Re: PATCH: Always use ll/sc for mips

[Jun Sun <jsun@mvista.com>]

Ralf Baechle wrote:

> On Thu, Jul 04, 2002 at 10:47:41AM +0200, Kevin D. Kissell wrote:
> 
> 
>>The R5900 kernel for the Playstation 2 does not use system
>>calls.  It uses a memory-mapped pseudo-device hack that
>>the guys at Sony came up with, which is much faster.  We
>>at MIPS came up with an even faster hack which uses 
>>the destruction of a "k" register value, but which requires 
>>the branch-likely instruction and thus only workson 
>>MIPS II CPUs and above (R39xxx, R4xxx, R5xxx,
>>but not the classic R3K).  See my message
>>"Re: patches for test-and-set without ll/sc" of January 22.
>>
>>I consider it to be very important for MIPS/Linux
>>that the embedded/workstation kernel and libraries
>>merge with the Playstation 2 "consumer" Linux, and
>>I don't think that will happen if we try to push the
>>PS2 people to use something far less efficient than
>>what they already have. "Entia non sunt multiplicanda 
>>praeter necessitatem", as a wise old guy once said,
>>but could we not consider a MIPS/Linux universe
>>where R3000 binaries use system calls, non-LL/SC
>>MIPSII+ binaries use k-register destruction, real,
>>manly, MIPS binaries use LL/SC instructions, and
>>where the MIPS/Linux kernel (a) supports an appropriate
>>system call, (b) makes a contract with userland to 
>>destroy k-regs predictably, and (c) contains the
>>emulation logic for LL/SC?  That should give us
>>full cross-platform binary compatibility, with optimal
>>performance on each platform when an appropriately
>>configured set of libraries and tools is installed.
>>
> 
> No, Sony's ABI isn't MP proof and will break silently on MP systems.  As
> such I can't consider it anything else but a hack.  sysmips(MIPS_ATOMIC_SET,
> ...) and ll/sc however are MP proof.
> 


sysmips(MIPS_ATOMIC_SET, ...) as it is is not MP-safe.  Two processors can set 
the variable at the same time since no spinlock is used to protect the access.

This is also a problem when I was writing preemptiable kernel patch.

Jun