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authorAndy Lutomirski <luto@kernel.org>2020-12-03 21:07:06 -0800
committerThomas Gleixner <tglx@linutronix.de>2020-12-09 09:37:43 +0100
commite45cdc71d1fa5ac3a57b23acc31eb959e4f60135 (patch)
treea49ce2b7a17945b7c43cb9d5e34d881bcd5d4735 /kernel/sched
parent758c9373d84168dc7d039cf85a0e920046b17b41 (diff)
membarrier: Execute SYNC_CORE on the calling thread
membarrier()'s MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is documented as syncing the core on all sibling threads but not necessarily the calling thread. This behavior is fundamentally buggy and cannot be used safely. Suppose a user program has two threads. Thread A is on CPU 0 and thread B is on CPU 1. Thread A modifies some text and calls membarrier(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE). Then thread B executes the modified code. If, at any point after membarrier() decides which CPUs to target, thread A could be preempted and replaced by thread B on CPU 0. This could even happen on exit from the membarrier() syscall. If this happens, thread B will end up running on CPU 0 without having synced. In principle, this could be fixed by arranging for the scheduler to issue sync_core_before_usermode() whenever switching between two threads in the same mm if there is any possibility of a concurrent membarrier() call, but this would have considerable overhead. Instead, make membarrier() sync the calling CPU as well. As an optimization, this avoids an extra smp_mb() in the default barrier-only mode and an extra rseq preempt on the caller. Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE") Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Link: https://lore.kernel.org/r/250ded637696d490c69bef1877148db86066881c.1607058304.git.luto@kernel.org
Diffstat (limited to 'kernel/sched')
-rw-r--r--kernel/sched/membarrier.c51
1 files changed, 33 insertions, 18 deletions
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 1c278dff4f2d..9d8df34bea75 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -194,7 +194,8 @@ static int membarrier_private_expedited(int flags, int cpu_id)
return -EPERM;
}
- if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1)
+ if (flags != MEMBARRIER_FLAG_SYNC_CORE &&
+ (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1))
return 0;
/*
@@ -213,8 +214,6 @@ static int membarrier_private_expedited(int flags, int cpu_id)
if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_id))
goto out;
- if (cpu_id == raw_smp_processor_id())
- goto out;
rcu_read_lock();
p = rcu_dereference(cpu_rq(cpu_id)->curr);
if (!p || p->mm != mm) {
@@ -229,16 +228,6 @@ static int membarrier_private_expedited(int flags, int cpu_id)
for_each_online_cpu(cpu) {
struct task_struct *p;
- /*
- * Skipping the current CPU is OK even through we can be
- * migrated at any point. The current CPU, at the point
- * where we read raw_smp_processor_id(), is ensured to
- * be in program order with respect to the caller
- * thread. Therefore, we can skip this CPU from the
- * iteration.
- */
- if (cpu == raw_smp_processor_id())
- continue;
p = rcu_dereference(cpu_rq(cpu)->curr);
if (p && p->mm == mm)
__cpumask_set_cpu(cpu, tmpmask);
@@ -246,12 +235,38 @@ static int membarrier_private_expedited(int flags, int cpu_id)
rcu_read_unlock();
}
- preempt_disable();
- if (cpu_id >= 0)
+ if (cpu_id >= 0) {
+ /*
+ * smp_call_function_single() will call ipi_func() if cpu_id
+ * is the calling CPU.
+ */
smp_call_function_single(cpu_id, ipi_func, NULL, 1);
- else
- smp_call_function_many(tmpmask, ipi_func, NULL, 1);
- preempt_enable();
+ } else {
+ /*
+ * For regular membarrier, we can save a few cycles by
+ * skipping the current cpu -- we're about to do smp_mb()
+ * below, and if we migrate to a different cpu, this cpu
+ * and the new cpu will execute a full barrier in the
+ * scheduler.
+ *
+ * For SYNC_CORE, we do need a barrier on the current cpu --
+ * otherwise, if we are migrated and replaced by a different
+ * task in the same mm just before, during, or after
+ * membarrier, we will end up with some thread in the mm
+ * running without a core sync.
+ *
+ * For RSEQ, don't rseq_preempt() the caller. User code
+ * is not supposed to issue syscalls at all from inside an
+ * rseq critical section.
+ */
+ if (flags != MEMBARRIER_FLAG_SYNC_CORE) {
+ preempt_disable();
+ smp_call_function_many(tmpmask, ipi_func, NULL, true);
+ preempt_enable();
+ } else {
+ on_each_cpu_mask(tmpmask, ipi_func, NULL, true);
+ }
+ }
out:
if (cpu_id < 0)