linux-kernel - Re: [PATCH v4 06/11] futex: Allow to re-allocate the private hash bucket.

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Message-ID: <87jzc6b5wt.ffs@tglx>
Date: Wed, 11 Dec 2024 16:22:58 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 06/11] futex: Allow to re-allocate the private hash
 bucket.

On Tue, Dec 10 2024 at 23:27, Thomas Gleixner wrote:
> On Tue, Dec 03 2024 at 17:42, Sebastian Andrzej Siewior wrote:
> realloc_or_restore_hash()

swap_hash()
{
         old_hash = current->mm->futex.hash;
         new_hash = current->mm->futex.new_hash;

         rehash(new_hash, old_hash);
         rcu_assign_pointer(current->mm->futex.hash, new_hash);
         rcu_free(old_hash);
}

> rehash(new_hash, old_hash)
> {
>         // old_hash is marked dead, so new waiters cannot queue
>         // themself and are stuck on the hash_mutex.
>
>         for (i = 0; i < old_hash->size; i++) {
> 		hb1 = &old_hash->queues[i];
>
>                 // Protect the old bucket against unqueue(), as it does
>                 // not try to get a reference on the hash bucket. It
>                 // solely relies on q->lock_ptr.
>                 spin_lock(&hb1->lock);
>
> 		plist_for_each_entry_safe(q, tmp, hb1, list) {
> 			hb2 = hash_bucket(new_hash, &q->key);
> 			// Nesting is safe as this is a one time operation
>                         spin_lock_nested(&hb2->lock);
>
>                         plist_del(&q->list, &hb->chain);
>
>                         // Redirect the lock pointer to the new hash
>                         // bucket. See unqueue().
> 			q->lock_ptr = &hb2->lock;
>
>                         plist_add(&q->list, &hb->chain);
>                 }
>         }
> }
>
> fork()
> {
>         if (hash_needs_resize()) 
>         	hash_putref(mm->futex.hash);

         futex_validate_hash();


futex_validate_hash()
{
         guard(mutex)();
         if (!hash_needs_resize(mm->futex.hash))
         	return;
         if (mm->futex.new_hash && !hash_needs_resize(mm->futex.new_hash))
         	return;

         new = alloc();
         if (!new)
               return;
         current->mm.futex_new_hash = new;
         if (!rcuput_ref(mm->futex.hash->ref))
               return;

         swap_hash();
}