Messages in this thread |  | | | Date | Wed, 01 Apr 2009 12:14:24 +0200 | | From | Eric Dumazet <> | | Subject | Re: [PATCH] x86: percpu_to_op() misses memory and flags clobbers |
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Jeremy Fitzhardinge a écrit : > Eric Dumazet wrote: >> While playing with new percpu_{read|write|add|sub} stuff in network tree, >> I found x86 asm was a litle bit optimistic. >> >> We need to tell gcc that percpu_{write|add|sub|or|xor} are modyfing >> memory and possibly eflags. We could add another parameter to >> percpu_to_op() >> to separate the plain "mov" case (not changing eflags), >> but let keep it simple for the moment. >> > > Did you observe an actual failure that this patch fixed? >
Not in current tree, as we dont use yet percpu_xxxx() very much. If deployed for SNMP mibs with hundred of call sites, can you guarantee it will work as is ?
>> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> >> >> diff --git a/arch/x86/include/asm/percpu.h >> b/arch/x86/include/asm/percpu.h >> index aee103b..fd4f8ec 100644 >> --- a/arch/x86/include/asm/percpu.h >> +++ b/arch/x86/include/asm/percpu.h >> @@ -82,22 +82,26 @@ do { \ >> case 1: \ >> asm(op "b %1,"__percpu_arg(0) \ >> : "+m" (var) \ >> - : "ri" ((T__)val)); \ >> + : "ri" ((T__)val) \ >> + : "memory", "cc"); \ >> > > This shouldn't be necessary. The "+m" already tells gcc that var is a > memory input and output, and there are no other memory side-effects > which it needs to be aware of; clobbering "memory" will force gcc to > reload all register-cached memory, which is a pretty hard hit. I think > all asms implicitly clobber "cc", so that shouldn't have any effect, but > it does no harm.
So, we can probably cleanup many asms in tree :)
static inline void __down_read(struct rw_semaphore *sem) { asm volatile("# beginning down_read\n\t" LOCK_PREFIX " incl (%%eax)\n\t" /* adds 0x00000001, returns the old value */ " jns 1f\n" " call call_rwsem_down_read_failed\n" "1:\n\t" "# ending down_read\n\t" : "+m" (sem->count) : "a" (sem) : "memory", "cc"); }
> > Now, its true that the asm isn't actually modifying var itself, but > %gs:var, which is a different location. But from gcc's perspective that > shouldn't matter because var makes a perfectly good proxy for that > location, and will make sure it correctly order all accesses to var. > > I'd be surprised if this were broken, because we'd be seeing all sorts > of strange crashes all over the place. We've seen it before when the > old x86-64 pda code didn't have proper constraints on its asm statements.
I was not saying it is broken, but a "litle bit optimistic" :)
Better be safe than sorry, because those errors are very hard to track, since it depends a lot on gcc being aggressive or not. I dont have time to test all gcc versions all over there.
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