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smp: Alloc percpu csd data in smpcfd_prepare_cpu() only once

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smp_call_function_many_cond() uses per-CPU CSD objects when queueing
callbacks to remote CPUs, and the wait path later dereferences those CSDs
from csd_lock_wait().

Making the wait path preemptible allows the initiating task to be
preempted or migrated before it waits for completion. A target CPU can be
offlined in that window. If smpcfd_dead_cpu() frees the target CPU's
per-CPU CSD storage, csd_lock_wait() can later dereference freed memory.

One way to protect the CSD storage is to free it via RCU or after a
synchronization step in the CPU offline path, but that would add
unnecessary complexity and can delay CPU shutdown.

Allocate the per-CPU CSD storage the first time a CPU comes up and keep
it allocated when the CPU is offlined. This allows csd_lock_wait() to
access the CSD even when the target CPU is offlined after preemption is
re-enabled and before the wait is invoked.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Muchun Song <muchun.song@linux.dev>
Link: https://patch.msgid.link/20260709122933.4021501-6-zhouchuyi@bytedance.com
This commit is contained in:
Chuyi Zhou
2026-07-16 09:24:55 +02:00
committed by Thomas Gleixner
parent 9a560af15f
commit 9f483e5b2f
1 changed files with 8 additions and 2 deletions
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kernel/smp.c
+8 -2
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@@ -64,7 +64,14 @@ int smpcfd_prepare_cpu(unsigned int cpu)
free_cpumask_var(cfd->cpumask);
return -ENOMEM;
}
cfd->csd = alloc_percpu(call_single_data_t);
/*
* Allocate the per-CPU CSD the first time a CPU comes up. It is
* not freed when the CPU is offlined, so csd_lock_wait() can access
* it even when the CPU was offlined after preemption was re-enabled.
*/
if (!cfd->csd)
cfd->csd = alloc_percpu(call_single_data_t);
if (!cfd->csd) {
free_cpumask_var(cfd->cpumask);
free_cpumask_var(cfd->cpumask_ipi);
@@ -80,7 +87,6 @@ int smpcfd_dead_cpu(unsigned int cpu)
free_cpumask_var(cfd->cpumask);
free_cpumask_var(cfd->cpumask_ipi);
free_percpu(cfd->csd);
return 0;
}