[Boqun Feng Cc'd] On Fri, Dec 16, 2022 at 03:26:21AM -0800, Linus Torvalds wrote: > On Thu, Dec 15, 2022 at 7:41 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > CPU1: ptrace(2) > > ptrace_check_attach() > > read_lock(&tasklist_lock); > > > > CPU2: setpgid(2) > > write_lock_irq(&tasklist_lock); > > spins > > > > CPU1: takes an interrupt that would call kill_fasync(). grep and the > > first instance of kill_fasync() is in hpet_interrupt() - it's not > > something exotic. IRQs disabled on CPU2 won't stop it. > > kill_fasync(..., SIGIO, ...) > > kill_fasync_rcu() > > read_lock_irqsave(&fa->fa_lock, flags); > > send_sigio() > > read_lock_irqsave(&fown->lock, flags); > > read_lock(&tasklist_lock); > > > > ... and CPU1 spins as well. > > Nope. See kernel/locking/qrwlock.c: [snip rwlocks are inherently unfair, queued ones are somewhat milder, but all implementations have writers-starving behaviour for read_lock() at least when in_interrupt()] D'oh... Consider requested "Al, you are a moron" duly delivered... I plead having been on way too low caffeine and too little sleep ;-/ Looking at the original report, looks like the scenario there is meant to be the following: CPU1: read_lock(&tasklist_lock) tasklist_lock grabbed CPU2: get an sg write(2) feeding request to libata; host->lock is taken, request is immediately completed and scsi_done() is about to be called. host->lock grabbed CPU3: write_lock_irq(&tasklist_lock) spins on tasklist_lock until CPU1 gets through. CPU2: get around to kill_fasync() called by sg_rq_end_io() and to grabbing tasklist_lock inside send_sigio() spins, since it's not in an interrupt and there's a pending writer host->lock is held, spin until CPU3 gets through. CPU1: take an interrupt, which on libata will try to grab host->lock tasklist_lock is held, spins on host->lock until CPU2 gets through Am I reading it correctly?
