linux-kernel - Re: [RFC 3/4] trace_uprobe: Support SDT markers having semaphore

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Message-ID: <20180307085705.GL25181@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Mar 2018 09:57:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Cc:     mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, mhiramat@...nel.org,
        ananth@...ux.vnet.ibm.com, naveen.n.rao@...ux.vnet.ibm.com,
        srikar@...ux.vnet.ibm.com, oleg@...hat.com
Subject: Re: [RFC 3/4] trace_uprobe: Support SDT markers having semaphore

On Wed, Mar 07, 2018 at 02:16:13PM +0530, Ravi Bangoria wrote:
> > W.T.H. is that called a semaphore? afaict its just a usage-counter.
> 
> I totally agree with you. But it's not me who named it semaphore :)
> 
> Please refer to "Semaphore Handling" section at:
> https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
> 
> We can surly name it differently in the kernel code and document it
> properly in the Documents/tracing/

Yes, just because the SDT people failed their CS101 class doesn't mean
we need to perpetuate that failure. Please name it sensibly in our code.

> > Also, would it not be much better to do userspace jump-labels for this?
> > That completely avoids the dynamic branch at the SDT site.
> >
> 
> Userspace jump-label is a good idea but...
> 
> Semaphore logic has already became a kinda ABI now. Tools like bcc,
> gdb, systemtap etc. flip the semaphore while probing the marker.

*groan*.. maybe suggest it for the next version; it appears we're
already at SDTv3, so surely there will be a v4 too.