From 314cdbefd1fd0a7acf3780e9628465b77ea6a836 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Wed, 30 Jan 2008 13:31:21 +0100 Subject: x86: FIFO ticket spinlocks Introduce ticket lock spinlocks for x86 which are FIFO. The implementation is described in the comments. The straight-line lock/unlock instruction sequence is slightly slower than the dec based locks on modern x86 CPUs, however the difference is quite small on Core2 and Opteron when working out of cache, and becomes almost insignificant even on P4 when the lock misses cache. trylock is more significantly slower, but they are relatively rare. On an 8 core (2 socket) Opteron, spinlock unfairness is extremely noticable, with a userspace test having a difference of up to 2x runtime per thread, and some threads are starved or "unfairly" granted the lock up to 1 000 000 (!) times. After this patch, all threads appear to finish at exactly the same time. The memory ordering of the lock does conform to x86 standards, and the implementation has been reviewed by Intel and AMD engineers. The algorithm also tells us how many CPUs are contending the lock, so lockbreak becomes trivial and we no longer have to waste 4 bytes per spinlock for it. After this, we can no longer spin on any locks with preempt enabled and cannot reenable interrupts when spinning on an irq safe lock, because at that point we have already taken a ticket and the would deadlock if the same CPU tries to take the lock again. These are questionable anyway: if the lock happens to be called under a preempt or interrupt disabled section, then it will just have the same latency problems. The real fix is to keep critical sections short, and ensure locks are reasonably fair (which this patch does). Signed-off-by: Nick Piggin Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar --- arch/x86/Kconfig | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'arch/x86/Kconfig') diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index db434f8..1992b8f 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -20,8 +20,7 @@ config X86 def_bool y config GENERIC_LOCKBREAK - def_bool y - depends on SMP && PREEMPT + def_bool n config GENERIC_TIME def_bool y -- cgit v1.1