On Fri, 28 Feb 2014, Heiko Carstens wrote:
On Fri, Feb 28, 2014 at 02:12:22PM +1100, Finn Thain wrote:
Data read fault at 0x00000000 in Super Data (pc=0x3afec) BAD
KERNEL BUSERR
I think futex_init needs to do set_fs(USER_DS), since futex is about
user data.
Rhetorical question: does it make sense to explicitly switch to
USER_DS when we know that current->mm may be invalid at the time?
Given that the presence of USER_DS or KERNEL_DS at initcall-time
varies between architectures, perhaps this question must be left to
arch implementors.
All architectures start with KERNEL_DS, otherwise the first couple of
system calls within kernel_init() would all fail and the system wouldn't
boot.
I was alluding to the assertion that "USER_DS is the normal operating
state": https://lkml.org/lkml/2014/1/15/476
Moreover, architectures that have no need for a run-time test for
usable futex_value ops have no real interest in such questions.
AFAICT, only mips needs the run-time test.
For every other arch the availability of futex_value ops is known at
compile-time. We don't need to pursue universal answers to questions
like these:
1) the correctness of switching to USER_DS with mm->current == NULL
I "fixed" s390, so it should survive an early switch to USER_DS and a
subsequent failing user space access now.
Anyway,
The discussion of these questions in various threads on various lists
did not resolve anything. Again, this is probably because all of these
questions are actually arch implementation issues.
So it seems to me that the solution to the futex_init() problem is to
evaluate the futex_cmpxchg_enabled assignment at compile-time where
possible, and use the run-time test only on mips. Thoughts?
Such a patch exists already (bottom of message):
http://marc.info/?l=linux-next&m=138875880028607&w=2
Thanks. I gather that you don't need this patch on s390 any more. Though
it does promise to save some cycles on any architecture that can optimize
away all the "if (!futex_cmpxchg_enabled) return" tests.
With your patch, the nak'd patch* from Geert would be obsolete unless it
was useful to mips (or deemed correct by fiat).
Finn
* http://marc.info/?l=linux-kernel&m=138675863423989&w=2
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