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seqlock: add raw_seqcount_try_begin

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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@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-1-surenb@google.com
This commit is contained in:
Suren Baghdasaryan 2024-11-22 09:44:14 -08:00 committed by Peter Zijlstra
parent b4943b8bfc
commit 96450ead16
1 changed files with 22 additions and 0 deletions
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22
include/linux/seqlock.h
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@ -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