writeback: report a Tasks-RCU quiescent state per cgwb drain pass

Overview

In the Linux kernel, the following vulnerability has been resolved:

writeback: report a Tasks-RCU quiescent state per cgwb drain pass

cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling cleanup_offline_cgwb() until it returns false, with a cond_resched() between passes. On a CONFIG_PREEMPTION kernel that cond_resched() does nothing: _cond_resched() is a plain "return 0", and under PREEMPT_DYNAMIC the full and lazy modes disable it. Since commit 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are the only two models on the architectures with PREEMPT_LAZY support, arm64 and x86 among them, so the drain loop never reports a Tasks-RCU quiescent state.

A worker draining a cgwb with millions of attached inodes runs for minutes. On a 6.18 arm64 host in lazy mode the cgwb worker drained one dying cgroup's writeback domain for over 11 minutes. A BPF program unlink (bpf_trampoline_unlink_prog -> bpf_trampoline_update -> unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks()) waited on that grace period while holding the trampoline mutex, 42 tasks queued behind it in D state, and the hung task detector fired at 614 s and panicked the host. Any BPF or ftrace detach during a long drain inherits the drain's length.

Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out anybody who calls sycnrhonize_rcu_tasks(). We put this in a do { } while loop because if we have many small cgroups cleanup_offline_cgwb() will return false and we will never call cond_resched_tasks_rcu_qs(), creating the same problem.

Affected

• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < 863049b4018237d1590cd13b16e9fe9c0faad2c6
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < 42462b5bb0dec57db7f85d0f77655c737d45754d
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < ebf1555ad2821be06f87a88ffd9a2cab8d2200e1
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < 4eb889649eaf65f4a5727b04ba1c017b7d05bb16
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < fd3bf32279df98751364b3609eb100a3e42f253b
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < 260f586a80e7f624dd7f069898a75a904f34d373
• Linux >= c22d70a162d3cc177282c4487be4d54876ca55c8 < 407a5d205179a4ab186571b0e16ec42725dc77bc
• Linux 5.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.