| Date | Tue, 21 Jul 2020 00:05:06 +0200 | From | Pavel Machek <> | Subject | Re: [PATCH 4.19 130/133] printk: queue wake_up_klogd irq_work only if per-CPU areas are ready |
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Hi!
> From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> > > commit ab6f762f0f53162d41497708b33c9a3236d3609e upstream. > > printk_deferred(), similarly to printk_safe/printk_nmi, does not > immediately attempt to print a new message on the consoles, avoiding > calls into non-reentrant kernel paths, e.g. scheduler or timekeeping, > which potentially can deadlock the system. > > Those printk() flavors, instead, rely on per-CPU flush irq_work to print > messages from safer contexts. For same reasons (recursive scheduler or > timekeeping calls) printk() uses per-CPU irq_work in order to wake up > user space syslog/kmsg readers. > > However, only printk_safe/printk_nmi do make sure that per-CPU areas > have been initialised and that it's safe to modify per-CPU irq_work. > This means that, for instance, should printk_deferred() be invoked "too > early", that is before per-CPU areas are initialised, printk_deferred() > will perform illegal per-CPU access. > > Lech Perczak [0] reports that after commit 1b710b1b10ef ("char/random: > silence a lockdep splat with printk()") user-space syslog/kmsg readers > are not able to read new kernel messages.
Is this still needed in 4.19? 1b710b1b10ef was reverted in 4.19, so there should not be any user-visible problems...
Best regards, Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html [unhandled content-type:application/pgp-signature] |