lkml.org 
[lkml]   [2022]   [Aug]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
    On 2022-08-23 19:15:43 [+0200], Vlastimil Babka wrote:
    > > +#define slub_local_irq_save(flags) local_irq_save(flags)
    > > +#define slub_local_irq_restore(flags) local_irq_restore(flags)
    >
    > Note these won't be neccessary anymore after
    > https://lore.kernel.org/linux-mm/20220823170400.26546-6-vbabka@suse.cz/T/#u

    Okay, let me postpone that one and rebase what is left on top.

    > > @@ -482,7 +488,7 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
    > > void *freelist_new, unsigned long counters_new,
    > > const char *n)
    > > {
    > > - if (!IS_ENABLED(CONFIG_PREEMPT_RT))
    > > + if (use_lockless_fast_path())
    > > lockdep_assert_irqs_disabled();
    >
    > This test would stay after the patch I referenced above. But while this
    > change will keep testing the technically correct thing, the name would be
    > IMHO misleading here, as this is semantically not about the lockless fast
    > path, but whether we need to have irqs disabled to avoid a deadlock due to
    > irq incoming when we hold the bit_spin_lock() and its handler trying to
    > acquire it as well.

    Color me confused. Memory is never allocated in-IRQ context on
    PREEMPT_RT. Therefore I don't understand why interrupts must be disabled
    for the fast path (unless that comment only applied to !RT).

    It could be about preemption since spinlock, local_lock don't disable
    preemption and so another allocation on the same CPU is possible. But
    then you say "we hold the bit_spin_lock()" and this one disables
    preemption. This means nothing can stop the bit_spin_lock() owner from
    making progress and since there is no memory allocation in-IRQ, we can't
    block on the same bit_spin_lock() on the same CPU.

    Sebastian

    \
     
     \ /
      Last update: 2022-08-24 15:27    [W:4.064 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site