From 86f40fa6a4675ecfe554c360ab022294742c9a01 Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Tue, 11 Mar 2025 19:18:04 +0100 Subject: [PATCH] fs: dodge an atomic in putname if ref == 1 While the structure is refcounted, the only consumer incrementing it is audit and even then the atomic operation is only needed when it interacts with io_uring. If putname spots a count of 1, there is no legitimate way for anyone to bump it. If audit is disabled, the count is guaranteed to be 1, which consistently elides the atomic for all path lookups. If audit is enabled, it still manages to elide the last decrement. Note the patch does not do anything to prevent audit from suffering atomics. See [1] and [2] for a different approach. Benchmarked on Sapphire Rapids issuing access() (ops/s): before: 5106246 after: 5269678 (+3%) Link 1: https://lore.kernel.org/linux-fsdevel/20250307161155.760949-1-mjguzik@gmail.com/ Link 2: https://lore.kernel.org/linux-fsdevel/20250307164216.GI2023217@ZenIV/ Signed-off-by: Mateusz Guzik Link: https://lore.kernel.org/r/20250311181804.1165758-1-mjguzik@gmail.com Reviewed-by: Jens Axboe Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- fs/namei.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 207898ee9fd5..3209d65b5fc0 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -275,14 +275,19 @@ EXPORT_SYMBOL(getname_kernel); void putname(struct filename *name) { + int refcnt; + if (IS_ERR_OR_NULL(name)) return; - if (WARN_ON_ONCE(!atomic_read(&name->refcnt))) - return; + refcnt = atomic_read(&name->refcnt); + if (refcnt != 1) { + if (WARN_ON_ONCE(!refcnt)) + return; - if (!atomic_dec_and_test(&name->refcnt)) - return; + if (!atomic_dec_and_test(&name->refcnt)) + return; + } if (name->name != name->iname) { __putname(name->name); -- 2.50.1