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In some profiler use cases, it is necessary to map an address to the backing file, e.g., a shared library. bpf_find_vma helper provides a flexible way to achieve this. bpf_find_vma maps an address of a task to the vma (vm_area_struct) for this address, and feed the vma to an callback BPF function. The callback function is necessary here, as we need to ensure mmap_sem is unlocked. It is necessary to lock mmap_sem for find_vma. To lock and unlock mmap_sem safely when irqs are disable, we use the same mechanism as stackmap with build_id. Specifically, when irqs are disabled, the unlocked is postponed in an irq_work. Refactor stackmap.c so that the irq_work is shared among bpf_find_vma and stackmap helpers. Signed-off-by: Song Liu <songliubraving@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Tested-by: Hengqi Chen <hengqi.chen@gmail.com> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20211105232330.1936330-2-songliubraving@fb.com
66 lines
1.7 KiB
C
66 lines
1.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only */
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/* Copyright (c) 2021 Facebook
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*/
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#ifndef __MMAP_UNLOCK_WORK_H__
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#define __MMAP_UNLOCK_WORK_H__
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#include <linux/irq_work.h>
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/* irq_work to run mmap_read_unlock() in irq_work */
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struct mmap_unlock_irq_work {
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struct irq_work irq_work;
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struct mm_struct *mm;
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};
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DECLARE_PER_CPU(struct mmap_unlock_irq_work, mmap_unlock_work);
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/*
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* We cannot do mmap_read_unlock() when the irq is disabled, because of
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* risk to deadlock with rq_lock. To look up vma when the irqs are
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* disabled, we need to run mmap_read_unlock() in irq_work. We use a
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* percpu variable to do the irq_work. If the irq_work is already used
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* by another lookup, we fall over.
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*/
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static inline bool bpf_mmap_unlock_get_irq_work(struct mmap_unlock_irq_work **work_ptr)
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{
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struct mmap_unlock_irq_work *work = NULL;
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bool irq_work_busy = false;
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if (irqs_disabled()) {
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if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
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work = this_cpu_ptr(&mmap_unlock_work);
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if (irq_work_is_busy(&work->irq_work)) {
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/* cannot queue more up_read, fallback */
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irq_work_busy = true;
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}
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} else {
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/*
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* PREEMPT_RT does not allow to trylock mmap sem in
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* interrupt disabled context. Force the fallback code.
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*/
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irq_work_busy = true;
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}
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}
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*work_ptr = work;
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return irq_work_busy;
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}
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static inline void bpf_mmap_unlock_mm(struct mmap_unlock_irq_work *work, struct mm_struct *mm)
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{
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if (!work) {
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mmap_read_unlock(mm);
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} else {
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work->mm = mm;
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/* The lock will be released once we're out of interrupt
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* context. Tell lockdep that we've released it now so
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* it doesn't complain that we forgot to release it.
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*/
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rwsem_release(&mm->mmap_lock.dep_map, _RET_IP_);
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irq_work_queue(&work->irq_work);
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}
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}
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#endif /* __MMAP_UNLOCK_WORK_H__ */
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