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 <peterz@xxxxxxxxxxxxx> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> --- Applies over Linus' ToT Changes since v2 [1] - Added SOB, per David Hildenbrand - Added SOB, per Liam Howlett [1] https://lore.kernel.org/all/20241121162826.987947-1-surenb@xxxxxxxxxx/ include/linux/seqlock.h | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) 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 base-commit: 28eb75e178d389d325f1666e422bc13bbbb9804c -- 2.47.0.371.ga323438b13-goog
