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Date:   Wed, 7 Oct 2020 15:29:45 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shuah Khan <shuah@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Aleksandr Nogikh <nogikh@google.com>,
        Nazime Hande Harputluoglu <handeharputlu@google.com>
Subject: Re: [PATCH] kcov, usb, vhost: specify contexts for remote coverage
 sections
Message-ID: <20201007192945.GB468921@rowland.harvard.edu>
References: <8c71349c3cd9698b8edcfbfc9631c5dcc3b29a37.1602091732.git.andreyknvl@google.com>
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On Wed, Oct 07, 2020 at 07:30:51PM +0200, Andrey Konovalov wrote:
> Currently there's a KCOV remote coverage collection section in
> __usb_hcd_giveback_urb(). Initially that section was added based on the
> assumption that usb_hcd_giveback_urb() can only be called in interrupt
> context as indicated by a comment before it.
> 
> As it turns out, it's actually valid to call usb_hcd_giveback_urb() in task
> context, provided that the caller turned off the interrupts; USB/IP actually
> does that. This can lead to a nested KCOV remote coverage collection
> sections both trying to collect coverage in task context. This isn't
> supported by KCOV, and leads to a WARNING.
> 
> The approach this patch takes is to annotate every call of kcov_remote_*()
> callbacks with the context those callbacks are supposed to be executed in.
> If the current context doesn't match the mask provided to a callback,
> that callback is ignored. KCOV currently only supports collecting remote
> coverage in two contexts: task and softirq.
> 
> As the result, the coverage from USB/IP related usb_hcd_giveback_urb() calls
> won't be collected, but the WARNING is fixed.
> 
> A potential future improvement would be to support nested remote coverage
> collection sections, but this patch doesn't address that.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---

> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1646,9 +1646,9 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
>  
>  	/* pass ownership to the completion handler */
>  	urb->status = status;
> -	kcov_remote_start_usb((u64)urb->dev->bus->busnum);
> +	kcov_remote_start_usb((u64)urb->dev->bus->busnum, KCOV_CONTEXT_SOFTIRQ);
>  	urb->complete(urb);
> -	kcov_remote_stop();
> +	kcov_remote_stop(KCOV_CONTEXT_SOFTIRQ);

This isn't right.  __usb_hcd_giveback_urb() can execute in pretty much
any context; its constraint is that interrupts must be disabled.

Alan Stern