From 365f34483be33a9d0151c06ac39627d7927210d9 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Oct 2024 09:11:00 -0700 Subject: srcu: Renaming in preparation for additional reader flavor Currently, there are only two flavors of readers, normal and NMI-safe. A number of fields, functions, and types reflect this restriction. This renaming-only commit prepares for the addition of light-weight (as in memory-barrier-free) readers. OK, OK, there is also a drive-by white-space fixeup! Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 21 ++++++++++----------- 1 file changed, 10 insertions(+), 11 deletions(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 835bbb2d1f88..06728ef6f32a 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -181,10 +181,9 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp) #define SRCU_NMI_SAFE 0x2 #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU) -void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe); +void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor); #else -static inline void srcu_check_nmi_safety(struct srcu_struct *ssp, - bool nmi_safe) { } +static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) { } #endif @@ -245,7 +244,7 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) { int retval; - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); retval = __srcu_read_lock(ssp); srcu_lock_acquire(&ssp->dep_map); return retval; @@ -262,7 +261,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp { int retval; - srcu_check_nmi_safety(ssp, true); + srcu_check_read_flavor(ssp, true); retval = __srcu_read_lock_nmisafe(ssp); rcu_try_lock_acquire(&ssp->dep_map); return retval; @@ -274,7 +273,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp) { int retval; - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); retval = __srcu_read_lock(ssp); return retval; } @@ -303,7 +302,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp) static inline int srcu_down_read(struct srcu_struct *ssp) __acquires(ssp) { WARN_ON_ONCE(in_nmi()); - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); return __srcu_read_lock(ssp); } @@ -318,7 +317,7 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp) { WARN_ON_ONCE(idx & ~0x1); - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); srcu_lock_release(&ssp->dep_map); __srcu_read_unlock(ssp, idx); } @@ -334,7 +333,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) __releases(ssp) { WARN_ON_ONCE(idx & ~0x1); - srcu_check_nmi_safety(ssp, true); + srcu_check_read_flavor(ssp, true); rcu_lock_release(&ssp->dep_map); __srcu_read_unlock_nmisafe(ssp, idx); } @@ -343,7 +342,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) static inline notrace void srcu_read_unlock_notrace(struct srcu_struct *ssp, int idx) __releases(ssp) { - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); __srcu_read_unlock(ssp, idx); } @@ -360,7 +359,7 @@ static inline void srcu_up_read(struct srcu_struct *ssp, int idx) { WARN_ON_ONCE(idx & ~0x1); WARN_ON_ONCE(in_nmi()); - srcu_check_nmi_safety(ssp, false); + srcu_check_read_flavor(ssp, false); __srcu_read_unlock(ssp, idx); } -- cgit v1.2.3 From c071b8e5351453c2cb2d12f425d928f3a24ed2d3 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Oct 2024 09:11:03 -0700 Subject: srcu: Improve srcu_read_lock{,_nmisafe}() comments This commit adds some additional usage constraints to the kernel-doc headers of srcu_read_lock() and srcu_read_lock_nmi_safe(). Suggested-by: Andrii Nakryiko Signed-off-by: Paul E. McKenney Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 06728ef6f32a..46fd06b212ba 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -235,10 +235,13 @@ static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flav * a mutex that is held elsewhere while calling synchronize_srcu() or * synchronize_srcu_expedited(). * - * Note that srcu_read_lock() and the matching srcu_read_unlock() must - * occur in the same context, for example, it is illegal to invoke - * srcu_read_unlock() in an irq handler if the matching srcu_read_lock() - * was invoked in process context. + * The return value from srcu_read_lock() must be passed unaltered + * to the matching srcu_read_unlock(). Note that srcu_read_lock() and + * the matching srcu_read_unlock() must occur in the same context, for + * example, it is illegal to invoke srcu_read_unlock() in an irq handler + * if the matching srcu_read_lock() was invoked in process context. Or, + * for that matter to invoke srcu_read_unlock() from one task and the + * matching srcu_read_lock() from another. */ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) { @@ -256,6 +259,10 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) * * Enter an SRCU read-side critical section, but in an NMI-safe manner. * See srcu_read_lock() for more information. + * + * If srcu_read_lock_nmisafe() is ever used on an srcu_struct structure, + * then none of the other flavors may be used, whether before, during, + * or after. */ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp) { -- cgit v1.2.3 From 05829be27fe6f64e0675dc3be3a12d43b52492e1 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Oct 2024 09:11:04 -0700 Subject: srcu: Create CPP macros for normal and NMI-safe SRCU readers This commit creates SRCU_READ_FLAVOR_NORMAL and SRCU_READ_FLAVOR_NMI C-preprocessor macros for srcu_read_lock() and srcu_read_lock_nmisafe(), respectively. These replace the old true/false values that were previously passed to srcu_check_read_flavor(). In addition, the srcu_check_read_flavor() function itself requires a bit of rework to handle bitmasks instead of true/false values. Signed-off-by: Paul E. McKenney Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Signed-off-by: Paul E. McKenney Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 20 ++++++++------------ include/linux/srcutree.h | 4 ++++ kernel/rcu/srcutree.c | 21 +++++++++++---------- 3 files changed, 23 insertions(+), 22 deletions(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 46fd06b212ba..84daaa33ea0a 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -176,10 +176,6 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp) #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ -#define SRCU_NMI_UNKNOWN 0x0 -#define SRCU_NMI_UNSAFE 0x1 -#define SRCU_NMI_SAFE 0x2 - #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU) void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor); #else @@ -247,7 +243,7 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) { int retval; - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); retval = __srcu_read_lock(ssp); srcu_lock_acquire(&ssp->dep_map); return retval; @@ -268,7 +264,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp { int retval; - srcu_check_read_flavor(ssp, true); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NMI); retval = __srcu_read_lock_nmisafe(ssp); rcu_try_lock_acquire(&ssp->dep_map); return retval; @@ -280,7 +276,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp) { int retval; - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); retval = __srcu_read_lock(ssp); return retval; } @@ -309,7 +305,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp) static inline int srcu_down_read(struct srcu_struct *ssp) __acquires(ssp) { WARN_ON_ONCE(in_nmi()); - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); return __srcu_read_lock(ssp); } @@ -324,7 +320,7 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp) { WARN_ON_ONCE(idx & ~0x1); - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); srcu_lock_release(&ssp->dep_map); __srcu_read_unlock(ssp, idx); } @@ -340,7 +336,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) __releases(ssp) { WARN_ON_ONCE(idx & ~0x1); - srcu_check_read_flavor(ssp, true); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NMI); rcu_lock_release(&ssp->dep_map); __srcu_read_unlock_nmisafe(ssp, idx); } @@ -349,7 +345,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx) static inline notrace void srcu_read_unlock_notrace(struct srcu_struct *ssp, int idx) __releases(ssp) { - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); __srcu_read_unlock(ssp, idx); } @@ -366,7 +362,7 @@ static inline void srcu_up_read(struct srcu_struct *ssp, int idx) { WARN_ON_ONCE(idx & ~0x1); WARN_ON_ONCE(in_nmi()); - srcu_check_read_flavor(ssp, false); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL); __srcu_read_unlock(ssp, idx); } diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index ab7d8d215b84..79ad809c7f03 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -43,6 +43,10 @@ struct srcu_data { struct srcu_struct *ssp; }; +/* Values for ->srcu_reader_flavor. */ +#define SRCU_READ_FLAVOR_NORMAL 0x1 // srcu_read_lock(). +#define SRCU_READ_FLAVOR_NMI 0x2 // srcu_read_lock_nmisafe(). + /* * Node in SRCU combining tree, similar in function to rcu_data. */ diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index b85da944d794..602b4b8c4b89 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -463,7 +463,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx) mask = mask | READ_ONCE(sdp->srcu_reader_flavor); } WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)), - "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp); + "Mixed reader flavors for srcu_struct at %ps.\n", ssp); return sum; } @@ -703,20 +703,21 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct); */ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) { - int reader_flavor_mask = 1 << read_flavor; - int old_reader_flavor_mask; + int old_read_flavor; struct srcu_data *sdp; - /* NMI-unsafe use in NMI is a bad sign */ - WARN_ON_ONCE(!read_flavor && in_nmi()); + /* NMI-unsafe use in NMI is a bad sign, as is multi-bit read_flavor values. */ + WARN_ON_ONCE((read_flavor != SRCU_READ_FLAVOR_NMI) && in_nmi()); + WARN_ON_ONCE(read_flavor & (read_flavor - 1)); + sdp = raw_cpu_ptr(ssp->sda); - old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor); - if (!old_reader_flavor_mask) { - old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask); - if (!old_reader_flavor_mask) + old_read_flavor = READ_ONCE(sdp->srcu_reader_flavor); + if (!old_read_flavor) { + old_read_flavor = cmpxchg(&sdp->srcu_reader_flavor, 0, read_flavor); + if (!old_read_flavor) return; } - WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask); + WARN_ONCE(old_read_flavor != read_flavor, "CPU %d old state %d new state %d\n", sdp->cpu, old_read_flavor, read_flavor); } EXPORT_SYMBOL_GPL(srcu_check_read_flavor); #endif /* CONFIG_PROVE_RCU */ -- cgit v1.2.3 From 6364dd8191d27230176ac4b1b4daaecaf4807399 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Oct 2024 09:11:05 -0700 Subject: srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite() This patch adds srcu_read_lock_lite() and srcu_read_unlock_lite(), which dispense with the read-side smp_mb() but also are restricted to code regions that RCU is watching. If a given srcu_struct structure uses srcu_read_lock_lite() and srcu_read_unlock_lite(), it is not permitted to use any other SRCU read-side marker, before, during, or after. Another price of light-weight readers is heavier weight grace periods. Such readers mean that SRCU grace periods on srcu_struct structures used by light-weight readers will incur at least two calls to synchronize_rcu(). In addition, normal SRCU grace periods for light-weight-reader srcu_struct structures never auto-expedite. Note that expedited SRCU grace periods for light-weight-reader srcu_struct structures still invoke synchronize_rcu(), not synchronize_srcu_expedited(). Something about wishing to keep the IPIs down to a dull roar. The srcu_read_lock_lite() and srcu_read_unlock_lite() functions may not (repeat, *not*) be used from NMI handlers, but if this is needed, an additional flavor of SRCU reader can be added by some future commit. [ paulmck: Apply Alexei Starovoitov expediting feedback. ] [ paulmck: Apply kernel test robot feedback. ] Signed-off-by: Paul E. McKenney Tested-by: kernel test robot Cc: Alexei Starovoitov Cc: Andrii Nakryiko Cc: Peter Zijlstra Cc: Kent Overstreet Cc: Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 51 +++++++++++++++++++++++++++++- include/linux/srcutree.h | 1 + kernel/rcu/srcutree.c | 82 +++++++++++++++++++++++++++++++++++++++++------- 3 files changed, 122 insertions(+), 12 deletions(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 84daaa33ea0a..4ba96e2cfa40 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -56,6 +56,13 @@ void call_srcu(struct srcu_struct *ssp, struct rcu_head *head, void cleanup_srcu_struct(struct srcu_struct *ssp); int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp); void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp); +#ifdef CONFIG_TINY_SRCU +#define __srcu_read_lock_lite __srcu_read_lock +#define __srcu_read_unlock_lite __srcu_read_unlock +#else // #ifdef CONFIG_TINY_SRCU +int __srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp); +void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) __releases(ssp); +#endif // #else // #ifdef CONFIG_TINY_SRCU void synchronize_srcu(struct srcu_struct *ssp); #define SRCU_GET_STATE_COMPLETED 0x1 @@ -179,7 +186,7 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp) #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU) void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor); #else -static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) { } +#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0) #endif @@ -249,6 +256,32 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) return retval; } +/** + * srcu_read_lock_lite - register a new reader for an SRCU-protected structure. + * @ssp: srcu_struct in which to register the new reader. + * + * Enter an SRCU read-side critical section, but for a light-weight + * smp_mb()-free reader. See srcu_read_lock() for more information. + * + * If srcu_read_lock_lite() is ever used on an srcu_struct structure, + * then none of the other flavors may be used, whether before, during, + * or after. Note that grace-period auto-expediting is disabled for _lite + * srcu_struct structures because auto-expedited grace periods invoke + * synchronize_rcu_expedited(), IPIs and all. + * + * Note that srcu_read_lock_lite() can be invoked only from those contexts + * where RCU is watching. Otherwise, lockdep will complain. + */ +static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp) +{ + int retval; + + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE); + retval = __srcu_read_lock_lite(ssp); + rcu_try_lock_acquire(&ssp->dep_map); + return retval; +} + /** * srcu_read_lock_nmisafe - register a new reader for an SRCU-protected structure. * @ssp: srcu_struct in which to register the new reader. @@ -325,6 +358,22 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx) __srcu_read_unlock(ssp, idx); } +/** + * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure. + * @ssp: srcu_struct in which to unregister the old reader. + * @idx: return value from corresponding srcu_read_lock(). + * + * Exit a light-weight SRCU read-side critical section. + */ +static inline void srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) + __releases(ssp) +{ + WARN_ON_ONCE(idx & ~0x1); + srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE); + srcu_lock_release(&ssp->dep_map); + __srcu_read_unlock(ssp, idx); +} + /** * srcu_read_unlock_nmisafe - unregister a old reader from an SRCU-protected structure. * @ssp: srcu_struct in which to unregister the old reader. diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index 79ad809c7f03..8074138cbd62 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -46,6 +46,7 @@ struct srcu_data { /* Values for ->srcu_reader_flavor. */ #define SRCU_READ_FLAVOR_NORMAL 0x1 // srcu_read_lock(). #define SRCU_READ_FLAVOR_NMI 0x2 // srcu_read_lock_nmisafe(). +#define SRCU_READ_FLAVOR_LITE 0x4 // srcu_read_lock_lite(). /* * Node in SRCU combining tree, similar in function to rcu_data. diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 602b4b8c4b89..8632a3caeb33 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -429,20 +429,29 @@ static bool srcu_gp_is_expedited(struct srcu_struct *ssp) } /* - * Returns approximate total of the readers' ->srcu_lock_count[] values - * for the rank of per-CPU counters specified by idx. + * Computes approximate total of the readers' ->srcu_lock_count[] values + * for the rank of per-CPU counters specified by idx, and returns true if + * the caller did the proper barrier (gp), and if the count of the locks + * matches that of the unlocks passed in. */ -static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx) +static bool srcu_readers_lock_idx(struct srcu_struct *ssp, int idx, bool gp, unsigned long unlocks) { int cpu; + unsigned long mask = 0; unsigned long sum = 0; for_each_possible_cpu(cpu) { struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu); sum += atomic_long_read(&sdp->srcu_lock_count[idx]); + if (IS_ENABLED(CONFIG_PROVE_RCU)) + mask = mask | READ_ONCE(sdp->srcu_reader_flavor); } - return sum; + WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)), + "Mixed reader flavors for srcu_struct at %ps.\n", ssp); + if (mask & SRCU_READ_FLAVOR_LITE && !gp) + return false; + return sum == unlocks; } /* @@ -473,6 +482,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx) */ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx) { + bool did_gp = !!(raw_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE); unsigned long unlocks; unlocks = srcu_readers_unlock_idx(ssp, idx); @@ -482,13 +492,16 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx) * unlock is counted. Needs to be a smp_mb() as the read side may * contain a read from a variable that is written to before the * synchronize_srcu() in the write side. In this case smp_mb()s - * A and B act like the store buffering pattern. + * A and B (or X and Y) act like the store buffering pattern. * - * This smp_mb() also pairs with smp_mb() C to prevent accesses - * after the synchronize_srcu() from being executed before the - * grace period ends. + * This smp_mb() also pairs with smp_mb() C (or, in the case of X, + * Z) to prevent accesses after the synchronize_srcu() from being + * executed before the grace period ends. */ - smp_mb(); /* A */ + if (!did_gp) + smp_mb(); /* A */ + else + synchronize_rcu(); /* X */ /* * If the locks are the same as the unlocks, then there must have @@ -546,7 +559,7 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx) * which are unlikely to be configured with an address space fully * populated with memory, at least not anytime soon. */ - return srcu_readers_lock_idx(ssp, idx) == unlocks; + return srcu_readers_lock_idx(ssp, idx, did_gp, unlocks); } /** @@ -750,6 +763,47 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx) } EXPORT_SYMBOL_GPL(__srcu_read_unlock); +/* + * Counts the new reader in the appropriate per-CPU element of the + * srcu_struct. Returns an index that must be passed to the matching + * srcu_read_unlock_lite(). + * + * Note that this_cpu_inc() is an RCU read-side critical section either + * because it disables interrupts, because it is a single instruction, + * or because it is a read-modify-write atomic operation, depending on + * the whims of the architecture. + */ +int __srcu_read_lock_lite(struct srcu_struct *ssp) +{ + int idx; + + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite()."); + idx = READ_ONCE(ssp->srcu_idx) & 0x1; + this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter); /* Y */ + barrier(); /* Avoid leaking the critical section. */ + return idx; +} +EXPORT_SYMBOL_GPL(__srcu_read_lock_lite); + +/* + * Removes the count for the old reader from the appropriate + * per-CPU element of the srcu_struct. Note that this may well be a + * different CPU than that which was incremented by the corresponding + * srcu_read_lock_lite(), but it must be within the same task. + * + * Note that this_cpu_inc() is an RCU read-side critical section either + * because it disables interrupts, because it is a single instruction, + * or because it is a read-modify-write atomic operation, depending on + * the whims of the architecture. + */ +void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) +{ + barrier(); /* Avoid leaking the critical section. */ + this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter); /* Z */ + RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite()."); +} +EXPORT_SYMBOL_GPL(__srcu_read_unlock_lite); + #ifdef CONFIG_NEED_SRCU_NMI_SAFE /* @@ -1134,6 +1188,8 @@ static void srcu_flip(struct srcu_struct *ssp) * it stays until either (1) Compilers learn about this sort of * control dependency or (2) Some production workload running on * a production system is unduly delayed by this slowpath smp_mb(). + * Except for _lite() readers, where it is inoperative, which + * means that it is a good thing that it is redundant. */ smp_mb(); /* E */ /* Pairs with B and C. */ @@ -1152,7 +1208,8 @@ static void srcu_flip(struct srcu_struct *ssp) /* * If SRCU is likely idle, in other words, the next SRCU grace period - * should be expedited, return true, otherwise return false. + * should be expedited, return true, otherwise return false. Except that + * in the presence of _lite() readers, always return false. * * Note that it is OK for several current from-idle requests for a new * grace period from idle to specify expediting because they will all end @@ -1181,6 +1238,9 @@ static bool srcu_should_expedite(struct srcu_struct *ssp) unsigned long tlast; check_init_srcu_struct(ssp); + /* If _lite() readers, don't do unsolicited expediting. */ + if (this_cpu_read(ssp->sda->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE) + return false; /* If the local srcu_data structure has callbacks, not idle. */ sdp = raw_cpu_ptr(ssp->sda); spin_lock_irqsave_rcu_node(sdp, flags); -- cgit v1.2.3 From 768b1f87098a4a586353898b074989808b1b27ad Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 15 Oct 2024 09:11:12 -0700 Subject: srcu: Improve srcu_read_lock_lite() kernel-doc comment Where RCU is watching is where it is OK to invoke rcu_read_lock(). Reported-by: Andrii Nakryiko Signed-off-by: Paul E. McKenney Acked-by: Andrii Nakryiko Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 4ba96e2cfa40..bab1dae3f69e 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -270,7 +270,8 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp) * synchronize_rcu_expedited(), IPIs and all. * * Note that srcu_read_lock_lite() can be invoked only from those contexts - * where RCU is watching. Otherwise, lockdep will complain. + * where RCU is watching, that is, from contexts where it would be legal + * to invoke rcu_read_lock(). Otherwise, lockdep will complain. */ static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp) { -- cgit v1.2.3 From 174dd22a781b97cb355b1037f0931436a254d3be Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 11 Nov 2024 09:38:26 -0800 Subject: srcu: Remove smp_mb() from srcu_read_unlock_lite() The srcu_read_unlock_lite() function invokes __srcu_read_unlock() instead of __srcu_read_unlock_lite(), which means that it is doing an unnecessary smp_mb(). This is harmless other than the performance degradation. This commit therefore switches to __srcu_read_unlock_lite(). Reported-by: Neeraj Upadhyay Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@amd.com/ Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()") Signed-off-by: Paul E. McKenney Cc: Frederic Weisbecker Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index bab1dae3f69e..56f83237de4d 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -372,7 +372,7 @@ static inline void srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) WARN_ON_ONCE(idx & ~0x1); srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE); srcu_lock_release(&ssp->dep_map); - __srcu_read_unlock(ssp, idx); + __srcu_read_unlock_lite(ssp, idx); } /** -- cgit v1.2.3 From 9407f5c3ec10c155aae61fc4496a7c17a96907b4 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 11 Nov 2024 16:28:51 -0800 Subject: srcu: Unconditionally record srcu_read_lock_lite() in ->srcu_reader_flavor Currently, srcu_read_lock_lite() uses the SRCU_READ_FLAVOR_LITE bit in ->srcu_reader_flavor to communicate to the grace-period processing in srcu_readers_active_idx_check() that the smp_mb() must be replaced by a synchronize_rcu(). Unfortunately, ->srcu_reader_flavor is not updated unless the kernel is built with CONFIG_PROVE_RCU=y. Therefore in all kernels built with CONFIG_PROVE_RCU=n, srcu_readers_active_idx_check() incorrectly uses smp_mb() instead of synchronize_rcu() for srcu_struct structures whose readers use srcu_read_lock_lite(). This commit therefore causes Tree SRCU srcu_read_lock_lite() to unconditionally update ->srcu_reader_flavor so that srcu_readers_active_idx_check() can make the correct choice. Reported-by: Neeraj Upadhyay Closes: https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c57e9@amd.com/ Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and srcu_read_unlock_lite()") Signed-off-by: Paul E. McKenney Cc: Frederic Weisbecker Reviewed-by: Neeraj Upadhyay Signed-off-by: Frederic Weisbecker --- include/linux/srcu.h | 8 +------- include/linux/srcutiny.h | 3 +++ include/linux/srcutree.h | 21 +++++++++++++++++++++ kernel/rcu/srcutree.c | 6 ++---- 4 files changed, 27 insertions(+), 11 deletions(-) (limited to 'include/linux/srcu.h') diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 56f83237de4d..08339eb8a01c 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -183,12 +183,6 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp) #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ -#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU) -void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor); -#else -#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0) -#endif - /** * srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing @@ -277,7 +271,7 @@ static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp) { int retval; - srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE); + srcu_check_read_flavor_lite(ssp); retval = __srcu_read_lock_lite(ssp); rcu_try_lock_acquire(&ssp->dep_map); return retval; diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h index 4d96bbdb45f0..1321da803274 100644 --- a/include/linux/srcutiny.h +++ b/include/linux/srcutiny.h @@ -81,6 +81,9 @@ static inline void srcu_barrier(struct srcu_struct *ssp) synchronize_srcu(ssp); } +#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0) +#define srcu_check_read_flavor_lite(ssp) do { } while (0) + /* Defined here to avoid size increase for non-torture kernels. */ static inline void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf) diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index 778eb61542e1..490aeecc6bb4 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -248,4 +248,25 @@ static inline void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite()."); } +void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor); + +// Record _lite() usage even for CONFIG_PROVE_RCU=n kernels. +static inline void srcu_check_read_flavor_lite(struct srcu_struct *ssp) +{ + struct srcu_data *sdp = raw_cpu_ptr(ssp->sda); + + if (likely(READ_ONCE(sdp->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE)) + return; + + // Note that the cmpxchg() in srcu_check_read_flavor() is fully ordered. + __srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE); +} + +// Record non-_lite() usage only for CONFIG_PROVE_RCU=y kernels. +static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) +{ + if (IS_ENABLED(CONFIG_PROVE_RCU)) + __srcu_check_read_flavor(ssp, read_flavor); +} + #endif diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index a381b553cdca..5e2e53464794 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -712,11 +712,10 @@ void cleanup_srcu_struct(struct srcu_struct *ssp) } EXPORT_SYMBOL_GPL(cleanup_srcu_struct); -#ifdef CONFIG_PROVE_RCU /* * Check for consistent reader flavor. */ -void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) +void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) { int old_read_flavor; struct srcu_data *sdp; @@ -734,8 +733,7 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor) } WARN_ONCE(old_read_flavor != read_flavor, "CPU %d old state %d new state %d\n", sdp->cpu, old_read_flavor, read_flavor); } -EXPORT_SYMBOL_GPL(srcu_check_read_flavor); -#endif /* CONFIG_PROVE_RCU */ +EXPORT_SYMBOL_GPL(__srcu_check_read_flavor); /* * Counts the new reader in the appropriate per-CPU element of the -- cgit v1.2.3