linux-kernel - Re: [PATCH v6 05/15] lockdep: Implement crossrelease feature

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Date:   Wed, 19 Apr 2017 16:25:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        willy@...radead.org, npiggin@...il.com, kernel-team@....com
Subject: Re: [PATCH v6 05/15] lockdep: Implement crossrelease feature

On Tue, Mar 14, 2017 at 05:18:52PM +0900, Byungchul Park wrote:
> +struct hist_lock {
> +	/*
> +	 * Each work of workqueue might run in a different context,
> +	 * thanks to concurrency support of workqueue. So we have to
> +	 * distinguish each work to avoid false positive.
> +	 */
> +	unsigned int		work_id;
>  };

> @@ -1749,6 +1749,14 @@ struct task_struct {
>  	struct held_lock held_locks[MAX_LOCK_DEPTH];
>  	gfp_t lockdep_reclaim_gfp;
>  #endif
> +#ifdef CONFIG_LOCKDEP_CROSSRELEASE
> +#define MAX_XHLOCKS_NR 64UL
> +	struct hist_lock *xhlocks; /* Crossrelease history locks */
> +	unsigned int xhlock_idx;
> +	unsigned int xhlock_idx_soft; /* For backing up at softirq entry */
> +	unsigned int xhlock_idx_hard; /* For backing up at hardirq entry */
> +	unsigned int work_id;
> +#endif
>  #ifdef CONFIG_UBSAN
>  	unsigned int in_ubsan;
>  #endif

> +/*
> + * Crossrelease needs to distinguish each work of workqueues.
> + * Caller is supposed to be a worker.
> + */
> +void crossrelease_work_start(void)
> +{
> +	if (current->xhlocks)
> +		current->work_id++;
> +}

> +/*
> + * Only access local task's data, so irq disable is only required.
> + */
> +static int same_context_xhlock(struct hist_lock *xhlock)
> +{
> +	struct task_struct *curr = current;
> +
> +	/* In the case of hardirq context */
> +	if (curr->hardirq_context) {
> +		if (xhlock->hlock.irq_context & 2) /* 2: bitmask for hardirq */
> +			return 1;
> +	/* In the case of softriq context */
> +	} else if (curr->softirq_context) {
> +		if (xhlock->hlock.irq_context & 1) /* 1: bitmask for softirq */
> +			return 1;
> +	/* In the case of process context */
> +	} else {
> +		if (xhlock->work_id == curr->work_id)
> +			return 1;
> +	}
> +	return 0;
> +}

I still don't like work_id; it doesn't have anything to do with
workqueues per se, other than the fact that they end up using it.

It's a history generation id; touching it completely invalidates our
history. Workqueues need this because they run independent work from the
same context.

But the same is true for other sites. Last time I suggested
lockdep_assert_empty() to denote all suck places (and note we already
have lockdep_sys_exit() that hooks into the return to user path).