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Re: [RFC][PATCH 0/2] Make sparcv8 work again on cas enabled hardware

On 2016-11-02 11:05, Torvald Riegel wrote:
On Tue, 2016-11-01 at 12:51 -0400, David Miller wrote:
From: Torvald Riegel <>
Date: Tue, 01 Nov 2016 17:46:41 +0100

On Tue, 2016-11-01 at 12:09 -0400, David Miller wrote:
From: Torvald Riegel <>
Date: Tue, 01 Nov 2016 16:59:44 +0100

On Tue, 2016-11-01 at 16:07 +0100, Andreas Larsson wrote:
Any comments are most welcome. The spin lock based sparcv8 semaphore
implementation is currently unchanged by this patchset, but I would say
that that should go as well.


I think tossing out all of the ldstub based v8 code is not wise.

I was envisioning adding code to use ldstub on v8 when CAS is not
available in order to maintain the status quo of what worked and
was functional before the changes which introduced this problem
for v8 in the first place.

Having that in place until the kernel-side atomics could be
implemented, propagated, and supported in glibc would be a nice
intermediate state compared to what we have now.

How do you intend to make the synchronization primitives work whose
implementation requires a CAS and for which nobody has provided an
alternative implementation that does not require CAS?

The pure userland version will do what has been done for decades,
by using a spinlock that protects the word we want to do atomic
operations upon.  A hash table of spinlocks is another option.

I know about the available techniques; my question was rather aimed at
who's going to do the work, in which rough stages, and when.

An external table of locks does not work for process-shared
synchronization.  Do you plan to not support that, and abort() when
someone tries to create a process-shared condvar, for example?

Or do you intend to write sparc-specific versions of all the concurrent
data structures that are process-shared?  Note that in the new condvar,
for example, there's no unused space in pthread_cond_t that could be
used for a spinlock.  So you'd have to reorganize quite a bit.

If you want sparc-specific versions, who's going to implement them, and
when?  What do we do in the meantime?

[resent due to failure on my part to remove standard signature]

Linux kernel commit 8b30ca73b7cc7f2177cfc4e8274d2ebdba328cd5 added
sys_sendmsg and sys_recvmsg to the sys_call_table in
arch/sparc/kernel/systbls_32.S. So sparc32 kernels prior to Linux 4.4 do
not support them as straight up system calls.

Best regards,
Andreas Larsson

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