On 9/3/21 09:03, Vlastimil Babka wrote: > On 9/3/21 08:22, Mike Galbraith wrote: >>> > so that when you read that function on its own, it's clear that the >>> > lock is always held over that critical section - and the issue is that >>> > perhaps the lock was already taken by the caller. >>> >>> Actually that "already taken" becomes "caller does not need it/can't > > Meant to say "... later in the series becomes ...". > >>> even take the local lock as it's not local" (it's a cpu hot remove >>> handler on behalf of another, dead cpu). >>> >>> So would it work with something like the following cleanup on top later >>> after proper testing? (now just compile tested). >> >> To my eyeballs, below duplication of a couple lines of initialization >> needed by the lockless function is less icky than the double return. > > Yeah, that's better, thanks Mike. Formal patch below, also added to my git branch: https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-local-lock-v5r1 ----8<---- >From b67952ce67528f3ebeaae58e0eae22a6dbae64b5 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka <vbabka@xxxxxxx> Date: Fri, 3 Sep 2021 12:59:25 +0200 Subject: [PATCH] mm, slub: remove conditional locking parameter from flush_slab() flush_slab() is called either as part of work scheduled on given live cpu, or called as a cleanup for another cpu that went offline. In the first case it needs to hold the cpu_slab->lock local lock when updating the protected kmem_cache_cpu fields. This is now achieved by a "bool lock" parameter. To avoid the conditional locking, we can instead lock unconditionally in flush_slab() for live cpus, and opencode the variant without locking in __flush_cpu_slab() for the dead cpus. Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Suggested-by: Mike Galbraith <efault@xxxxxx> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> --- mm/slub.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index df1ac8aff86f..77fe3d6d2065 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2566,15 +2566,13 @@ static inline void unfreeze_partials_cpu(struct kmem_cache *s, #endif /* CONFIG_SLUB_CPU_PARTIAL */ -static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c, - bool lock) +static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c) { unsigned long flags; void *freelist; struct page *page; - if (lock) - local_lock_irqsave(&s->cpu_slab->lock, flags); + local_lock_irqsave(&s->cpu_slab->lock, flags); freelist = c->freelist; page = c->page; @@ -2583,8 +2581,7 @@ static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c, c->freelist = NULL; c->tid = next_tid(c->tid); - if (lock) - local_unlock_irqrestore(&s->cpu_slab->lock, flags); + local_unlock_irqrestore(&s->cpu_slab->lock, flags); if (page) deactivate_slab(s, page, freelist); @@ -2595,9 +2592,17 @@ static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c, static inline void __flush_cpu_slab(struct kmem_cache *s, int cpu) { struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu); + struct page *page = c->page; + void *freelist = c->freelist; - if (c->page) - flush_slab(s, c, false); + c->page = NULL; + c->freelist = NULL; + c->tid = next_tid(c->tid); + + if (page) { + deactivate_slab(s, page, freelist); + stat(s, CPUSLAB_FLUSH); + } unfreeze_partials_cpu(s, c); } @@ -2625,7 +2630,7 @@ static void flush_cpu_slab(struct work_struct *w) c = this_cpu_ptr(s->cpu_slab); if (c->page) - flush_slab(s, c, true); + flush_slab(s, c); unfreeze_partials(s); } -- 2.33.0
