linux-kernel - Re: BUG: ib_mad ftrace event unsupported migration

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Message-ID: <20221102181900.5bc9812f@rorschach.local.home>
Date:   Wed, 2 Nov 2022 18:19:00 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Leonid Ravich <lravich@...il.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Leonid Ravich <leonid.ravich@...anetworks.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yigal Korman <yigal.korman@...anetworks.com>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>
Subject: Re: BUG:  ib_mad ftrace event unsupported migration

On Wed, 2 Nov 2022 22:01:17 +0200
Leonid Ravich <lravich@...il.com> wrote:

> disagree, without CONFIG_PREEMPTION (which is the default case in some
> destros) we will not get any warning, because there will not be
> preamption disable.

I test all for my code (NON_PREEMPT, VOLUNTEER_PREEMPT, PREEMPT) and
with and without lockdep enabled.

This would be a bug if you called kmalloc(X, GFP_KERNEL) in *any* non
preempt section.

> 
> second issue I see and maybe it is only me, is that the assuption of
> atomicity in trace is not a common knowledge for trace users.     

Well, I suppose we could add more documentation. Would that help? Where
would you see it? In the sample code?

I advise not even grabbing locks in trace events, because in most cases
lockdep will not catch any issues with them (it will be hidden unless
the trace event is enabled).

-- Steve