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MIPS: Fix race on setting and getting cpu_online_mask
While testing cpu hoptlug (cpu down and up in loops) on kernel 4.4, it was observed that occasionally check for cpu online will fail in kernel/cpu.c, _cpu_up: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/cpu.c?h=v4.4.79#n485 518 /* Arch-specific enabling code. */ 519 ret = __cpu_up(cpu, idle); 520 521 if (ret != 0) 522 goto out_notify; 523 BUG_ON(!cpu_online(cpu)); Reason is race between start_secondary and _cpu_up. cpu_callin_map is set before cpu_online_mask. In __cpu_up, cpu_callin_map is waited for, but cpu online mask is not, resulting in race in which secondary processor started and set cpu_callin_map, but not yet set the online mask,resulting in above BUG being hit. Upstream differs in the area. cpu_online check is in bringup_wait_for_ap, which is after cpu reached AP_ONLINE_IDLE,where secondary passed its start function. Nonetheless, fix makes start_secondary safe and not depending on other locks throughout the code. It protects as well against cpu_online checks put in between sometimes in the future. Fix this by moving completion after all flags are set. Signed-off-by: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com> Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/16925/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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@ -376,9 +376,6 @@ asmlinkage void start_secondary(void)
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cpumask_set_cpu(cpu, &cpu_coherent_mask);
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notify_cpu_starting(cpu);
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complete(&cpu_running);
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synchronise_count_slave(cpu);
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set_cpu_online(cpu, true);
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set_cpu_sibling_map(cpu);
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@ -386,6 +383,9 @@ asmlinkage void start_secondary(void)
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calculate_cpu_foreign_map();
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complete(&cpu_running);
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synchronise_count_slave(cpu);
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/*
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* irq will be enabled in ->smp_finish(), enabling it too early
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* is dangerous.
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