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srcu: Remove SRCU-lite implementation

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This commit removes the SRCU-lite implementation, which has been replaced
by SRCU-fast.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" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
This commit is contained in:
Paul E. McKenney
2025-07-01 17:23:29 -07:00
committed by Neeraj Upadhyay (AMD)
parent 3aea745a2a
commit 623baa01d5
3 changed files with 2 additions and 86 deletions
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47
include/linux/srcu.h
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@@ -46,11 +46,11 @@ int init_srcu_struct(struct srcu_struct *ssp);
/* Values for SRCU Tree srcu_data ->srcu_reader_flavor, but also used by rcutorture. */
#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().
// 0x4 // SRCU-lite is no longer with us.
#define SRCU_READ_FLAVOR_FAST 0x8 // srcu_read_lock_fast().
#define SRCU_READ_FLAVOR_ALL (SRCU_READ_FLAVOR_NORMAL | SRCU_READ_FLAVOR_NMI | \
SRCU_READ_FLAVOR_FAST) // All of the above.
#define SRCU_READ_FLAVOR_SLOWGP (SRCU_READ_FLAVOR_LITE | SRCU_READ_FLAVOR_FAST)
#define SRCU_READ_FLAVOR_SLOWGP SRCU_READ_FLAVOR_FAST
// Flavors requiring synchronize_rcu()
// instead of smp_mb().
void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
@@ -299,33 +299,6 @@ static inline struct srcu_ctr __percpu *srcu_down_read_fast(struct srcu_struct *
return __srcu_read_lock_fast(ssp);
}
/**
* 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, 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)
{
int retval;
srcu_check_read_flavor_force(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.
@@ -434,22 +407,6 @@ static inline void srcu_up_read_fast(struct srcu_struct *ssp, struct srcu_ctr __
__srcu_read_unlock_fast(ssp, scp);
}
/**
* 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_lite().
*
* 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_lite(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.

3
include/linux/srcutiny.h
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@@ -93,9 +93,6 @@ static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_
__srcu_read_unlock(ssp, __srcu_ptr_to_ctr(ssp, scp));
}
#define __srcu_read_lock_lite __srcu_read_lock
#define __srcu_read_unlock_lite __srcu_read_unlock
static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
{
synchronize_srcu(ssp);

38
include/linux/srcutree.h
View File

@@ -278,44 +278,6 @@ static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_
RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
}
/*
* 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.
*/
static inline int __srcu_read_lock_lite(struct srcu_struct *ssp)
{
struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite().");
this_cpu_inc(scp->srcu_locks.counter); /* Y */
barrier(); /* Avoid leaking the critical section. */
return __srcu_ptr_to_ctr(ssp, scp);
}
/*
* 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.
*/
static inline void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
{
barrier(); /* Avoid leaking the critical section. */
this_cpu_inc(__srcu_ctr_to_ptr(ssp, idx)->srcu_unlocks.counter); /* Z */
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 reader usage even for CONFIG_PROVE_RCU=n kernels. This is