From 18808354b79622ed11857e41f9044ba17aec5b01 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 29 Sep 2015 17:47:18 -0400 Subject: percpu_ref: unify staggered atomic switching wait behavior When an atomic or percpu switching starts before the previous atomic switching finishes, the taken behaviors are * If the new atomic switching has confirmation callback, it waits for the previous atomic switching to complete. * If the new percpu switching is the first percpu switching following the previous atomic switching, it waits the previous atomic switching to complete. No percpu_ref user depends on these subtleties. The only meaningful part is that, if the caller ensures that atomic switching isn't in progress, mode switching operations can be issued from any context. This patch pulls the wait logic to the top of both switching functions so that they always wait for the previous atomic switching to complete. This makes the behavior simpler and consistent for both directions and will help allowing concurrent invocations of mode switching functions. Signed-off-by: Tejun Heo --- lib/percpu-refcount.c | 31 +++++++++++++++++++++---------- 1 file changed, 21 insertions(+), 10 deletions(-) (limited to 'lib') diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index 599a78c..c3617a8 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -161,15 +161,19 @@ static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref) static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref, percpu_ref_func_t *confirm_switch) { + /* + * If the previous ATOMIC switching hasn't finished yet, wait for + * its completion. If the caller ensures that ATOMIC switching + * isn't in progress, this function can be called from any context. + * Do an extra confirm_switch test to circumvent the unconditional + * might_sleep() in wait_event(). + */ + if (ref->confirm_switch) + wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); + if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) { - if (confirm_switch) { - /* - * Somebody else already set ATOMIC. Wait for its - * completion and invoke @confirm_switch() directly. - */ - wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); + if (confirm_switch) confirm_switch(ref); - } return; } @@ -180,7 +184,6 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref, * Non-NULL ->confirm_switch is used to indicate that switching is * in progress. Use noop one if unspecified. */ - WARN_ON_ONCE(ref->confirm_switch); ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch; percpu_ref_get(ref); /* put after confirmation */ @@ -192,13 +195,21 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref) unsigned long __percpu *percpu_count = percpu_count_ptr(ref); int cpu; + /* + * If the previous ATOMIC switching hasn't finished yet, wait for + * its completion. If the caller ensures that ATOMIC switching + * isn't in progress, this function can be called from any context. + * Do an extra confirm_switch test to circumvent the unconditional + * might_sleep() in wait_event(). + */ + if (ref->confirm_switch) + wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); + BUG_ON(!percpu_count); if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) return; - wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); - atomic_long_add(PERCPU_COUNT_BIAS, &ref->count); /* -- cgit v1.1