linux-kernel - Re: [patch V2 2/7] futex: Hash private futexes per process

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Date:	Thu, 19 May 2016 14:21:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process

On Thu, May 05, 2016 at 08:44:04PM -0000, Thomas Gleixner wrote:
> +struct futex_hash {
> +	struct raw_spinlock		lock;

	raw_spinlock_t ?

> +	unsigned int			hash_bits;
> +	struct futex_hash_bucket	*hash;
> +};

> +static void futex_populate_hash(unsigned int hash_bits)
> +{
> +	struct mm_struct *mm = current->mm;
> +	struct futex_hash_bucket *hb = NULL;
> +
> +	/*
> +	 * We don't need an explicit smp_mb() when the hash is populated
> +	 * because before we dereference mm->futex_hash.hash_bits in the hash
> +	 * function we have an smp_mb() in futex_get_key_refs() already.
> +	 */
> +	if (mm->futex_hash.hash)
> +		return;
> +
> +	/*
> +	 * If we failed to allocate a hash on the fly, fall back to the global
> +	 * hash.
> +	 */
> +	hb = futex_alloc_hash(hash_bits);
> +	if (!hb)
> +		hb = FUTEX_USE_GLOBAL_HASH;
> +
> +	raw_spin_lock(&mm->futex_hash.lock);
> +	/* We might have raced with another task allocating the hash. */
> +	if (!mm->futex_hash.hash) {
> +		mm->futex_hash.hash_bits = hash_bits;
> +		/*
> +		 * Ensure that the above is visible before we store
> +		 * the pointer.
> +		 */
> +		smp_wmb(); /* (A0) Pairs with (B) */
> +		mm->futex_hash.hash = hb;

		smp_store_release(&mm->futex_hash.hash, hb); ?

> +		hb = NULL;
> +	}
> +	raw_spin_unlock(&mm->futex_hash.lock);
> +	kfree(hb);
> +}