On 2022-02-11 18:07:32 [+0100], Jason A. Donenfeld wrote: > On PREEMPT_RT, it's problematic to take spinlocks from hard irq > handlers. We can fix this by deferring to a workqueue the dumping of > the fast pool into the input pool. > > We accomplish this with some careful rules on fast_pool->count: > > - When it's incremented to >= 64, we schedule the work. > - If the top bit is set, we never schedule the work, even if >= 64. > - The worker is responsible for setting it back to 0 when it's done. > > There are two small issues around using workqueues for this purpose that > we work around. > > The first issue is that mix_interrupt_randomness() might be migrated to > another CPU during CPU hotplug. This issue is rectified by checking that > it hasn't been migrated (after disabling irqs). If it has been migrated, > then we set the count to zero, so that when the CPU comes online again, > it can requeue the work. As part of this, we switch to using an > atomic_t, so that the increment in the irq handler doesn't wipe out the > zeroing if the CPU comes back online while this worker is running. > > The second issue is that, though relatively minor in effect, we probably > want to make sure we get a consistent view of the pool onto the stack, > in case it's interrupted by an irq while reading. To do this, we don't > reenable irqs until after the copy. There are only 18 instructions > between the cli and sti, so this is a pretty tiny window. > > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Theodore Ts'o <tytso@xxxxxxx> > Cc: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> > Cc: Jonathan Neuschäfer <j.neuschaefer@xxxxxxx> > Reviewed-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Acked-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > --- > Sebastian - as requested, we now disable irqs for a very short 18 > instructions rather than fishing into migrate_disable() and upsetting > PeterZ. Might this be the lucky patch? -Jason I think we good. I'm not going to comment on the 90 char wide comment :) Sebastian
