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> > I think we may still have some issues. > Case 1: >   1. one bpf program is running in process context, >      bpf_timer_start() is called and timer->lock is taken >   2. timer softirq is triggered and this callback is called ___bpf_spin_lock is actually irqsave version of spin_lock. So this race is not possible. > Case 2: >   1. this callback is called, timer->lock is taken >   2. a nmi happens and some bpf program is called (kprobe, tracepoint, >      fentry/fexit or perf_event, etc.) and that program calls >      bpf_timer_start() > > So we could have deadlock in both above cases? Shouldn't be possible either because bpf timers are not allowed in nmi-bpf-progs. I'll double check that it's the case. Pretty much the same restrictions are with bpf_spin_lock. > >> +    /* callback_fn and prog need to match. They're updated together >> +     * and have to be read under lock. >> +     */ >> +    prog = t->prog; >> +    callback_fn = t->callback_fn; >> + >> +    /* wrap bpf subprog invocation with prog->refcnt++ and -- to make >> +     * sure that refcnt doesn't become zero when subprog is executing. >> +     * Do it under lock to make sure that bpf_timer_start doesn't drop >> +     * prev prog refcnt to zero before timer_cb has a chance to bump it. >> +     */ >> +    bpf_prog_inc(prog); >> +    ____bpf_spin_unlock(&timer->lock); >> + >> +    /* bpf_timer_cb() runs in hrtimer_run_softirq. It doesn't migrate >> and >> +     * cannot be preempted by another bpf_timer_cb() on the same cpu. >> +     * Remember the timer this callback is servicing to prevent >> +     * deadlock if callback_fn() calls bpf_timer_cancel() on the same >> timer. >> +     */ >> +    this_cpu_write(hrtimer_running, t); > > This is not protected by spinlock, in bpf_timer_cancel() and > bpf_timer_cancel_and_free(), we have spinlock protected read, so > there is potential race conditions if callback function and > helper/bpf_timer_cancel_and_free run in different context? what kind of race do you see? This is per-cpu var and bpf_timer_cb is in softirq while timer_cancel/cancel_and_free are calling it under spin_lock_irqsave... so they cannot race because softirq and bpf_timer_cb will run after start/canel/cancel_free will do unlock_irqrestore. >> +    prev = t->prog; >> +    if (prev != prog) { >> +        if (prev) >> +            /* Drop pref prog refcnt when swapping with new prog */ > > pref -> prev > >> +            bpf_prog_put(prev); > > Maybe we want to put the above two lines with {}? you mean add {} because there is a comment ? I don't think the kernel coding style considers comment as a statement. >> +    if (this_cpu_read(hrtimer_running) != t) >> +        hrtimer_cancel(&t->timer); > > We could still have race conditions here when > bpf_timer_cancel_and_free() runs in process context and callback in > softirq context. I guess we might be okay. No, since this check is under spin_lock_irsave. > But if bpf_timer_cancel_and_free() in nmi context, not 100% sure > whether we have issues or not. timers shouldn't be available to nmi-bpf progs. There will be all sorts of issues. The underlying hrtimer implementation cannot deal with nmi either.