync, so our smp barriers do just that. but >> >> the non-smp barriers do not, and so the frozen/thawed processes >> >> randomly get stuck in the wrong task state. >> >> >> >> thinking about it, shouldnt the freezer code be using smp barriers ? >> > >> > Yes, it should, but rmb() and wmb() are supposed to be SMP barriers. >> > >> > Or do you mean something different? >> >> then what's the diff between smp_rmb() and rmb() ? >> >> this is what i'm proposing: >> --- a/kernel/freezer.c >> +++ b/kernel/freezer.c >> @@ -17,7 +17,7 @@ static inline void frozen_process(void) >> { >> if (!unlikely(current->flags & PF_NOFREEZE)) { >> current->flags |= PF_FROZEN; >> - wmb(); >> + smp_wmb(); >> } >> clear_freeze_flag(current); >> } >> @@ -93,7 +93,7 @@ bool freeze_task(struct task_struct *p, bool sig_only) >> * the task as frozen and next clears its TIF_FREEZE. >> */ >> if (!freezing(p)) { >> - rmb(); >> + smp_rmb(); >> if (frozen(p)) >> return false; > > smp_rmb() is NOP on uniprocessor. > > I believe the code is correct as is.
that isnt what the code / documentation says. unless i'm reading them wrong, both seem to indicate that the proposed patch is what we actually want.
include/linux/compiler-gcc.h: #define barrier() __asm__ __volatile__("": : :"memory")
include/asm-generic/system.h: #define mb() asm volatile ("": : :"memory") #define rmb() mb() #define wmb() asm volatile ("": : :"memory")
#ifdef CONFIG_SMP #define smp_mb() mb() #define smp_rmb() rmb() #define smp_wmb() wmb() #else #define smp_mb() barrier() #define smp_rmb() barrier() #define smp_wmb() barrier() #endif
Documentation/memory-barriers.txt: SMP memory barriers are reduced to compiler barriers on uniprocessor compiled systems because it is assumed that a CPU will appear to be self-consistent, and will order overlapping accesses correctly with respect to itself.
[!] Note that SMP memory barriers _must_ be used to control the ordering of references to shared memory on SMP systems, though the use of locking instead is sufficient. -mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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