Commit 82efd569 authored by Vlastimil Babka's avatar Vlastimil Babka Committed by Linus Torvalds
Browse files

locking/local_lock: fix _Generic() matching of local_trylock_t 



Michael Larabel reported [1] a nginx performance regression in v6.15-rc3
and bisected it to commit 51339d99 ("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: 51339d99 ("locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type")
Reported-by: default avatarMichael Larabel <Michael@phoronix.com>
Closes: https://www.phoronix.com/review/linux-615-nginx-regression/2

 [1]
Signed-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 0251ddbf

include/linux/local_lock_internal.h

+4 −4
Original line number Diff line number Diff line
@@ -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)					\