kernel/linux.git/include/linux/srcutree.h, branch v6.18.21

srcu: Document __srcu_read_{,un}lock_fast() implicit RCU readers

2025-09-15T12:16:27+00:00

This commit documents the implicit RCU readers that are implied by the this_cpu_inc() and atomic_long_inc() operations in __srcu_read_lock_fast() and __srcu_read_unlock_fast(). While in the area, fix the documentation of the memory pairing of atomic_long_inc() in __srcu_read_lock_fast(). [ paulmck: Apply Joel Fernandes feedback. ] Signed-off-by: Paul E. McKenney Cc: Mathieu Desnoyers Cc: Steven Rostedt Cc: Sebastian Andrzej Siewior Cc:

srcu: Add srcu_read_lock_fast_notrace() and srcu_read_unlock_fast_notrace()

2025-08-14T22:25:48+00:00

This commit adds no-trace variants of the srcu_read_lock_fast() and srcu_read_unlock_fast() functions for tracing use. [ paulmck: Apply notrace feedback from Joel Fernandes, Steven Rostedt, and Mathieu Desnoyers. ] [ paulmck: Apply excess-notrace feedback from Boqun Feng. ] Link: https://lore.kernel.org/all/20250721162433.10454-1-paulmck@kernel.org Signed-off-by: Paul E. McKenney Reviewed-by: Joel Fernandes Cc: Mathieu Desnoyers Cc: Steven Rostedt Cc: Sebastian Andrzej Siewior Cc:

srcu: Move rcu_is_watching() checks to srcu_read_{,un}lock_fast()

2025-08-14T22:25:48+00:00

The rcu_is_watching() warnings are currently in the SRCU-tree implementations of __srcu_read_lock_fast() and __srcu_read_unlock_fast(). However, this makes it difficult to create _notrace variants of srcu_read_lock_fast() and srcu_read_unlock_fast(). This commit therefore moves these checks to srcu_read_lock_fast(), srcu_read_unlock_fast(), srcu_down_read_fast(), and srcu_up_read_fast(). Signed-off-by: Paul E. McKenney Reviewed-by: Joel Fernandes Cc: Mathieu Desnoyers Cc: Steven Rostedt Cc: Sebastian Andrzej Siewior Cc:

srcu: Remove SRCU-lite implementation

2025-07-16T04:21:08+00:00

This commit removes the SRCU-lite implementation, which has been replaced by SRCU-fast. Both SRCU-lite and SRCU-fast provide faster readers by dropping the smp_mb() call from their lock and unlock primitives, but incur a pair of added RCU grace periods during the SRCU grace period. There is a trivial mapping from the SRCU-lite API to that of SRCU-fast, so there should be no transition issues. [ paulmck: Apply Christoph Hellwig feedback. ] Signed-off-by: "Paul E. McKenney" Signed-off-by: Neeraj Upadhyay (AMD)

srcu: Make SRCU-fast also be NMI-safe

2025-02-05T15:12:06+00:00

BPF uses rcu_read_lock_trace() in NMI context, so srcu_read_lock_fast() must be NMI-safe if it is to have any chance of addressing RCU Tasks Trace use cases. This commit therefore causes srcu_read_lock_fast() and srcu_read_unlock_fast() to use atomic_long_inc() instead of this_cpu_inc() on architectures that support NMIs but do not have NMI-safe implementations of this_cpu_inc(). Note that both x86 and arm64 have NMI-safe implementations of this_cpu_inc(), and thus do not pay the performance penalty inherent in atomic_inc_long(). It is tempting to use this trick to fold srcu_read_lock_nmisafe() into srcu_read_lock(), but this would need careful thought, review, and performance analysis. Though those smp_mb() calls might well make performance a non-issue. Reported-by: Alexei Starovoitov Signed-off-by: Paul E. McKenney Signed-off-by: Boqun Feng

srcu: Add SRCU-fast readers

2025-02-05T15:12:05+00:00

This commit adds srcu_read_{,un}lock_fast(), which is similar to srcu_read_{,un}lock_lite(), but avoids the array-indexing and pointer-following overhead. On a microbenchmark featuring tight loops around empty readers, this results in about a 20% speedup compared to RCU Tasks Trace on my x86 laptop. Please note that SRCU-fast has drawbacks compared to RCU Tasks Trace, including: o Lack of CPU stall warnings. o SRCU-fast readers permitted only where rcu_is_watching(). o A pointer-sized return value from srcu_read_lock_fast() must be passed to the corresponding srcu_read_unlock_fast(). o In the absence of readers, a synchronize_srcu() having _fast() readers will incur the latency of at least two normal RCU grace periods. o RCU Tasks Trace priority boosting could be easily added. Boosting SRCU readers is more difficult. SRCU-fast also has a drawback compared to SRCU-lite, namely that the return value from srcu_read_lock_fast()-fast is a 64-bit pointer and that from srcu_read_lock_lite() is only a 32-bit int. [ paulmck: Apply feedback from Akira Yokosawa. ] Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Move SRCU Tree/Tiny definitions from srcu.h

2025-02-05T15:12:05+00:00

There are a couple of definitions under "#ifdef CONFIG_TINY_SRCU" in include/linux/srcu.h. There is no point in them being there, so this commit moves them to include/linux/srcutiny.h and include/linux/srcutree.c, thus eliminating that #ifdef. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Pull integer-to-pointer conversion into __srcu_ctr_to_ptr()

2025-02-05T15:12:05+00:00

This commit abstracts the srcu_read_unlock*() integer-to-pointer conversion into a new __srcu_ctr_to_ptr(). This will be used in rcutorture for testing an srcu_read_unlock_fast() that avoids array-indexing overhead by taking a pointer rather than an integer. [ paulmck: Apply kernel test robot feedback. ] Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Pull pointer-to-integer conversion into __srcu_ptr_to_ctr()

2025-02-05T15:12:05+00:00

This commit abstracts the srcu_read_lock*() pointer-to-integer conversion into a new __srcu_ptr_to_ctr(). This will be used in rcutorture for testing an srcu_read_lock_fast() that returns a pointer rather than an integer. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng

srcu: Rename srcu_check_read_flavor_lite() to srcu_check_read_flavor_force()

2025-02-05T15:12:05+00:00

This commit renames the srcu_check_read_flavor_lite() function to srcu_check_read_flavor_force() and adds a read_flavor argument in order to support an srcu_read_lock_fast() variant that is to avoid array indexing in both the lock and unlock primitives. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Boqun Feng