Re: [PATCH rcu 05/12] srcu: Standardize srcu_data pointers to "sdp" and similar

From: Paul E. McKenney
Date: Mon Oct 14 2024 - 20:29:13 EST

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On Mon, Oct 14, 2024 at 09:49:52AM -0700, Paul E. McKenney wrote:
> On Mon, Oct 14, 2024 at 02:45:50PM +0530, Neeraj Upadhyay wrote:
> > On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
> > > This commit changes a few "cpuc" variables to "sdp" to align wiht usage
> > > elsewhere.
> > >
> >
> > s/wiht/with/
>
> Good eyes!

And fixed.

> > This commit is doing a lot more than renaming "cpuc".
>
> Indeed, it does look like I forgot to commit between two changes.
>
> It looks like this commit log goes with the changes to the
> functions srcu_readers_lock_idx(), srcu_readers_unlock_idx(), and
> srcu_readers_active(). With the exception of the change from "NMI-safe"
> to "reader flavors in the WARN_ONCE() string in srcu_readers_unlock_idx().
>
> How would you suggest that I split up the non-s/cpuc/sdp/ changes?

As a first step, I split it three ways, as you can see on the updated "dev"
branch in the -rcu tree.

Thoughts?

Thanx, Paul

> > - Neeraj
> >
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> > > Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > > Cc: <bpf@xxxxxxxxxxxxxxx>
> > > ---
> > > include/linux/srcu.h | 35 ++++++++++++++++++----------------
> > > include/linux/srcutree.h | 4 ++++
> > > kernel/rcu/srcutree.c | 41 ++++++++++++++++++++--------------------
> > > 3 files changed, 44 insertions(+), 36 deletions(-)
> > >
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index 06728ef6f32a4..84daaa33ea0ab 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
> > > @@ -235,16 +231,19 @@ 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)
> > > {
> > > 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;
> > > @@ -256,12 +255,16 @@ 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)
> > > {
> > > 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;
> > > @@ -273,7 +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;
> > > }
> > > @@ -302,7 +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);
> > > }
> > >
> > > @@ -317,7 +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);
> > > }
> > > @@ -333,7 +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);
> > > }
> > > @@ -342,7 +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);
> > > }
> > >
> > > @@ -359,7 +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 ab7d8d215b84b..79ad809c7f035 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 abe55777c4335..4c51be484b48a 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -438,9 +438,9 @@ static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
> > > unsigned long sum = 0;
> > >
> > > for_each_possible_cpu(cpu) {
> > > - struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
> > > + struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> > >
> > > - sum += atomic_long_read(&cpuc->srcu_lock_count[idx]);
> > > + sum += atomic_long_read(&sdp->srcu_lock_count[idx]);
> > > }
> > > return sum;
> > > }
> > > @@ -456,14 +456,14 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
> > > unsigned long sum = 0;
> > >
> > > for_each_possible_cpu(cpu) {
> > > - struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
> > > + struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> > >
> > > - sum += atomic_long_read(&cpuc->srcu_unlock_count[idx]);
> > > + sum += atomic_long_read(&sdp->srcu_unlock_count[idx]);
> > > if (IS_ENABLED(CONFIG_PROVE_RCU))
> > > - mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
> > > + 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;
> > > }
> > >
> > > @@ -564,12 +564,12 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > > unsigned long sum = 0;
> > >
> > > for_each_possible_cpu(cpu) {
> > > - struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
> > > + struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> > >
> > > - sum += atomic_long_read(&cpuc->srcu_lock_count[0]);
> > > - sum += atomic_long_read(&cpuc->srcu_lock_count[1]);
> > > - sum -= atomic_long_read(&cpuc->srcu_unlock_count[0]);
> > > - sum -= atomic_long_read(&cpuc->srcu_unlock_count[1]);
> > > + sum += atomic_long_read(&sdp->srcu_lock_count[0]);
> > > + sum += atomic_long_read(&sdp->srcu_lock_count[1]);
> > > + sum -= atomic_long_read(&sdp->srcu_unlock_count[0]);
> > > + sum -= atomic_long_read(&sdp->srcu_unlock_count[1]);
> > > }
> > > 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 */
> >

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