| Message ID | 20230512205255.875713771@linutronix.de (mailing list archive) |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | [V4,01/37] x86/smpboot: Cleanup topology_phys_to_logical_pkg()/die() | expand |
--- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -238,8 +238,6 @@ static void notrace start_secondary(void x86_cpuinit.early_percpu_clock_init(); smp_callin(); - /* otherwise gcc will move up smp_processor_id before the cpu_init */ - barrier(); /* Check TSC synchronization with the control CPU: */ check_tsc_sync_target();
Peter stumbled over the barrier() after the invocation of smp_callin() in start_secondary(): "...this barrier() and it's comment seem weird vs smp_callin(). That function ends with an atomic bitop (it has to, at the very least it must not be weaker than store-release) but also has an explicit wmb() to order setup vs CPU_STARTING. There is no way the smp_processor_id() referred to in this comment can land before cpu_init() even without the barrier()." The barrier() along with the comment was added in 2003 with commit d8f19f2cac70 ("[PATCH] x86-64 merge") in the history tree. One of those well documented combo patches of that time which changes world and some more. The context back then was: /* * Dont put anything before smp_callin(), SMP * booting is too fragile that we want to limit the * things done here to the most necessary things. */ cpu_init(); smp_callin(); + /* otherwise gcc will move up smp_processor_id before the cpu_init */ + barrier(); Dprintk("cpu %d: waiting for commence\n", smp_processor_id()); Even back in 2003 the compiler was not allowed to reorder that smp_processor_id() invocation before the cpu_init() function call. Especially not as smp_processor_id() resolved to: asm volatile("movl %%gs:%c1,%0":"=r" (ret__):"i"(pda_offset(field)):"memory"); There is no trace of this change in any mailing list archive including the back then official x86_64 list discuss@x86-64.org, which would explain the problem this change solved. The debug prints are gone by now and the the only smp_processor_id() invocation today is farther down in start_secondary() after locking vector_lock which itself prevents reordering. Even if the compiler would be allowed to reorder this, the code would still be correct as GSBASE is set up early in the assembly code and is valid when the CPU reaches start_secondary(), while the code at the time when this barrier was added did the GSBASE setup in cpu_init(). As the barrier has zero value, remove it. Reported-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20230509100421.GU83892@hirez.programming.kicks-ass.net --- V4: New patch --- arch/x86/kernel/smpboot.c | 2 -- 1 file changed, 2 deletions(-)