Re: [PATCH v4] sched/psi: fix race between file release and pressure write

[Edward Adam Davis]

On Tue, 14 Apr 2026 10:29:20 +0800, Chen Ridong wrote:
> CPU0: write memory.pressure               CPU1: write cgroup.pressure=0
>   ==================================       ==================================
> 
>   kernfs_fop_write_iter()
>     kernfs_get_active_of(of)
>     pressure_write()
>       cgroup_kn_lock_live(memory.pressure)
>         cgroup_tryget(cgrp)
>         kernfs_break_active_protection(kn)
>         ... blocks on cgroup_mutex
> 
>                                         cgroup_pressure_write()
>                                         cgroup_kn_lock_live(cgroup.pressure)
>                                         cgroup_file_show(memory.pressure, false)
>                                           kernfs_show(false)
>                                             kernfs_drain_open_files()
>                                               cgroup_file_release(of)
>                                                 kfree(ctx)
>                                                   of->priv = NULL
>                                         cgroup_kn_unlock()
> 
>       ... acquires cgroup_mutex
>       ctx = of->priv;        // may now be NULL
>       if (ctx->psi.trigger)  // NULL dereference
> 
> IIUC, for rmdir, 'of->priv cannot be NULL' may be true, but for the other patch
> shown above, it might not be.
Marvelous!

Edward
BR