might_sleep() output is pretty informative, but can be confusing at times
especially with PREEMPT_RCU when the check triggers due to a voluntary
sleep inside a RCU read side critical section:
BUG: sleeping function called from invalid context at kernel/test.c:110
in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 415, name: kworker/u112:52
Preemption disabled at: migrate_disable+0x33/0xa0
in_atomic() is 0, but it still tells that preemption was disabled at
migrate_disable(), which is completely useless because preemption is not
disabled. But the interesting information to decode the above, i.e. the RCU
nesting depth, is not printed.
That becomes even more confusing when might_sleep() is invoked from
cond_resched_lock() within a RCU read side critical section. Here the
expected preemption count is 1 and not 0.
BUG: sleeping function called from invalid context at kernel/test.c:131
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 415, name: kworker/u112:52
Preemption disabled at: test_cond_lock+0xf3/0x1c0
So in_atomic() is set, which is expected as the caller holds a spinlock,
but it's unclear why this is broken and the preempt disable IP is just
pointing at the correct place, i.e. spin_lock(), which is obviously not
helpful either.
Make that more useful in general:
- Print preempt_count() and the expected value
and for the CONFIG_PREEMPT_RCU case:
- Print the RCU read side critical section nesting depth
- Print the preempt disable IP only when preempt count
does not have the expected value.
So the might_sleep() dump from a within a preemptible RCU read side
critical section becomes:
BUG: sleeping function called from invalid context at kernel/test.c:110
in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 415, name: kworker/u112:52
preempt_count: 0, expected: 0
RCU nest depth: 1, expected: 0
and the cond_resched_lock() case becomes:
BUG: sleeping function called from invalid context at kernel/test.c:141
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 415, name: kworker/u112:52
preempt_count: 1, expected: 1
RCU nest depth: 1, expected: 0
which makes is pretty obvious what's going on. For all other cases the
preempt disable IP is still printed as before:
BUG: sleeping function called from invalid context at kernel/test.c: 156
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
preempt_count: 1, expected: 0
RCU nest depth: 0, expected: 0
Preemption disabled at:
[<ffffffff82b48326>] test_might_sleep+0xbe/0xf8
BUG: sleeping function called from invalid context at kernel/test.c: 163
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
preempt_count: 1, expected: 0
RCU nest depth: 1, expected: 0
Preemption disabled at:
[<ffffffff82b48326>] test_might_sleep+0x1e4/0x280
This also prepares to provide a better debugging output for RT enabled
kernels and their spinlock substitutions.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210923165358.181022656@linutronix.de
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