From 82efd569a8909f2b13140c1b3de88535aea0b051 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Wed, 23 Apr 2025 10:21:29 +0200 Subject: [PATCH] locking/local_lock: fix _Generic() matching of local_trylock_t MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Michael Larabel reported [1] a nginx performance regression in v6.15-rc3 and bisected it to commit 51339d99c013 ("locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type") The problem is the _Generic() usage with a default association that masks the fact that "local_trylock_t *" association is not being selected as expected. Replacing the default with the only other expected type "local_lock_t *" reveals the underlying problem: include/linux/local_lock_internal.h:174:26: error: ‘_Generic’ selector of type ‘__seg_gs local_lock_t *’ is not compatible with any association The local_locki's are part of __percpu structures and thus the __percpu attribute is needed to associate the type properly. Add the attribute and keep the default replaced to turn any further mismatches into compile errors. The failure to recognize local_try_lock_t in __local_lock_release() means that a local_trylock[_irqsave]() operation will set tl->acquired to 1 (there's no _Generic() part in the trylock code), but then local_unlock[_irqrestore]() will not set tl->acquired back to 0, so further trylock operations will always fail on the same cpu+lock, while non-trylock operations continue to work - a lockdep_assert() is also not being executed in the _Generic() part of local_lock() code. This means consume_stock() and refill_stock() operations will fail deterministically, resulting in taking the slow paths and worse performance. Fixes: 51339d99c013 ("locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type") Reported-by: Michael Larabel Closes: https://www.phoronix.com/review/linux-615-nginx-regression/2 [1] Signed-off-by: Vlastimil Babka Acked-by: Alexei Starovoitov Signed-off-by: Linus Torvalds --- include/linux/local_lock_internal.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h index bf2bf40d7b18..8d5ac16a9b17 100644 --- a/include/linux/local_lock_internal.h +++ b/include/linux/local_lock_internal.h @@ -102,11 +102,11 @@ do { \ l = (local_lock_t *)this_cpu_ptr(lock); \ tl = (local_trylock_t *)l; \ _Generic((lock), \ - local_trylock_t *: ({ \ + __percpu local_trylock_t *: ({ \ lockdep_assert(tl->acquired == 0); \ WRITE_ONCE(tl->acquired, 1); \ }), \ - default:(void)0); \ + __percpu local_lock_t *: (void)0); \ local_lock_acquire(l); \ } while (0) @@ -171,11 +171,11 @@ do { \ tl = (local_trylock_t *)l; \ local_lock_release(l); \ _Generic((lock), \ - local_trylock_t *: ({ \ + __percpu local_trylock_t *: ({ \ lockdep_assert(tl->acquired == 1); \ WRITE_ONCE(tl->acquired, 0); \ }), \ - default:(void)0); \ + __percpu local_lock_t *: (void)0); \ } while (0) #define __local_unlock(lock) \ -- 2.49.0