Messages in this thread | | | Subject | Re: [PATCH v6 2/2] Output stall data in debugfs | From | Peter Zijlstra <> | Date | Thu, 11 Aug 2011 21:35:22 +0200 |
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On Wed, 2011-08-10 at 11:02 -0700, Alex Neronskiy wrote: > @@ -210,22 +236,27 @@ void touch_softlockup_watchdog_sync(void) > /* watchdog detector functions */ > static void update_hardstall(unsigned long stall, int this_cpu) > { > if (stall > hardstall_thresh && stall > worst_hardstall) { > unsigned long flags; > + spin_lock_irqsave(&hardstall_write_lock, flags); > + if (stall > worst_hardstall) { > + int write_ind = hard_read_ind; > + int locked = spin_trylock(&hardstall_locks[write_ind]); > + /* cannot wait, so if there's contention, > + * switch buffers */ > + if (!locked) > + write_ind = !write_ind; > + > worst_hardstall = stall; > + hardstall_traces[write_ind].nr_entries = 0; > + save_stack_trace(&hardstall_traces[write_ind]); > > + /* tell readers to use the new buffer from now on */ > + hard_read_ind = write_ind; > + if (locked) > + spin_unlock(&hardstall_locks[write_ind]); > + } > + spin_unlock_irqrestore(&hardstall_write_lock, flags); > } > }
That must be the most convoluted locking I've seen in a while.. OMG!
What's wrong with something like:
static void update_stall(struct stall *s, unsigned long stall) { if (stall <= s->worst) return;
again: if (!raw_spin_trylock(&s->lock[s->idx])) { s->idx ^= 1; goto again; }
if (stall <= s->worst) goto unlock;
s->worst = stall; s->trace[s->idx].nr_entries = 0; save_stack_trace(&s->trace[s->idx]);
unlock: raw_spin_unlock(&s->lock[s->idx]); }
And have your read side do:
static void show_stall_trace(struct seq_file *f, void *v) { struct stall *s = f->private; int i, idx = ACCESS_ONCE(s->idx);
mutex_lock(&stall_mutex);
raw_spin_lock(&s->lock[idx]); seq_printf(f, "stall: %d\n", s->worst); for (i = 0; i < s->trace[idx].nr_entries; i++) { seq_printf(f, "[<%pK>] %pS\n", (void *)s->trace->entries[i], (void *)s->trace->entries[i]); } raw_spin_unlock(&s->lock[idx]);
mutex_unlock(&stall_mutex); }
Yes its racy on s->worst, but who cares (if you do care you can keep a copy in s->delay[idx] or so). Also, it might be better to not do the spinlock but simply use an atomic bitop to set an in-use flag, there is no reason to disable preemption over the seq_printf() loop.
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