Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving aring buffer

From: Oleg Nesterov
Date: Fri Nov 01 2013 - 11:21:14 EST

On 11/01, Oleg Nesterov wrote:
> Hi Namhyung,
> Sorry if this was already discussed. But I can't really understand
> the idea of this per-cpu buffer...
> On 10/29, Namhyung Kim wrote:
> >
> > Fetching from user space should be done in a non-atomic context. So
> > use a per-cpu buffer and copy its content to the ring buffer
> > atomically. Note that we can migrate during accessing user memory
> > thus use a per-cpu mutex to protect concurrent accesses.
> And if the task migrates or just sleeps in page fault, another task
> which hits another uprobe on the same CPU should wait.
> Why we can't simply add trace_uprobe->buffer instead? Only to save
> some memory? But every uprobe is very expensive in this sense anyway.

Ah, please ignore... handler_chain() is not self-serialized, so
tu->buffer needs locking/waiting too.


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