|author||Ahmed S. Darwish <email@example.com>||2020-08-27 13:40:39 +0200|
|committer||Peter Zijlstra <firstname.lastname@example.org>||2020-09-10 11:19:28 +0200|
seqlock: Introduce seqcount_latch_t
Latch sequence counters are a multiversion concurrency control mechanism where the seqcount_t counter even/odd value is used to switch between two copies of protected data. This allows the seqcount_t read path to safely interrupt its write side critical section (e.g. from NMIs). Initially, latch sequence counters were implemented as a single write function above plain seqcount_t: raw_write_seqcount_latch(). The read side was expected to use plain seqcount_t raw_read_seqcount(). A specialized latch read function, raw_read_seqcount_latch(), was later added. It became the standardized way for latch read paths. Due to the dependent load, it has one read memory barrier less than the plain seqcount_t raw_read_seqcount() API. Only raw_write_seqcount_latch() and raw_read_seqcount_latch() should be used with latch sequence counters. Having *unique* read and write path APIs means that latch sequence counters are actually a data type of their own -- just inappropriately overloading plain seqcount_t. Introduce seqcount_latch_t. This adds type-safety and ensures that only the correct latch-safe APIs are to be used. Not to break bisection, let the latch APIs also accept plain seqcount_t or seqcount_raw_spinlock_t. After converting all call sites to seqcount_latch_t, only that new data type will be allowed. References: 9b0fd802e8c0 ("seqcount: Add raw_write_seqcount_latch()") References: 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()") References: aadd6e5caaac ("time/sched_clock: Use raw_read_seqcount_latch()") Signed-off-by: Ahmed S. Darwish <email@example.com> Signed-off-by: Peter Zijlstra (Intel) <firstname.lastname@example.org> Link: https://email@example.com
Diffstat (limited to 'Documentation/locking')
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
index 62c5ad98c11c..a334b584f2b3 100644
@@ -139,6 +139,24 @@ with the associated LOCKTYPE lock acquired.
Read path: same as in :ref:`seqcount_t`.
+Latch sequence counters (``seqcount_latch_t``)
+Latch sequence counters are a multiversion concurrency control mechanism
+where the embedded seqcount_t counter even/odd value is used to switch
+between two copies of protected data. This allows the sequence counter
+read path to safely interrupt its own write side critical section.
+Use seqcount_latch_t when the write side sections cannot be protected
+from interruption by readers. This is typically the case when the read
+side can be invoked from NMI handlers.
+Check `raw_write_seqcount_latch()` for more information.
Sequential locks (``seqlock_t``)