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Message-ID: <6b2af276-f592-061e-1c0b-b11ed57b49ea@oracle.com>
Date: Fri, 1 Jul 2022 09:37:56 -0500
From: john.p.donnelly@...cle.com
To: Waiman Long <longman@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, Hillf Danton <hdanton@...a.com>,
Mel Gorman <mgorman@...hsingularity.net>,
John Donnelly <john.p.donnelly@...cle.com>
Subject: Re: [PATCH v2] locking/rwsem: Allow slowpath writer to ignore handoff
bit if not set by first waiter
On 6/22/22 3:04 PM, Waiman Long wrote:
> With commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more
> consistent"), the writer that sets the handoff bit can be interrupted
> out without clearing the bit if the wait queue isn't empty. This disables
> reader and writer optimistic lock spinning and stealing.
>
> Now if a non-first writer in the queue is somehow woken up or a new
> waiter enters the slowpath, it can't acquire the lock. This is not the
> case before commit d257cc8cb8d5 as the writer that set the handoff bit
> will clear it when exiting out via the out_nolock path. This is less
> efficient as the busy rwsem stays in an unlock state for a longer time.
>
> In some cases, this new behavior may cause lockups as shown in [1] and
> [2].
>
> This patch allows a non-first writer to ignore the handoff bit if it
> is not originally set or initiated by the first waiter. This patch is
> shown to be effective in fixing the lockup problem reported in [1].
>
> [1] https://urldefense.com/v3/__https://lore.kernel.org/lkml/20220617134325.GC30825@techsingularity.net/__;!!ACWV5N9M2RV99hQ!MtLCpFzIovhhzojTACH793QVlr1c2eCOsHnjbuB6O3ydj5cqhlZUjVchbfnbMeD4kQ5AsgjVmX0hGfREWPrEAA$
> [2] https://urldefense.com/v3/__https://lore.kernel.org/lkml/3f02975c-1a9d-be20-32cf-f1d8e3dfafcc@oracle.com/__;!!ACWV5N9M2RV99hQ!MtLCpFzIovhhzojTACH793QVlr1c2eCOsHnjbuB6O3ydj5cqhlZUjVchbfnbMeD4kQ5AsgjVmX0hGfQCd2bHaQ$
>
> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
> Signed-off-by: Waiman Long <longman@...hat.com>
> Tested-by: Mel Gorman <mgorman@...hsingularity.net>
Acked-by: John Donnelly <john.p.donnelly@...cle.com>
> ---
> kernel/locking/rwsem.c | 30 ++++++++++++++++++++----------
> 1 file changed, 20 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 9d1db4a54d34..ffd6188d4a7c 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -335,8 +335,6 @@ struct rwsem_waiter {
> struct task_struct *task;
> enum rwsem_waiter_type type;
> unsigned long timeout;
> -
> - /* Writer only, not initialized in reader */
> bool handoff_set;
> };
> #define rwsem_first_waiter(sem) \
> @@ -459,10 +457,12 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
> * to give up the lock), request a HANDOFF to
> * force the issue.
> */
> - if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
> - time_after(jiffies, waiter->timeout)) {
> - adjustment -= RWSEM_FLAG_HANDOFF;
> - lockevent_inc(rwsem_rlock_handoff);
> + if (time_after(jiffies, waiter->timeout)) {
> + if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
> + adjustment -= RWSEM_FLAG_HANDOFF;
> + lockevent_inc(rwsem_rlock_handoff);
> + }
> + waiter->handoff_set = true;
> }
>
> atomic_long_add(-adjustment, &sem->count);
> @@ -599,7 +599,7 @@ rwsem_del_wake_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter,
> static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
> struct rwsem_waiter *waiter)
> {
> - bool first = rwsem_first_waiter(sem) == waiter;
> + struct rwsem_waiter *first = rwsem_first_waiter(sem);
> long count, new;
>
> lockdep_assert_held(&sem->wait_lock);
> @@ -609,11 +609,20 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
> bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>
> if (has_handoff) {
> - if (!first)
> + /*
> + * Honor handoff bit and yield only when the first
> + * waiter is the one that set it. Otherwisee, we
> + * still try to acquire the rwsem.
> + */
> + if (first->handoff_set && (waiter != first))
> return false;
>
> - /* First waiter inherits a previously set handoff bit */
> - waiter->handoff_set = true;
> + /*
> + * First waiter can inherit a previously set handoff
> + * bit and spin on rwsem if lock acquisition fails.
> + */
> + if (waiter == first)
> + waiter->handoff_set = true;
> }
>
> new = count;
> @@ -1027,6 +1036,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
> waiter.task = current;
> waiter.type = RWSEM_WAITING_FOR_READ;
> waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
> + waiter.handoff_set = false;
>
> raw_spin_lock_irq(&sem->wait_lock);
> if (list_empty(&sem->wait_list)) {
Hi,
ping .
Can we get this reviewed ?
The offending commit:
d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
is found in all these branches:
kernel_dot_org/linux-stable.git linux-5.15.y - 76723ed1fb89
kernel_dot_org/linux-stable.git linux-5.16.y - d257cc8cb8d5
kernel_dot_org/linux-stable.git linux-5.17.y - d257cc8cb8d5
kernel_dot_org/linux-stable.git linux-5.18.y - d257cc8cb8d5
kernel_dot_org/linux-stable.git master - d257cc8cb8d5
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