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Sergey Senozhatsky
ab6f762f0f
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 1b710b1b10 ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.
The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).
Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.
Link: https://lore.kernel.org/lkml/[email protected]/
Reported-by: Lech Perczak <[email protected]>
Signed-off-by: Sergey Senozhatsky <[email protected]>
Tested-by: Jann Horn <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: John Ogness <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>