From: Suren Baghdasaryan Date: Fri, 22 Nov 2024 17:44:14 +0000 (-0800) Subject: seqlock: add raw_seqcount_try_begin X-Git-Tag: block-6.14-20240131~52^2~16 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=96450ead16527cbef559b5bd046182e731228f95;p=linux-block.git 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: Peter Zijlstra Signed-off-by: Suren Baghdasaryan Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: David Hildenbrand Reviewed-by: Liam R. Howlett Link: https://lkml.kernel.org/r/20241122174416.1367052-1-surenb@google.com --- diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 5298765d6ca4..22c2c48b4265 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -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