netdev - Re: [PATCH net] ptp: vclock: use mutex to fix "sleep on atomic" bug

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Date:   Thu, 16 Feb 2023 14:05:56 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Íñigo Huguet <ihuguet@...hat.com>
Cc:     yangbo.lu@....com, mlichvar@...hat.com,
        gerhard@...leder-embedded.com, habetsm.xilinx@...il.com,
        ecree.xilinx@...il.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
        Yalin Li <yalli@...hat.com>
Subject: Re: [PATCH net] ptp: vclock: use mutex to fix "sleep on atomic" bug

On Thu, Feb 16, 2023 at 03:30:51PM +0100, Íñigo Huguet wrote:
> vclocks were using spinlocks to protect access to its timecounter and
> cyclecounter. Access to timecounter/cyclecounter is backed by the same
> driver callbacks that are used for non-virtual PHCs, but the usage of
> the spinlock imposes a new limitation that didn't exist previously: now
> they're called in atomic context so they mustn't sleep.
> 
> Some drivers like sfc or ice may sleep on these callbacks, causing
> errors like "BUG: scheduling while atomic: ptp5/25223/0x00000002"
> 
> Fix it replacing the vclock's spinlock by a mutex. It fix the mentioned
> bug and it doesn't introduce longer delays.

Thanks for taking this up...

> I've tested synchronizing various different combinations of clocks:
> - vclock->sysclock
> - sysclock->vclock
> - vclock->vclock
> - hardware PHC in different NIC -> vclock
> - created 4 vclocks and launch 4 parallel phc2sys processes

Could you please try it with lockdep enabled?
 
> @@ -43,16 +43,16 @@ static void ptp_vclock_hash_del(struct ptp_vclock *vclock)
>  static int ptp_vclock_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
>  {
>  	struct ptp_vclock *vclock = info_to_vclock(ptp);
> -	unsigned long flags;
>  	s64 adj;
>  
>  	adj = (s64)scaled_ppm << PTP_VCLOCK_FADJ_SHIFT;
>  	adj = div_s64(adj, PTP_VCLOCK_FADJ_DENOMINATOR);
>  
> -	spin_lock_irqsave(&vclock->lock, flags);
> +	if (mutex_lock_interruptible(&vclock->lock) < 0)
> +		return -EINTR;

Nit: please drop the '< 0' from the test.

> @@ -281,9 +280,10 @@ ktime_t ptp_convert_timestamp(const ktime_t *hwtstamp, int vclock_index)
>  		if (vclock->clock->index != vclock_index)
>  			continue;
>  
> -		spin_lock_irqsave(&vclock->lock, flags);
> +		if (mutex_lock_interruptible(&vclock->lock) < 0)
> +			break;

This is the only one that I'm not sure about.  The others are all
called from user context.  Clean lockdep run would help.

Thanks,
Richard