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Date:   Tue, 23 Jul 2019 18:30:48 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Jason Gunthorpe <jgg@mellanox.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
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Subject: Re: [PATCH 4/6] nouveau: unlock mmap_sem on all errors from
 nouveau_range_fault
Message-ID: <20190723163048.GD1655@lst.de>
References: <20190722094426.18563-1-hch@lst.de> <20190722094426.18563-5-hch@lst.de> <20190723151824.GL15331@mellanox.com>
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On Tue, Jul 23, 2019 at 03:18:28PM +0000, Jason Gunthorpe wrote:
> Hum..
> 
> The caller does this:
> 
> again:
> 		ret = nouveau_range_fault(&svmm->mirror, &range);
> 		if (ret == 0) {
> 			mutex_lock(&svmm->mutex);
> 			if (!nouveau_range_done(&range)) {
> 				mutex_unlock(&svmm->mutex);
> 				goto again;
> 
> And we can't call nouveau_range_fault() -> hmm_range_fault() without
> holding the mmap_sem, so we can't allow nouveau_range_fault to unlock
> it.

Goto again can only happen if nouveau_range_fault was successful,
in which case we did not drop mmap_sem.

Also:

>  	ret = hmm_range_fault(range, true);
>  	if (ret <= 0) {
>  		if (ret == 0)
>  			ret = -EBUSY;
> -		up_read(&range->vma->vm_mm->mmap_sem);
>  		hmm_range_unregister(range);

This would hold mmap_sem over hmm_range_unregister, which can lead
to deadlock if we call exit_mmap and then acquire mmap_sem again.