On 02/03/2010 04:44 AM, John David Anglin wrote:
> On Tue, 02 Feb 2010, Helge Deller wrote:
>
>> I wonder if we have some problems with the LWS code path in the kernel
>> regarding atomic locking with futexes?
>>
>> In arch/parisc/kernel/syscall.S we use a lock table called lws_lock_start[]
>> to guard the LWS code against other competing userspace processes.
>> I wonder if this really enough, esp. since we do implement futex syscalls
>> (e.g. clone/exit calls uses futex functions to change userspace values
>> because of CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID).
>>
>> Do we maybe need to protect the LWS code path with the same locking table
>> as the
>> generic kernel? Atomicity of futexes writing to userspace are not in sync
>> with the locking of the LWS/lws_lock_start[] code.
>>
>> I tried to come up with a patch for that which I attached here, but sadly
>> it hangs as soon as the init process is started on a 64bit/SMP kernel.
>> So either my thinking here is stupid, or I do have a stupid coding bug.
>>
>> Furthermore, the coding for futex_atomic_op_inuser() in
>> arch/parisc/include/asm/futex.h seems to miss real functionality.
>> I didn't looked closer into this though.
>
> While locking may be a problem, it is not the main reason the minifail
> program fails.  The program fails on my c3750 with a 32-bit UP kernel.

Hmm, I can't reproduce it at the moment with a 32bit UP kernel on my
c3000. The minifail3 test program I currently use (to avoid glibc issues)
is attached.

> There is no locking in this kernel.  The LWS code relies on the scheduler
> for atomicity.

yep. This should then work an a UP kernel.

> I tried disabling interrupts around the crucial three instructions but
> it didn't help.

Ugh.

Anyway, my current patch which compiles and runs fine is attached here as
well. Maybe you want to try it on your SMP builds? It includes the syscall.S
changes you sent last time too.
In this version of the patch I added on own LWS locking hash table for user-space accesses,
which is used in the LWS code and when some (probably not all) put_user()/
get_user() calls are made in the futex code.

Feedback still welcome.

Helge