Commit dba4761a authored Nov 22, 2024 by Suren Baghdasaryan Committed by Andrew Morton Jan 13, 2025

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.

Link: https://lkml.kernel.org/r/20241122174416.1367052-1-surenb@google.com


Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Sourav Panda <souravpanda@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent a474d843

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