On 11/12/2024 7:01 AM, Paul E. McKenney wrote:
> If srcu_read_lock_lite() is used on a given srcu_struct structure, then
> the grace-period processing must to synchronize_rcu() instead of smp_mb()
s/to/do/
> between the scans of the ->srcu_unlock_count[] and ->srcu_lock_count[]
> counters. Currently, it does that by testing the SRCU_READ_FLAVOR_LITE
> bit of the ->srcu_reader_flavor mask, which works well. But only if
> the CPU running that srcu_struct structure's grace period has previously
> executed srcu_read_lock_lite(), which might not be the case, especially
> just after that srcu_struct structure has been created and initialized.
>
> This commit therefore updates the srcu_readers_unlock_idx() function
> to OR together the ->srcu_reader_flavor masks from all CPUs, and
> then make the srcu_readers_active_idx_check() function that test the
> SRCU_READ_FLAVOR_LITE bit in the resulting mask.
>
> Note that the srcu_readers_unlock_idx() function is already scanning all
> the CPUs to sum up the ->srcu_unlock_count[] fields and that this is on
> the grace-period slow path, hence no concerns about the small amount of
> extra work.
>
> Reported-by: Neeraj Upadhyay <neeraj.upadh...@amd.com>
> Closes:
> https://lore.kernel.org/all/d07e8f4a-d5ff-4c8e-8e61-50db285c5...@amd.com/
> Fixes: c0f08d6b5a61 ("srcu: Add srcu_read_lock_lite() and
> srcu_read_unlock_lite()")
> Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> Cc: Frederic Weisbecker <frede...@kernel.org>
> ---
> kernel/rcu/srcutree.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 70979f294768c..5991381b44383 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -458,7 +458,7 @@ static bool srcu_readers_lock_idx(struct srcu_struct
> *ssp, int idx, bool gp, uns
> * Returns approximate total of the readers' ->srcu_unlock_count[] values
> * for the rank of per-CPU counters specified by idx.
> */
> -static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int
> idx)
> +static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int
> idx, unsigned long *rdm)
> {
> int cpu;
> unsigned long mask = 0;
> @@ -468,11 +468,11 @@ static unsigned long srcu_readers_unlock_idx(struct
> srcu_struct *ssp, int idx)
> struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
>
> sum += atomic_long_read(&sdp->srcu_unlock_count[idx]);
> - if (IS_ENABLED(CONFIG_PROVE_RCU))
> - mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
> + mask = mask | READ_ONCE(sdp->srcu_reader_flavor);
> }
> WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
> "Mixed reader flavors for srcu_struct at %ps.\n", ssp);
> + *rdm = mask;
> return sum;
> }
>
> @@ -482,10 +482,11 @@ 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 rdm;
> unsigned long unlocks;
>
> - unlocks = srcu_readers_unlock_idx(ssp, idx);
> + unlocks = srcu_readers_unlock_idx(ssp, idx, &rdm);
> + bool did_gp = !!(rdm & SRCU_READ_FLAVOR_LITE);
Move "did_gp" declaration up?
Reviewed-by: Neeraj Upadhyay <neeraj.upadh...@amd.com>
-Neeraj
