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From: Uros Bizjak <ubizjak@gmail.com>
To: iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org
Cc: Uros Bizjak <ubizjak@gmail.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Kevin Tian <kevin.tian@intel.com>,
	Joerg Roedel <joro@8bytes.org>,
	Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: [PATCH 3/3] iommufd: Use atomic_long_try_cmpxchg() in incr_user_locked_vm()
Date: Wed, 22 May 2024 10:26:49 +0200
Message-ID: <20240522082729.971123-3-ubizjak@gmail.com>
In-Reply-To: <20240522082729.971123-1-ubizjak@gmail.com>
References: <20240522082729.971123-1-ubizjak@gmail.com>
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Use atomic_long_try_cmpxchg() instead of
atomic_long_cmpxchg (*ptr, old, new) != old in incr_user_locked_vm().
cmpxchg returns success in ZF flag, so this change saves a compare
after cmpxchg (and related move instruction in front of cmpxchg).

Also, atomic_long_try_cmpxchg() implicitly assigns old *ptr
value to "old" when cmpxchg fails. There is no need to re-read
the value in the loop.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Will Deacon <will@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
---
 drivers/iommu/iommufd/pages.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/iommufd/pages.c b/drivers/iommu/iommufd/pages.c
index 528f356238b3..117f644a0c5b 100644
--- a/drivers/iommu/iommufd/pages.c
+++ b/drivers/iommu/iommufd/pages.c
@@ -809,13 +809,14 @@ static int incr_user_locked_vm(struct iopt_pages *pages, unsigned long npages)
 
 	lock_limit = task_rlimit(pages->source_task, RLIMIT_MEMLOCK) >>
 		     PAGE_SHIFT;
+
+	cur_pages = atomic_long_read(&pages->source_user->locked_vm);
 	do {
-		cur_pages = atomic_long_read(&pages->source_user->locked_vm);
 		new_pages = cur_pages + npages;
 		if (new_pages > lock_limit)
 			return -ENOMEM;
-	} while (atomic_long_cmpxchg(&pages->source_user->locked_vm, cur_pages,
-				     new_pages) != cur_pages);
+	} while (!atomic_long_try_cmpxchg(&pages->source_user->locked_vm,
+					  &cur_pages, new_pages));
 	return 0;
 }
 
-- 
2.45.1