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

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Message-ID: <20160519122406.GA3192@twins.programming.kicks-ass.net>
Date:	Thu, 19 May 2016 14:24:06 +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:
> +static struct futex_hash_bucket *hash_futex(union futex_key *key)
> +{
> +#ifdef CONFIG_FUTEX_PRIVATE_HASH
> +	struct mm_struct *mm = current->mm;
> +	unsigned int slot;
> +
> +	/*
> +	 * Futexes which use the per process hash have the lower bits cleared
> +	 */
> +	if (key->both.offset & (FUT_OFF_INODE | FUT_OFF_MMSHARED))
> +		return hash_global_futex(key);
> +
> +	slot = hash_long(key->private.address, mm->futex_hash.hash_bits);
> +	return &mm->futex_hash.hash[slot];

Do we want the option to WARN if we get collisions in this per-process
hash?

Because afaiu there is no guarantee what so ever this doesn't happen,
and collisions here can create the very same priority inversions as are
possible in the global hash.

Less likely etc.. more contained since its only the threads of the one
process that get tangled up, but still possible.

> +#else
> +	return hash_global_futex(key);
> +#endif
> +}