Commit 96450ead authored by Suren Baghdasaryan's avatar Suren Baghdasaryan Committed by Peter Zijlstra
Browse files

seqlock: add raw_seqcount_try_begin 



Add raw_seqcount_try_begin() to opens a read critical section of the given
seqcount_t if the counter is even. This enables eliding the critical
section entirely if the counter is odd, instead of doing the speculation
knowing it will fail.

Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarSuren Baghdasaryan <surenb@google.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-1-surenb@google.com
parent b4943b8b

include/linux/seqlock.h

+22 −0
Original line number Diff line number Diff line
@@ -318,6 +318,28 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex) 
	__seq;								\

})



/**

 * raw_seqcount_try_begin() - begin a seqcount_t read critical section

 *                            w/o lockdep and w/o counter stabilization

 * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants

 *

 * Similar to raw_seqcount_begin(), except it enables eliding the critical

 * section entirely if odd, instead of doing the speculation knowing it will

 * fail.

 *

 * Useful when counter stabilization is more or less equivalent to taking

 * the lock and there is a slowpath that does that.

 *

 * If true, start will be set to the (even) sequence count read.

 *

 * Return: true when a read critical section is started.

 */

#define raw_seqcount_try_begin(s, start)				\

({									\

	start = raw_read_seqcount(s);					\

	!(start & 1);							\

})



/**

 * raw_seqcount_begin() - begin a seqcount_t read critical section w/o

 *                        lockdep and w/o counter stabilization