locking/qspinlock/x86: Micro-optimize virt_spin_lock()
Optimize virt_spin_lock() to use simpler and faster: atomic_try_cmpxchg(*ptr, &val, new) instead of: atomic_cmpxchg(*ptr, val, new) == val The x86 CMPXCHG instruction returns success in the ZF flag, so this change saves a compare after the CMPXCHG. Also optimize retry loop a bit. atomic_try_cmpxchg() fails iff &lock->val != 0, so there is no need to load and compare the lock value again - cpu_relax() can be unconditinally called in this case. This allows us to generate optimized: 1f: ba 01 00 00 00 mov $0x1,%edx 24: 8b 03 mov (%rbx),%eax 26: 85 c0 test %eax,%eax 28: 75 63 jne 8d <...> 2a: f0 0f b1 13 lock cmpxchg %edx,(%rbx) 2e: 75 5d jne 8d <...> ... 8d: f3 90 pause 8f: eb 93 jmp 24 <...> instead of: 1f: ba 01 00 00 00 mov $0x1,%edx 24: 8b 03 mov (%rbx),%eax 26: 85 c0 test %eax,%eax 28: 75 13 jne 3d <...> 2a: f0 0f b1 13 lock cmpxchg %edx,(%rbx) 2e: 85 c0 test %eax,%eax 30: 75 f2 jne 24 <...> ... 3d: f3 90 pause 3f: eb e3 jmp 24 <...> Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: Waiman Long <longman@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: https://lore.kernel.org/r/20240422120054.199092-1-ubizjak@gmail.com
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