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srcu: Fix warning to permit SRCU-fast readers in NMI handlers

SRCU-fast is designed to be used in NMI handlers, even going so far
as to use atomic operations for architectures supporting NMIs but not
providing NMI-safe per-CPU atomic operations. However, the WARN_ON_ONCE()
in __srcu_check_read_flavor() complains if SRCU-fast is used in an NMI
handler. This commit therefore modifies that WARN_ON_ONCE() to avoid
such complaints.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Link: https://patch.msgid.link/8232efe8-a7a3-446c-af0b-19f9b523b4f7@paulmck-laptop
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

authored by

Paul E. McKenney and committed by
Steven Rostedt (Google) a77cb6a8 f7d32765

+2 -1
+2 -1
kernel/rcu/srcutree.c
··· 789 789 struct srcu_data *sdp; 790 790 791 791 /* NMI-unsafe use in NMI is a bad sign, as is multi-bit read_flavor values. */ 792 - WARN_ON_ONCE((read_flavor != SRCU_READ_FLAVOR_NMI) && in_nmi()); 792 + WARN_ON_ONCE(read_flavor != SRCU_READ_FLAVOR_NMI && 793 + read_flavor != SRCU_READ_FLAVOR_FAST && in_nmi()); 793 794 WARN_ON_ONCE(read_flavor & (read_flavor - 1)); 794 795 795 796 sdp = raw_cpu_ptr(ssp->sda);